EE6503 POWER ELECTRONICS L T P C

3 0 0 3

CO1: Ability to understand and analyze about linear and digital electronic circuits.

CO2: Ability to understand and analyze about phase controlled rectifiers.

CO3: Ability to understand and analyze about dc-dc converters.

CO4: Ability to understand and analyze about inverters and PWM.

CO5: Ability to understand and analyze about ac-ac converters

UNIT I POWER SEMI-CONDUCTOR DEVICES Study of switching devices, Diode, SCR, TRIAC, GTO, BJT, MOSFET, IGBT-Static

and Dynamic characteristics - Triggering and commutation circuit for SCR- Design of Driver

and snubber circuit.

UNIT II PHASE-CONTROLLED CONVERTERS 2-pulse,3-pulse and 6-pulseconverters– performance parameters –Effect of source inductance–– Gate

Circuit Schemes for Phase Control–Dual converters.

UNIT III DC TO DC CONVERTER Step-down and step-up chopper-control strategy–Forced commutated chopper–Voltage

commutated, Current commutated, Load commutated, Switched mode regulators- Buck,

boost, buck- boost converter, Introduction to Resonant Converters. UNIT IV INVERTERS

Single phase and three phase voltage source inverters(both1200modeand180

0mode)–Voltage&

harmonic control--PW M techniques: Sinusoidal PW M, modified sinusoidal PW M -

multiple PW M – Introduction to space vector modulation –Current source inverter.

UNIT V AC TO AC CONVERTERS Single phase and Three phase AC voltage controllers–Control strategy- Power Factor

Control – Multistage sequence control -single phase and three phase cyclo converters –

Introduction to Matrix converters.

UNIT 1

INTRODUCTION

Power Electronics is the art of converting electrical energy from one form to another

in an efficient, clean, compact, and robust manner for convenient utilisation.

Classification of Power Electronic Devices

Broad classifications of Power Electronic devices are as follows:

(A)Based on family

1. DIODES: PN Junction Diodes, Schottky dodes

2. TRANSISTOR: Power BJT (Bipolar Junction Transistor), MD (Monolithic

Darlington), Power MOSFET (Metal Oxide Semiconductor Field Effect Transistor),

IGBT (Insulated Gate Bipolar Transistor)

3. THYRISTORS: SCR (Silicon Controller Rectifier), GTO (Gate Turn Off Thyristor),

MCT (MOS-Controlled Thyristor) etc.

(B)Based on controllability

1. Uncontrolled Devices such as diodes: on and off sates controlled by the power circuit.

2. Pulse Driven Devices such as SCR and TRIAC: latched on by a control signal but

must be turned off by the power circuit.

3. Controllable Devices: Two types which are turned on and off by control signals.

i. Current-Driven Devices: BJT, MD, GTO

ii. Voltage-Driven Devices: MOSFET, IGBT, MCT.

All devices are operated in the switching mode which has the advantages of high efficiency

and high power capability.

(C) Based on material used

Classification of state-of-the-art power semiconductors (Silicon and Silicon-carbide)

SCR

The SCR is a four layer and three terminal device. The four

layers made of P and N layers, are arranged alternately such

that they form three junctions J1, J2 and J3. These junctions

are either alloyed or diffused based on the type of construction.

The outer layers (P and N-layers) are heavily doped whereas

middle P and N-layers are lightly doped. The gate terminal is

taken at the middle P-layer, anode is from outer P- layer and

cathode is from N- layer terminals. The SCR is made of silicon

because compared to germanium leakage current in silicon is very small.

To manufacture the SCR, three types of constructions

are used, namely the planar type, Mesa type and Press

pack type. For low power SCRs, planar construction is

used where all the junctions in an SCR are diffused. In

mesa type construction, junction J2 is formed by

diffusion method and thereby outer layers are alloyed to

it. This construction is mainly used for high power SCRs. To provide high mechanical

strength, the SCR is braced with plates made up of either molybdenum or tungsten. And one

of these plates is soldered to a copper stud which is further threaded to connect the heat sink.

Working or Modes of Operation of SCR

Depending on the biasing given to the SCR, the operation of SCR is divided into three

modes. They are

1. Forward blocking Mode

2. Forward Conduction Mode and

3. Reverse Blocking Mode

Forward Blocking Mode

SCR is connected such that the anode terminal is made positive with respect to cathode

while the gate terminal kept open.

Junctions J1 and J3 are forward biased and the junction J2 reverse biased.

Due to this, a small leakage current flows through the SCR.

Until the voltage applied across the SCR is more than the break over voltage of it, SCR

offers a very high resistance to the current flow.

Therefore, the SCR acts as a open switch in this mode by blocking forward current

flowing through the SCR

Forward Conduction Mode

SCR or thyristor comes into the conduction mode from blocking mode.

It can be done in two ways as either by applying positive pulse to gate terminal or by

increasing the forward voltage (or voltage across the anode and cathode) beyond the

break over voltage of the SCR.

Once any one of these methods is applied, the avalanche breakdown occurs at junction

J2. Therefore the SCR turns into conduction mode and acts as a closed switch thereby

current starts flowing through it

If the gate current value is high, the minimum will be the time to come in conduction

mode as Ig3 > Ig2 > Ig1. In this mode, maximum current flows through the SCR and its

value depends on the load resistance or impedance.

It is also noted that if gate current is increasing, the voltage required to turn ON the SCR

is less if gate biasing is preferred.

The current at which the SCR turns into conduction mode from blocking mode is called

as latching current (IL). And also when the forward current reaches to level at which the

SCR returns to blocking state is called as holding current (IH).

At this holding current level, depletion region starts to develop around junction J2.

Hence the holding current is slightly less than the latching current.

Reverse Blocking Mode

In this mode of operation,

cathode is made positive with

respect to anode.

J1 and J3 are reverse

biased and J2 is forward biased.

This reverse voltage

drives the SCR into reverse

blocking region results to flow a

small leakage current through it

and acts as an open switch.

So the device offers a high

impedance in this mode until the

voltage applied is less than the reverse breakdown voltage VBR of the SCR.

SCR Triggering or Turn ON Methods With a voltage applied to the SCR, if the anode is made positive with respect to the cathode,

the SCR becomes forward biased. Thus, the SCR comes into the forward blocking state. The

SCR can be made to conduct or switching into conduction mode is performed by any one of

the following methods.

1. Forward voltage triggering

2. Temperature triggering

3. dv/dt triggering

4. Light triggering

5. Gate triggering

1. Forward Voltage Triggering

By increasing the forward anode to cathode

voltage, the depletion layer width is also

increasing at junction J2. This also causes to

increase the minority charge carriers

accelerating voltage at junction J2. This further

leads to an avalanche breakdown of the junction

J2 at a forward breakover voltage VBO. At this

stage SCR turns into conduction mode and

hence a large current flow thorugh it with a low

voltage drop across it.

2. Temperature Triggering

The reverse leakage current depends on the temperature. If the temperature is increased to a

certain value, the number of hole-pairs also increases. This causes to increase the leakage

current and further it increases the current gains of the SCR. This starts the regenerative

action inside the SCR since the (α1 + α2) value approaches to unity (as the current gains

increases). By increasing the temperature at junction J2 causes the breakdown of the junction

and hence it conducts.

3. dv/dt Triggering

In forward blocking state junctions J1 and J3 are forward biased and J2 is reverse biased. So

the junction J2 behaves as a capacitor (of two conducting plates J1 and J3 with a dielectric

J2) due to the space charges in the depletion region. The charging current of the capacitor is

given as I = C dv/ dt

where dv/dt is the rate of change of applied voltage and C is the junction capacitance.

4. Light Triggering

An SCR turned ON by light radiation is also called as Light

Activated SCR (LASCR). This type of triggering is employed for

phase controlled converters in HVDC transmission systems. In this

method, light rays with appropriate wavelength and intensity are

allowed to strike the junction J2. These types of SCRs are

consisting a niche in the inner p-layer. Therefore, when the light

struck on this niche, electron-hole pairs are generated at the

junction J2 which provides additional charge carriers at the

junction leads to turn ON the SCR.

5.Gate Triggering

This is most common and efficient method to turn ON the SCR. When the SCR is forward

biased, a sufficient voltage at the gate terminal injects some electrons into the junction J2.

This result to increase reverse leakage current and hence the breakdown of junction J2 even

at the voltage lower than the VBO. Depends on the size of the SCR the gate current varies

from a few milli-amps to 200 milli amps or more. If the gate current applied is more, then

more electrons are injected into the junction J2 and results to come into the conduction state

at much lower applied voltage.

In gate triggering method, a positive voltage applied between the gate and the cathode

terminals. We can use three types of gate signals to turn On the SCR. Those are DC signal,

AC signal and pulse signal.

DC Gate Triggering

In this triggering, a sufficient DC voltage is applied between the gate and cathode terminals

in such a way that the gate is made positive with respect to the cathode. The gate current

drives the SCR into conduction mode.

AC Triggering

This is the most commonly used method for AC applications

where the SCR is employed for such applications as a switching

device. With the proper isolation between the power and control

circuit, the SCR is triggered by the phase-shift AC voltage

derived from the main supply. The firing angle is controlled by

changing the phase angle of the gate signal.

.

Pulse Triggering

The most popular method of triggering the SCR is the pulse triggering. In this method, gate is

supplied with single pulse or a train of pulses. The main advantage of this method is that gate

drive is discontinuous or doesn’t need continuous pulses to turn the SCR and hence gate

losses are reduced in greater amount by applying single or periodically appearing pulses. For

isolating the gate drive from the main supply, a pulse transformer is used.

Dynamic Switching Characteristics

The dynamic processes of the SCR are turn ON and turn OFF processes in which both

voltage and currents of an SCR vary with time. The transition from one state to another takes

finite time, but doesn’t take place instantaneously. The static or VI characteristics of the SCR

give no indication about the speed at which the SCR switched into forward conduction mode

from forward blocking mode. Hence the dynamic characteristics are sometimes more

important which gives the switching characteristics of the SCR.

Delay Time (td)

The delay time is measured from the instant at which the gate current reaches 90 percent of

its final value to the instant at which anode current reaches 10 percent of its final value. It can

also define as the time

between which anode voltage

falls from initial anode

voltage value Va to 0.9va.

Consider the below figure and

observe that, until the time td,

the SCR is in forward

blocking mode so the anode

current is the small leakage

current. When the gate signal

is applied (at 90 percent of Ig)

then the gate current is

reached to 0.1 Ia and also

correspondingly anode to

cathode voltage falls to

0.9Va.

Rise Time (tr)

This is the time taken by the

anode current to rise from 10

percent to 90 percent of its

final value. Also called as the

time required for the forward blocking voltage to fall from 0.9Va to 0.1Va. This rise time is

inversely proportional to the gate current and its rate of building up. Therefore, if high and

steep current pulses are applied at the gate reduces the rise time tr. Also, if the load is

inductive this rise time will be higher and for resistive and capacitive loads it is low.

During this time, turn ON losses in the SCR are high due to large anode current and high

anode voltage occurs simultaneously. This can result in the formation of local hot spots and

hence the SCR may be damaged.

Spread Time (ts)

This is the time taken by the anode current to rise from 0.9Ia to Ia. Also the time required for

the forward blocking voltage to fall from o.1Va to its ON-state voltage drop which is the

range of 1 to 1.5 volts. During this time anode current spread over the entire conducting

region of an SCR from a narrow conducting region. After the spreading time, a full anode

current flows through the device with small ON-state voltage drop.

Therefore, the total turn ON time,

Ton = tr + td + ts

The typical value of the turn ON time is in the order of 1 to 4 micro seconds depends on the

gate signal wave shapes and anode circuit parameters . To reduce the turn ON time of the

SCR, the amplitude of the gate pulse should be in the order of 3 to 5 times the minimum gate

current of the SCR.

TRIAC

“TRIAC” is a 4-layer, PNPN in the

positive direction and a NPNP in the

negative direction, three-terminal

bidirectional device that blocks current in

its “OFF” state acting like an open-circuit

switch, but unlike a conventional

thyristor, the triac can conduct current in

either direction when triggered by a single

gate pulse.

A triac has four possible triggering modes of operation as follows.

Ι + Mode = MT2 current positive (+ve), Gate current positive (+ve)

Ι – Mode = MT2 current positive (+ve), Gate current negative (-ve)

ΙΙΙ + Mode = MT2 current negative (-ve), Gate current positive (+ve)

ΙΙΙ – Mode = MT2 current negative (-ve), Gate current negative (-ve)

Triac I-V Characteristics Curves

In Quadrant Ι, the triac is usually triggered

into conduction by a positive gate current,

labelled above as mode Ι+. But it can also be

triggered by a negative gate current, modeΙ–.

Similarly, in Quadrant ΙΙΙ, triggering with a

negative gate current, –ΙG is also common,

mode ΙΙΙ– along with mode ΙΙΙ+. Modes Ι–

and ΙΙΙ+ are, however, less sensitive

configurations requiring a greater gate

current to cause triggering than the more

common triac triggering modes of Ι+ and ΙΙΙ–.

Also, just like silicon controlled rectifiers (SCR’s), triac’s also require a minimum holding

current IH to maintain conduction at the waveforms cross over point. Then even though the

two thyristors are combined into one single triac device, they still exhibit individual electrical

characteristics such as different breakdown voltages, holding currents and trigger voltage

levels exactly the same as we would expect from a single SCR device.

Operation In quadrants 1 and 2, MT2 is positive, and

current flows from MT2 to MT1 through P, N, P and N

layers. The N region attached to MT2 does not

participate significantly. In quadrants 3 and 4, MT2 is

negative, and current flows from MT1 to MT2, also

through P, N, P and N layers. The N region attached to

MT2 is active, but the N region attached to MT1 only

participates in the initial triggering, not the bulk current

flow.

In most applications, the gate current comes from MT2, so quadrants 1 and 3 are the only

operating modes (both gate and MT2 positive or negative against MT1). Other applications

with single polarity triggering from an IC or digital drive circuit operate in quadrants 2 and 3,

than MT1 is usually connected to positive voltage (e.g. +5V) and gate is pulled down to 0V

(ground).

Quadrant 1 : When MT2 and Gate being Positive with Respect to MT1

When this happens, current flows through the path P1-N1-P2-N2. Here, P1-N1 and

P2-N2 are forward biased but N1-P2 is reverse biased. The triac is said to be operated in

positively biased region. Positive gate with respect to MT1 forward biases P2-N2 and

breakdown occurs.

Quadrant 2 : When MT2 is Positive but Gate is Negative with Respect to MT1

The current flows through the path P1-N1-P2-N2. But P2-N3 is forward biased and current

carriers injected into P2 on the triac.

Quadrant 3 : When MT2 and Gate are Negative with Respect to MT1

Current flows through the path P2-N1-P1-N4. Two junctions P2-N1 and P1-N4 are forward

biased but the junction N1-P1 is reverse biased. The triac is said to be in the negatively biased

region.

Quadrant 4 : When MT2 is Negative but Gate is Positive with Respect to MT1

P2-N2 is forward biased at that condition. Current carriers are injected so the triac turns on.

This mode of operation has a disadvantage that it should not be used for high (di/dt) circuits.

Sensitivity of triggering in mode 2 and 3 is high and if marginal triggering capability is

required, negative gate pulses should be used. Triggering in mode 1 is more sensitive than

mode 2 and mode 3.

Gate turn-off thyristor

Normal thyristors (silicon-

controlled rectifiers) are not fully

controllable switches (a "fully

controllable switch" can be turned

on and off at will). Thyristors can only be turned ON and cannot be turned OFF. Thyristors are

switched ON by a gate signal, but even after the gate signal is de-asserted (removed), the thyristor

remains in the ON-state until any turn-off condition occurs (which can be the application of a

reverse voltage to the terminals, or when the current flowing through (forward current) falls

below a certain threshold value known as the "holding current"). Thus, a thyristor behaves like a

normal semiconductor diode after it is turned on or "fired".

Steady state output and gate characteristics

The latching current of a GTO is considerably higher than a thyristor of similar rating.

The forward leakage current is also considerably higher. In fact, if the gate current is not

sufficient to turn on a GTO it operates as a high voltage low gain transistor with considerable

anode current. It should be noted that a GTO can block rated forward voltage only when the gate

is negatively biased with respect to the cathode during forward blocking state. At least, a low

value resistance must be connected across the gate cathode terminal. Increasing the value of this

resistance reduces the forward blocking voltage of the GTO. Asymmetric GTOs have small (20-

30 V) reverse break down voltage. This may lead the device to operate in “reverse avalanche”

under certain conditions. This condition is not dangerous for the GTO provided the avalanche

time and current are small. The gate voltage during this period must remain negative..

The zone between the min and max curves reflects parameter variation between

individual GTOs. These characteristics are valid for DC and low frequency AC gate currents.

They do not give correct voltage when the GTO is turned on with high dia/dt and Gd/Idt. VG in

this case is much higher.

Dynamic characteristics of a GTO When the GTO is off the anode current is zero and VAK = Vd. To turn on the GTO, a

positive gate current pulse is

injected through the gate

terminal. A substantial gate

current ensure that all GTO

cathode segments are turned on

simultaneously and within a short

time. There is a delay between

the application of the gate pulse

and the fall of anode voltage,

called the turn on delay time td.

After this time the anode voltage

starts falling while the anode

current starts rising towards its

steady value IL. Within a further

time interval tr they reach 10% of

their initial value and 90% of

their final value respectively. tr is

called the current rise time (voltage fall time). Both td and maximum permissible on state Adidt

are very much gate current dependent. High value of I gM and digdt at turn on reduces these

times and increases maximum permissible on state Adidt . It should be noted that large value of

ig (IgM) and digdt are required during td and tr only. After this time period both vg and ig settles

down to their steady value. A minimum ON time period tON (min) is required for homogeneous

anode current conduction in the GTO. This time is also necessary for the GTO to be able to turn

off its rated anode current.

To turn off a GTO the

gate terminal is negatively biased

with respect to the cathode. With

the application of the negative

bias the gate current starts

growing in the negative direction.

However, the anode

voltage,current or the gate

voltage does not change

appreciably from their on state

levels for a further time period called the storage time (ts). The storage time increases with the

turn off anode current and decrease withgQdidt. During storage time the load current at the

cathode end is gradually diverted to the gate terminal. At the end of the storage time gate current

reaches its negative maximum value IgQ. At this point both the junctions J2 & J3 of the GTO

starts blocking voltage. Consequently, both the gate cathode and the anode cathode voltage starts

rising towards their final value while the anode current starts decreasing towards zero. At the end

of current fall time “tf” the anode current reaches 10% of its initial value after which both the

anode current and the gate current continues to flow in the form of a current tail for a further

duration of ttail. A GTO is normally used with a R-C turn off snubber. Therefore, VAK does not

start to rise appreciably till tf. At this point VAK starts rising rapidly and exceeds the dc voltage

Vd (VdM) (due to resonance of snubber capacitor with didt limiting inductor) before setting

down at its steady value Vd . A GTO should not be retriggered within a minimum off period off

(min) to avoid the risk of failure due to localized turn ON. GTOs have typically low turn off gain

in the range of 4-5

Bipolar Junction Transistor (BJT) Power BJT is used traditionally for many applications. However, IGBT (Insulated-Gate

Bipolar Transistor) and MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) have

replaced it for most of the applications but still they are used in some areas due to its lower

saturation voltage over the operating temperature range. IGBT and MOSFET have higher

input capacitance as compared to BJT. Thus, in case of IGBT and MOSFET, drive circuit

must be capable to charge and discharge the internal capacitances.

Although BJTs have lower input capacitance as compared to MOSFET or

IGBT, BJTs are considerably slower in response due to low input

impedance. BJTs use more silicon for the same drive performance.

In the case of MOSFET studied earlier, power BJT is different in

configuration as compared to simple planar BJT. In planar BJT, collector

and emitter is on the same side of the wafer while in power BJT it is on the

opposite edges. This is done to increase the power-handling capability of

BJT.

(a) NPN BJT (b) PNP BJT Input Characteristics and Output Characteristics

A forward-

biased p-n junction

has two capacitances

named depletion layer

capacitance and

diffused capacitance.

While a reverse bias

junction has only a

depletion capacitance

in action. Value of

these capacitances depends on

the junction voltage and

construction of the transistor.

These capacitances come into

role during the transient

operation i.e. switching

operations. Due to these

capacitances, transistor does not

turn on or turn off instantly.

Switching characteristics

of power BJT is shown in

Fig.26. As the positive base

voltage is applied, base current

starts to flow but there is no

collector current for some time.

This time is known as the delay

time(td) required to charge the

junction capacitance of the base

to emitter to 0.7 V approx.

(known as forward-bias

voltage). For t > td, collector

current starts rising and

VCE starts to drop with the

magnitude of 9/10th of its peak

value. This time is called rise

time, required to turn on the transistor. The transistor remains on so long as the collector

current is at least of this value.

For turning off the BJT, polarity of the base voltage is reversed and thus the base

current polarity will also be changed as shown in Fig. 26. The base current required during

the steady-state operation is more than that required to saturate the transistor. Thus, excess

minority carrier charges are stored in the base region which needs to be removed during the

turn-off process. The time required to nullify this charge is the storage time, ts. Collector

current remains at the same value for this time. After this, collector current starts decreasing

and base-to-emitter junction charges to the negative polarity; base current also get reduced.

MOSFET

Enhancement mode Depletion mode

COMPARISON OF THE KEY FEATURES OF N-CHANNEL AND P-CHANNEL MOSFETS

PARAMETER N-CHANNEL P-CHANNEL

Source / drain material N-Type P-Type

Channel material P-Type N-Type

Threshold voltage Vth negative doping dependent

Substrate material P-Type P-Type

Inversion layer carriers Electrons Holes

As already

implied the key

factor of the

MOSFET is the

fact that the gate

is insulated from

the channel by a

thin oxide layer.

This forms one of

the key elements of its structure.

For an N-channel device the current flow is carried by electrons and in the diagram

below it can be seen that the drain and source are formed using N+ regions which provide

good conductivity for these regions.

In some structures the N+ regions are formed using ion implantation after the gate area

has been formed. In this way, they are self-aligned to the gate.

The gate to source and gate to drain overlap are required to ensure there is a continuous

channel. Also the device is often symmetrical and therefore source and drain can be

interchanged. On some higher power designs this may not always be the case.

It can be seen from the diagram that the substrate is the opposite type to the channel,

i.e. P-type rather than N-type, etc. This is done to achieve source and drain isolation. The

oxide over the channel is normally grown thermally as this ensure good interfacing with the

substrate and the most common gate material is polysilicon, although some metals and

silicides can be used.

MOSFET Characteristics Curves

The minimum ON-state gate voltage

required to ensure that the MOSFET

remains “ON” when carrying the selected

drain current can be determined from the

v-i transfer curves above. When VIN is

HIGH or equal to VDD, the MOSFET Q-

point moves to point A along the load line.

The drain current ID increases to its

maximum value due to a reduction in the channel resistance. ID becomes a constant value

independent of VDD, and is dependent only on VGS. Therefore, the transistor behaves like a

closed switch but the channel ON-resistance does not reduce fully to zero due to

its RDS(on) value, but gets very small.

Likewise, when VIN is LOW or reduced to zero, the MOSFET Q-point moves from

point A to point B along the load line. The channel resistance is very high so the transistor

acts like an open circuit and no current flows through the channel. So if the gate voltage of

the MOSFET toggles between two values, HIGH and LOW the MOSFET will behave as a

“single-pole single-throw” (SPST) solid state switch.

There are basically three regions in which MOSFETs can operate:

Cut-off region: In this region of the MOSFET is in a non-conducting state, i.e. turned

OFF - channel current IDS = 0. The gate voltage VGS is less than the threshold voltage

required for conduction.

Linear region: In this linear region the channel is conducting and controlled by the

gate voltage. For the MOSFET to be in this state the VGS must be greater than the threshold

voltage and also the voltage across the channel, VDS must be greater than VGS.

Saturation region: In this region the MOSFET is turned hard on. The voltage drop

for a MOSFET is typically lower than that of a bipolar transistor and as a result power

MOSFETs are widely used for switching large currents.

Switching characteristics of a MOSFET

Power MOSFETs

are often used as

switching devices.

The switching

characteristic of a

power MOSFET

depends on the

capacitances

between gate to

source(Cgs), gate to

drain (Cgd) and

drain to source

(Cds). It also

depends on the

impedance of the

gate drive circuit. During turn-on there is a turn-on delay (td on), which is the time required

for the input capacitance to charge to threshold voltage level . During the rise time (tr),

charges to full gate voltage and the device operate in the linear region (ON state). During

rise time drain current ID rises from zero to full on state current.

• Total turn-on time, t on = td on + tr

MOSFET can be turned off by discharging capacitance.td_off is the turn-off delay time

required for input capacitance to discharge from to . Fall time GSC()dofftGSC1VGSPVft is

the time required for input capacitance to discharge from to threshold voltage . During fall

time tf drain current falls from DI to zero. Figure below shows the switching waveforms of

power MOSFET.

Insulated Gate Bipolar Transistor (IGBT) The major difference

with the corresponding

MOSFET cell structure

lies in the addition of a p+

injecting layer. This layer

forms a pn junction with

the drain layer and injects

minority carriers into it.

The n type drain layer

itself may have two

different doping levels.

The lightly doped n-

region is called the drain

drift region. Doping level and width of this layer sets the forward blocking voltage (determined

by the reverse break down voltage of J2) of the device.

The doping level and physical geometry of the p type body region however, is

considerably different from that of a MOSFET in order to defeat the latch up action of a parasitic

thyristor embedded in the

IGBT structure.

The IGBT cell has a

parasitic p-n-p-n thyristor

structure embedded into it.

The constituent p-n-p

transistor, n-p-n transistor

and the driver MOSFET are

shown by dotted lines in

this figure. Important

resistances in the current

flow path are also indicated.

Steady state characteristics of an IGBT

When the gate emitter voltage is below the threshold voltage only a very small leakage

current flows though the device while the collector – emitter voltage almost equals the supply

voltage (point C in Fig ). The device, under this condition is said to be operating in the cut off

region. The maximum forward voltage the device can withstand in this mode (marked VCES in

Fig) is determined by the avalanche break down voltage of the body – drain p-n junction. Unlike

a BJT, however, this break down voltage is independent of the collector current as shown in Fig

7.4(a). IGBTs of Non-punch through design can block a maximum reverse voltage (VRM) equal

to VCES in the cut off mode. However, for Punch Through IGBTs VRM is negligible (only a few

tens of volts) due the presence of the heavily doped n+ drain buffer layer.

Switching characteristics of IGBT Similarity of these waveforms with those of a MOSFET is obvious. To turn on the IGBT the gate

drive voltage changes

from –Vgg to +Vgg. The

gate emitter voltage vgE

follows Vgg with a time

constant τ1. Since the

drain source voltage of

the drive MOSFET is

large the gate drain

capacitor assumes the

lower value CGD1

. The collector current ic does not start increasing till vgE reaches the threshold voltage

vgE(th). Thereafter, ic increases following the transfer characteristics of the device till vgE

reaches a value vgEIL corresponding to ic = iL. This period is called the current rise time tri. The

free wheeling diode current falls from IL to zero during this period. After ic reaches IL, vgE

becomes clamped at vgE IL similar to a MOSFET. vCE also starts falling during this period. First

vCE falls rapidly (tfv1) and afterwards the fall of vCE slows down considerably. Two factors

contribute to the slowing down of voltage fall. First the gate-drain capacitance Cgd will increase

in the MOSFET portion of the IGBT at low drain-source voltages. Second, the pnp transistor

portion of the IGBT traverses the active region to its on state more slowly than the MOSFET

portion of the IGBT. Once the pnp transistor is fully on after tfv2, the on state voltage of the

device settles down to vCE(sat). The turn ON process ends here.

The turn off process of an IGBT follows the inverse sequence of turn ON with one major

difference. Once vgE goes below vgE(th) the drive MOSFET of the IGBT equivalent circuit turns

off. During this period (tfi1) the device current falls rapidly. However, when the drive MOSFET

turns off, some amount of current continues of flow through the output p-n-p transistor due to

stored charge in its base. Since there is no reverse voltage applied to the IGBT terminals that

could generate a negative drain current, there is no possibility for removing the stored charge by

carrier sweep-out. The only way these excess carriers can be removed is by recombination within

the IGBT. During this recombination period (tfi2) the remaining current in the IGBT decays

relatively slowly forming a current fail. A long tfi2 is undesirable, because the power dissipation

in this interval will be large due to full collector-emitter voltage. tfi2 can be reduced by

decreasing the excess carrier life time in the p-n-p transistor base. However, in the process, on

state losses will increase. Therefore, judicious design trade offs are made in a practical IGBT to

give minimum total loss.

SCR Firing Circuits

Resistance Firing Circuit

The circuit below shows the

resistance triggering of SCR where it is

employed to drive the load from the input

AC supply. Resistance and diode

combination circuit acts as a gate control

circuitry to switch the SCR in the desired

condition.

As the positive voltage applied,

the SCR is forward biased and doesn’t

conduct until its gate current is more than minimum gate current of the SCR.

When the gate current is applied by varying the resistance R2 such that the gate current

should be more than the minimum value of gate current, the SCR is turned ON. And

hence the load current starts flowing through the SCR.

The SCR remains ON until the anode current is equal to the holding current of the

SCR. And it will switch OFF when the voltage applied is zero. So the load current is

zero as the SCR acts as open switch.

The diode protects the gate drive circuit from reverse gate voltage during the negative

half cycle of the input. And Resistance R1 limits the current flowing through the gate

terminal and its value is such that the gate current should not exceed the maximum

gate current.

It is the simplest and economical type of triggering but limited for few applications due

to its disadvantages.

In this, the triggering angle is limited to 90 degrees only. Because the applied voltage

is maximum at 90 degrees so the gate current has to reach minimum gate current value

somewhere between zero to 90 degrees.

Resistance – Capacitacne (RC) Firing Circuit

The limitation of

resistance firing circuit can be

overcome by the RC triggering

circuit which provides the

firing angle control from 0 to

180 degrees. By changing the

phase and amplitude of the gate

current, a large variation of

firing angle is obtained using this circuit.

Below figure shows the RC triggering circuit consisting of two diodes with an RC

network connected to turn the SCR.

By varying the variable resistance, triggering or firing angle is controlled in a full

positive half cycle of the input signal.

During the negative half cycle of the input signal, capacitor charges with lower plate

positive through diode D2 up to the maximum supply voltage Vmax. This voltage

remains at -Vmax across the capacitor till supply voltage attains zero crossing.

During the positive half cycle of the input, the SCR becomes forward biased and the

capacitor starts charging through variable resistance to the triggering voltage value of

the SCR.

When the capacitor charging voltage is equal to the gate trigger voltage, SCR is turned

ON and the capacitor holds a small voltage. Therefore the capacitor voltage is helpful

for triggering the SCR even after 90 degrees of the input waveform.

In this, diode D1 prevents the negative voltage between the gate and cathode during

the negative half cycle of the input through diode D2.

UJT Firing Circuit

It is the most common

method of triggering the SCR

because the prolonged pulses

at the gate using R and RC

triggering methods cause more

power dissipation at the gate

so by using UJT (Uni Junction

Transistor) as triggering

device the power loss is limited as it produce a train of pulses.

The RC network is connected to the emitter terminal of the UJT which forms the

timing circuit. The capacitor is fixed while the resistance is variable and hence the

charging rate of the capacitor depends on the variable resistance means that the

controlling of the RC time constant.

When the voltage is applied, the capacitor starts charging through the variable

resistance. By varying the resistance value voltage across the capacitor get varied.

Once the capacitor voltage is equal to the peak value of the UJT, it starts conducting

and hence produce a pulse output till the voltage across the capacitor equal to the

valley voltage Vv of the UJT. This process repeats and produces a train of pulses at

base terminal 1.

The pulse output at the base terminal 1 is used to turn ON the SCR at predetermined

time intervals.

SCR Turn ON Methods

From the above equation, if (α1 + α2) is equal to one then Ia becomes infinite. That means

anode current suddenly rises to a high value and latches into conduction mode from non-

conductive state. This is called regenerative action of SCR. So for triggering of SCR the gate

current value (α1 + α2) must approach to unity. From the obtained equation the conditions to

turn the SCR into turn ON are

1. The leakage current through the SCR will increase when the temperature of the device is

very high. This turns the SCR into conduction.

2. When the current flowing through the device is extremely small then α1 and α2 are very

small. The conditions for break over voltage are the larger values of electron multiplication

factor Mn and hole multiplication factor Mp near the junction J2. Therefore the by increasing

the voltage across the device to break over voltage VBO causes the junction J2 breakdown

and thereby the SCR is turned ON.

3. And also by increasing α1 and α2 break over condition is achieved. The current gains of

the transistors depend on the value of Ig so by increasing Ig, SCR can be turned ON.

SCR Turn OFF Methods

An SCR cannot be turned OFF through the gate terminal like turning ON process. To turn

OFF the SCR, anode current must be reduced to a level below the holding current level of the

SCR. The process of turning OFF the SCR is called as commutation. Two major types of

commutating the SCR are,

1. Natural Commutation and

2. Forced Commutation

Forced commutation is again classified into several types such as

Class A Commutation

Class B Commutation

Class C Commutation

Class D Commutation

Class E Commutation

Advantages of SCR

1. As compared with electromechanical or mechanical switch, SCR has no moving parts.

Hence, with a high efficiency it can deliver noiseless operation.

2. The switching speed is very high as it can perform 1 nano operations per second.

3. These can be operated at high voltage and current ratings with a small gate current.

4. More suitable for AC operations because at every zero position of the AC cycle the

SCR will automatically switch OFF.

5. Small in size, hence easy to mount and trouble free service.

EE6503 POWER ELECTRONICS L T P C

3 0 0 3

CO2: Ability to understand and analyze about phase controlled rectifiers.

UNIT II PHASE-CONTROLLED CONVERTERS 2-pulse,3-pulse and 6-pulseconverters– performance parameters –Effect of source inductance–– Gate

Circuit Schemes for Phase Control–Dual converters.

UNIT II PHASE-CONTROLLED CONVERTERS

(2-pulse converter)

Single phase fully controlled bridge converter

The single

phase fully controlled

rectifier allows conversion

of single phase AC into

DC.

Used in

various applications such

as battery charging, speed

control of DC motors and

front end of UPS

(Uninterruptible Power

Supply) and SMPS

(Switched Mode Power Supply).

All four devices used are thyristors.

The turn-on instants of these devices are dependent on the firing signals that are

given. Turn-off happens when the current through the device reaches zero and it

is reverse biased at least for duration equal to the turn-off time of the device

specified in the data sheet.

In positive half cycle thyristors T1 & T2 are fired at an angle α .

When T1 & T2 conducts

Vo=Vs

IO=is=Vo/R=Vs/R

In negative half cycle of input voltage, SCR’s T3 &T4 are triggered at an angle

of (π+α)

Here output current & supply current are in opposite direction

∴ is=-io

T3 & T4 becomes off at 2π.

The average (dc) output voltage can be determined by using the expression

The output voltage waveform consists of two output pulses during the input supply time

period between 0 & 2π radians . In the continuous load current operation of a single

phase full converter (assuming constant load current) each thyristor conduct

for π radians (180º) after it is triggered. When thyristors T1 and T2 are triggered at ωt=

α T1 and T2 conduct from α to (π + α ) and the output voltage follows the input supply

voltage. Therefore output voltage V0=Vm sin ωt for ωt= α to (π + α ).

Hence the average or dc output voltage can be calculated as

CONTROL CHARACTERISTIC OF SINGLE PHASE FULL CONVERTER

From the control characteristic that by varying the trigger angle α we can vary

the output dc voltage across the load. Thus it is possible to control the dc output

voltage by changing the trigger angle α .

For trigger angle α in the

range of 0 to 90 degrees (ie: 0≤ α ≤90º)

,Vdc is positive and the average dc load

current Idc is also positive.

The average or dc output

power Pdc is positive; hence the circuit

operates as a controlled rectifier to convert

ac supply voltage into dc output power

which is fed to the load.

For trigger angle α > 90º,cos α becomes negative and as a result the average dc

output voltage Vdc becomes negative, but the load current flows in the same

positive direction i.e.,Idc is positive .

Hence the output power becomes negative. This means that the power flows

from the load circuit to the input ac source. This is referred to as line

commutated inverter operation.

During the inverter mode operation for α > 90º the load energy can be fed back

from the load circuit to the input ac source

TWO QUADRANT OPERATION OF A SINGLE PHASE FULL CONVERTER

The figure

shows the two regions

of single phase full

converter operation in

the Vdc versus Idc plane.

In the first quadrant when the trigger angle α< 90º,Vdc and Idc are both

positive and the converter operates as a controlled rectifier and converts the ac

input power into dc output power.

The power flows from the input source to the load circuit. This is the normal

controlled rectifier operation where Pdc is positive.

When the trigger angle is increased above 90º , Vdc becomes negative but Idc is

positive and the average output power

(dc output power) Pdc becomes

negative and the power flows from the

load circuit to the input source.

The operation occurs in

the fourth quadrant where Vdc is

negative and Idc is positive. The

converter operates as a line commutated

inverter.

Single phase Semi Converter

Single Phase Semi

Controlled Rectifier is used to convert the

AC voltage to DC voltage, both the

positive and Negative half cycle are

converted.

This circuit includes two

SCR`s and two Diodes for this operation.

One SCR and one diode

conducts for positive half cycle and other

one SCR and diode conducts for negative

half cycle to convert the AC voltage to DC

voltage. In positive half cycle SCR

T1 is triggered and diode D2 conducts in

forward bias,

In negative half cycle SCR

T3 is triggered and diode D1 Conducts in

forward bias, T1 and T3 turns OFF, when the input voltage to SCR reaches zero.

By using Zero Crossing Detection method, the pulse applied to SCR is derived

from the input AC voltage.

3-Pulse Converter

Consider a three-phase 3-pulse converter, where each of the thyristor is in

conduction mode during the third of the supply cycle.

The earliest time a thyristor is triggered into conduction is at 30° in reference to

the phase voltage.

Its operation is explained using three thyristors and three diodes.

When the thyristors T1, T2 and T3 are replaced by diodes D1, D2 and D3,

conduction will begin at angle 30° in respect to the phase voltages Van, Vbn and

Vcn respectively.

The firing angle α is measured initially at 30° in reference to the phase voltage

corresponding to it.

The current can only flow in one direction through the thyristor, which is similar

to inverter mode of functioning where power flows from the DC side to the AC

side.

In addition, the voltage in the thyristors is controlled by controlling the firing

angle. This is achieved when α = 0(possible in a rectifier).

The 3-pulse converter acts as an inverter and a rectifier.

The 3-phase input supply is applied through the star connected supply

transformer as shown in the figure. The common neutral point of the supply is

connected to one end of the load while the other end of the load connected to the

common cathode point.

When the thyristor T1 is triggered at ωt=(∏/6 + α)=(30° + α) , the phase voltage

Van appears across the load when T1conducts. The load current flows through

the supply phase winding 'a-n' and through thyristor T1 as long as T1 conducts.

When thyristor T2 is triggered at ωt=(5∏/6α), T1 becomes reverse biased and

turns-off. The load current flows through the thyristor and through the supply

phase winding 'b-n' . When T2 conducts the phase voltage vbn appears across

the load until the thyristor T3 is triggered .

When the thyristor T3 is triggered at ωt=(3∏/2 + α)=(270°+α) , T2 is reversed

biased and hence T2 turns-off. The phase voltage Van appears across the load

when T3 conducts.

When T1 is triggered again at the beginning of the next input cycle the thyristor

T3 turns off as it is reverse biased naturally as soon as T1 is triggered.

The figure shows the 3-phase input supply voltages, the output voltage which

appears across the load, and the load current assuming a constant and ripple free

load current for a highly inductive load and the current through the thyristor T1.

For a purely resistive load where the load inductance ‘L = 0’ and the trigger

angle α >(∏/6) , the load current appears as discontinuous load current and each

thyristor is naturally commutated when the polarity of the corresponding phase

supply voltage reverses.

The frequency of output ripple frequency for a 3-Phase Half Wave Converter is fs,

where fs is the input supply frequency

The 3-PHASE HALF WAVE CONVERTER is not normally used in practical

converter systems because of the disadvantage that the supply current

waveforms contain dc components (i.e., the supply current waveforms have an

average or dc value).

The trigger angle α for the thyristor T1 is measured from the cross over point.

The thyristor T1 is forward biased during the period ωt=30° to 150° , when the

phase supply voltage van has higher amplitude than the other phase supply

voltages.

Hence T1 can be triggered between 30° to 150°. When the thyristor T1 is

triggered at a trigger angle α, the average or dc output voltage for continuous

load current is calculated using the equation

6-Pulse Converter The figure below shows a six-pulse bridge controlled converter connected to a

three-phase source. In this converter, the number of pulses is twice that of

phases, that is p = 2m. Using the same converter configuration, it is possible to

combine two bridges of the six-pulse to obtain a twelve or more pulses

converter.

When

commutation is not available, two

diodes will conduct at any

particular time. Furthermore, to

obtain a voltage drop across the

load, two diodes must be at

positioned at opposite legs of the

bridge. For example, diodes 3

and 6 cannot be ON at the same time. Therefore, the voltage drop across the DC

load is a combination of line voltage VL from the three-phase source.

It is important to note that more the number of pulses, the greater the utilization

of the converter. In addition, the fewer the number of pulses the lesser the

utilization of the converter.

The three phase full converter is extensively used in industrial power

applications upto about 120kW output power level, where two quadrant

operations is required. The figure shows a three phase full converter with highly

inductive load. This circuit is also known as three phase full wave bridge or as a

six pulse converter.

The

thyristors are

triggered at an

interval of (∏/3)

radians (i.e. at an

interval of 30°). The

frequency of output

ripple voltage is

6fs and the filtering

requirement is less

than that of three

phase semi and half

wave converters.

At

ωt=(∏/6 +α) ,

thyristor is already

conducting when the

thyristor is turned on by applying the gating signal to the gate of . During the

time period ωt=(∏/6 +α) to (∏/2 +α), thyristors and conduct together and the

line to line supply voltage appears across the load.

At ωt=(∏/2 +α), the thyristor T2 is triggered and T6 is reverse biased

immediately and T6 turns off due to natural commutation. During the time

period ωt=(∏/ +α) to (5∏/6 +α), thyristor T1 and T2 conduct together and the

line to line supply voltage appears across the load.

The thyristors are numbered in the circuit diagram corresponding to the order in

which they are triggered. The trigger sequence (firing sequence) of the thyristors

is 12, 23, 34, 45, 56, 61, 12, 23, and so on. The figure shows the waveforms of

three phase input supply voltages, output voltage, the thyristor current

through T1 and T4, the supply current through the line ‘a’.

For any current to flow in

the load at least one device from the top

group (T1, T3, T5) and one from the

bottom group (T2, T4, T6) must conduct.

From symmetry

consideration it can be argued that each

thyristor conducts for 120° of the input

cycle.

The thyristors are fired in

the sequence T1 → T2 → T3 → T4 → T5

→ T6 → T1 with 60° interval between

each firing.

The thyristors on the same phase leg are fired at an interval of 180° and hence

cannot conduct simultaneously.

So there is only six possible conduction mode for the converter in the

continuous conduction mode of operation. These are T1T2, T2T3, T3T4, T4T5,

T5T6, T6T1.

Each conduction mode is of 60° duration and appears in the sequence

mentioned.

Performance parameters Form factor of f(f

FF) : Form factor of ‘f ‘ is defined as

Ripple factor of f(f

RF) : Ripple factor of f is defined as

Ripple factor can be used as a measure of the deviation of the output voltage and current of a

rectifier from ideal dc.

Peak to peak ripple of f: By definition

Fundamental component of f(F

1): It is the RMS value of the sinusoidal component in the

Fourier series expression of f with frequency 1/T.

K

th

harmonic component of f(FK

): It is the RMS value of the sinusoidal component in the

Fourier series expression of f with frequency K/T.

Crest factor of f(C

f) : By definition

Distortion factor of f(DF

f) : By definition

Total Harmonic Distortion of f(THDf): The amount of distortion in the waveform of f is

quantified by means of the index Total Harmonic Distortion (THD). By definition

Displacement Factor of a Rectifier (DPF): If vi and i

i are the per phase input voltage and input

current of a rectifier respectively, then the Displacement Factor of a rectifier is defined as.

DPF = cosφ1 Where φ1 is the phase angle between the fundamental components of v

i and i

i.

Power factor of a rectifier (PF): As for any other equipment, the definition of the power factor

of a rectifier is

PF =

if the per phase input voltage and current of a rectifier are vi and i

i respectively then

In terms of THDii

Pulse number of a rectifier (p): Refers to the number of output voltage/current pulses in a

single time period of the input ac supply voltage. Mathematically, pulse number of a rectifier is

given by

Commutation in a rectifier: Refers to the process of transfer of current from one device

(diode or thyristor) to the other in a rectifier. The device from which the current is transferred

is called the “out going device” and the device to which the current is transferred is called the

“incoming device”. The incoming device turns on at the beginning of commutation while the

out going device turns off at the end of commutation.

Commutation failure: Refers to the situation where the out going device fails to turn off at

the end of commutation and continues to conduct current.

Firing angle of a rectifier (α): Used in connection with a controlled rectifier using thyristors.

It refers to the time interval from the instant a thyristor is forward biased to the instant when a

gate pulse is actually applied to it. This time interval is expressed in radians by multiplying it

with the input supply frequency in rad/sec. It should be noted that different thyristors in a

rectifier circuit may have different firing angles. However, in the steady state operation, they

are usually the same.

Extinction angle of a rectifier (γ): Also used in connection with a controlled rectifier. It

refers to the time interval from the instant when the current through an outgoing thyristor

becomes zero (and a negative voltage applied across it) to the instant when a positive voltage

is reapplied. It is expressed in radians by multiplying the time interval with the input supply

frequency (ω) in rad/sec. The extinction time (γ/ω) should be larger than the turn off time of

the thyristor to avoid commutation failure.

Overlap angle of a rectifier (μ): The commutation process in a practical rectifier is not

instantaneous. During the period of commutation, both the incoming and the outgoing

devices conduct current simultaneously. This period, expressed in radians, is called the

overlap angle “μ” of a rectifier. It is easily verified that α + μ + γ = π radian.

Effect of source inductance The analysis of most converters is usually simplified under ideal conditions (no

source impedance). However, this assumption is not justified since source impedance

is normally inductive with a negligible resistive element.

Source inductance has a significant impact on the converter performance because its

presence alters the output voltage of the converter. As a result, the output voltage

reduces as the load current reduces. In addition, the input current and output voltage

waveforms change significantly.

Source inductance effect on a converter is analyzed in the following two ways.

Effect on Single Phase converter Assuming that the converter

operates in conduction mode and the

ripple from the load current is

negligible, the open circuit voltage

becomes equal to average DC output at

a firing angle of α.

The diagram below shows a

fully controlled converter with source

in single phase. The thyristors T3 and

T4 are assumed to be in conduction

mode when t = 0.

On the other hand,

T1 and T2fire when ωt = α

Where −

Vi = input voltage

Ii = input current

Vo = output voltage

Io = output voltage

When there is no

source inductance,

commutation will occur at

T3 and T4. Immediately

thyristors T1 and T2 are

switched ON. This will lead

the input polarity to change

instantaneously.

In the presence of

source inductance, change

of polarity and commutation does not occur instantaneously. Thus, T3and T4 do not

commutate as soon as T1 and T2 are switched ON.

At some interval, all the four thyristors will be conducting. This conducting interval is

called the overlap interval (μ).

The overlap during commutation reduces the DC output voltage and the angle of

extinction γ resulting in failed commutation when αis close to 180°. This is shown by

the waveform below.

Effect on Three Phase converter Just like the single-phase converter, there are no instantaneous commutations due to

the presence of the source inductances.

Taking the source inductances into consideration, the effects (qualitative) on the

converter performance is the same as in a single phase converter. This is shown in the

diagram below.

Gate Circuit Schemes for Phase Control

Single phase bridge recti�ers utilize thyristor or silicon controlled recti�er (S.C.R.) as

switching devices which is explained in. To turn on a thyristor, various control schemes are

used to generate gate pulses or �ring pulses which are supplied between gate and cathode of

the thyristor [2]. The number of degrees from the beginning of the cycle when the thyristor is

gated or switched on is referred to as the �ring angle, α and when the thyristor is turned off is

known as extinction angle. The thyristors of bridge recti�er are switched on and off in proper

sequence by using control electronics and gate driver circuits to get a controlled dc output

voltage. For this a sinusoidal ac voltage is supplied to control circuit and the same supply is

given to bridge recti�er circuit through isolation and synchronization block The cosine

control �ring scheme has an advantage that it linearizes the transfer characteristic of bridge

recti�er by using an indirect control variable as a substitute for �ring angle. This scheme

also provides automatic negative feedback to the change in input ac supply. The input ac

voltage of peak value, Vm is transformed to a low level ac voltage, Vao of 9 V is obtained by

using a 230/9 V, 50 Hz step down transformer. The cosine wave generator integrates Vao to

obtain a cosine wave of peak value, Em which is compared with a variable dc control voltage,

EC by using a comparator. The comparator output is fed to monostable block. The

monostable output is modulated at high frequency to obtain the �ring pulses. After proper

ampli�cation and isolation, these �ring pulses are supplied to the thyristors of the bridge

recti�er circuit.

Features of firing circuit: Firing circuits should produce trigger pulses for the thyristor at appropriate instants.

There needs to be an electrical isolation between firing circuits and the thyristor. It is

achieved using or an opto-isolator.

Types of firing circuit:

R-Firing circuit:

RC firing circuit:

UJT firing circuit:

Firing Angle:

The number of degrees from the beginning of the cycle when SCR is switched on is firing

angle. Any conducting at a particular point on the ac source voltage. The particular point is

defined as the firing angle cycle the SCR is gated ON, the greater will be the voltage applied

to the load.

Dual converters

Obviously since converter-I and converter-II are connected in antiparallel they must produce

the same dc voltage. This requires that the firing angles of these two converters be related as

a2 = pi – a1

Although the dc voltages produced by these converters are equal the output voltages do not

match on an instantaneous basis. Therefore to avoid a direct short circuit between two

different supply lines the two converters must never be gated simultaneously. Converter-I

receives gate pulses when the load current is positive. Gate pulses to converter-II are blocked

at that time. For negative load current converter-II thyristors are fired while converter-I gate

pulses are blocked. Thus there is no circulating current flowing through the converters and

therefore it is called the non-circulating current type dual converter. It requires precise

sensing of the zero crossing of the output current which may pose a problem particularly at

light load due to possible discontinuous conduction. To overcome this problem an interphase

reactor may be incorporated between the two converters. With the interphase reactor in place

both the converters can be gated simultaneously with

a2 = pi – a1

The resulting converter is called the circulating current type dual converter.

The inductance in between the converters has been included to limit circulating

harmonic current. In both these figures CONV – I and CONV – II have identical

construction and are also fired at the same firing angle a. Their input supplies also have same

Average output voltage of Single-phase converter = 2Vm COSα/ π

Average output voltage of Three-phase converter = 3Vm COSα/ π

For converter 1, the average output voltage, V01= Vmax COSα1

For converter 2, the average output voltage, V02= Vmax COSα2

The Output voltage is given by,

The firing angle can never be greater than 180. So, α1+ α2= 1800

Modes of Operation of Dual Converter

There are two functional modes: Non-circulating current mode and circulating mode.

Non-Circulating Current Mode

One converter will perform at a time. There is no circulating current between the

converters.

During the converter 1 operation, the firing angle (α1) will be 0<α1< 900 (Vdc and Idc are

positive)

During the converter 2 operation, firing angle (α2) will be 0<α2< 900 (Vdc and Idc are

negative)

Circulating Current Mode

In this mode, both converters will be in the ON condition at the same time. So circulating

current is present.

The firing angles are adjusted such that α1+ α2=1800. Firing angle of converter 1 is α1

and firing angle of converter 2 is α2.

In this mode, the Converter 1 works as a controlled rectifier when the firing angle is

0<α1< 900 and Converter 2 works as an inverter when the firing angle is 90

0 <α2< 180

0.

In this condition, Vdc and Idc are positive.

Converter 1 works as an inverter when firing angle be 900 <α1< 180

0and Converter 2

works as a controlled rectifier when the firing angle is 0<α2< 900 in this condition, Vdc

and Idc are negative.

Applications of Dual Converter

Direction and Speed control of DC motors.

Applicable wherever, the reversible DC is required.

Industrial variable speed DC drives.

EE6503 POWER ELECTRONICS L T P C

3 0 0 3 CO3: Ability to understand and analyze about dc-dc converters.

UNIT III DC TO DC CONVERTER Step-down and step-up chopper-control strategy–Forced commutated chopper–Voltage

commutated, Current commutated, Load commutated, Switched mode regulators- Buck,

boost, buck- boost converter, Introduction to Resonant Converters.

UNIT III DC TO DC CONVERTER

Chopper / DC to DC Converter DC to DC converter is very much needed nowadays as many industrial applications

are dependent upon DC voltage source. The performance of these applications will be

improved if we use a variable DC supply. It will help to improve controllability of the

equipments also. Examples of such applications are subway cars, trolley buses, battery

operated vehicles etc.

Chopper is a

basically static power

electronics device which

converts fixed DC

voltage/power to variable

DC voltage or power. It is

nothing but a high speed

switch which connects and

disconnects the load from

source at a high rate to get

variable or chopped voltage

at the output.

Chopper can

increase or decrease the DC

voltage level at its opposite

side. So, chopper serves the same purpose in DC circuit transfers in case of ac circuit.

So it is also known as DC transformer.

Devices used in Chopper

Low power application : GTO, IGBT, Power BJT, Power

MOSFET etc.

High power application : Thyristor or SCR. These devices are

represented as a switch in a dotted box for simplicity.

When it is closed current can flow in the direction of

arrow only.

Classification of Choppers:

(a) Depending upon the direction of the output current and voltage, the converters can be

classified into five classes namely

Class A [One-quadrant Operation] (1st quadrant only)

Class B [One-quadrant Operation] (2nd quadrant only)

Class C [Two-quadrant Operation] (1,2 quadrants only)

Class D [Two-quadrant Operation] (1,4 quadrants only)

Class E [Four-quadrant Operation] (All four quadrants)

In some text books it will be named as Class A, B,C... whereas in some books it will be be

Type A, B, C...

(b) Based turn off process (commutation process)

Natural Commutated Chopper ( Occurs in AC input circuits)

Forced Commutated Chopper (Occurs in DC input circuits)

The forced commutation type is further classified as

Voltage Commutated Chopper (Ex. Jones Chopper)

Current Commutated Chopper (Ex. Morgan Chopper)

Click here to know in detail about the commutation types

(c) Based on the output voltage of the output, the choppers are classified as

(i) Step-Down Chopper

In this case the average output voltage is less than the input voltage. It is also known as step

down converter

(ii) Step-Up Chopper

Here the average output voltage is more than the input voltage. It is also known as step up

converter

(iii) Step-Up/Down Chopper

This type of converter produces an output voltage that is either lower or higher than the input

voltage

(d) Depending upon the power loss occurred during turn ON/OFF of the switching device,

the choppers are classified into two categories namely

(i) Hard switched Converter

Here the power loss is high during the switching (ON to OFF and OFF to ON) as a result of

the non zero voltage and current on the power switches.

(ii) Soft switched or resonant converters

In this type of choppers, the power loss is low at the time of switching as a result of zero

voltage and/or zero current on the switches.

1) Step down Chopper :

Step down chopper as Buck converted is used to

reduce the i/p voltage level at the output side. Circuit

diagram of a step down chopper is shown in the

adjacent figure.

When CH is turned ON, Vs directly appears across the

load as shown in figure. So Vo = VS. When CH is

turned off, Vs is disconnected from the load. So

output voltage Vo = 0. The voltage waveform of step

down chopper is shown below: TON → It is the

interval in which chopper is in ON state. TOFF → It

is the interval in which chopper is in OFF state. VS →

Source or input voltage. Vo → Output or load voltage. T → Chopping period = TON + TOFF

Operation of Step Down Chopper with Resistive Load

When CH is ON, Vo = VS When CH

is OFF, Vo = 0

Where, D is duty

cycle = TON/T. TON can be varied from 0 to T, so 0 ≤ D ≤ 1. Hence output voltage Vo can be

varied from 0 to VS.

So, we can

conclude that output voltage is always less than the input voltage and hence the name step

down chopper is justified. The output voltage and current waveform of step down chopper

with resistive load is shown below.

Operation Of Step Down Chopper with Inductive Load When CH is ON, Vo = VS When CH is OFF, Vo = 0

During ON time of chopper

Therefore,

peak to peak load current,

During OFF Time of Chopper

If inductance value of L is very large, so load current will be continuous in nature. When CH

is OFF inductor reverses its polarity and discharges. This current freewheels through diode

FD. By equating (i) and (ii)

So, from (i) we get,

The output voltage and current waveform of step

down chopper with inductive load is shown below

2) Step up Chopper or Boost Converter : Step up chopper or boost

converter is used to increase the input voltage level of its output side. Its circuit

diagram and waveforms are shown below in figure.

Operation of Step up Chopper

When CH is ON it short circuits the load. Hence

output voltage during TON is zero. During this

period inductor gets charged. So, VS = VL

Where, ΔI is the peak to peak inductor current.

When CH is OFF inductor L discharges through

the load.

We will get summation of both source voltage VS and

inductor Voltage VL as output voltage, i.e.

Now, by equating (iii) and

(iv), As we can vary TON

from 0 to T, so 0 ≤ D ≤ 1. Hence VO can be varied from VS to ∞. It is clear that output

voltage is always greater than the input voltage and hence it boost up or increase the voltage

level.

3) Buck-Boost Converter or Step Up Step Down Converter With the help of Buck-Boost converter we can increase or decrease the input voltage level at

its output side as per our requirement. The circuit diagram of this converter is shown below.

Operation of Buck-Boost Converter When CH is ON source voltage will be applied across inductor L and it will be charged. So

VL = VS

When chopper is OFF inductor L reverses its polarity and discharges through load and diode,

So. By evaluating

(v) and (vi) we get,

Taking magnitude we get, D can be varied from 0 to one. When, D = 0; Vo = 0

When D = 0.5, Vo = VS When, D = 1, Vo = ∞ Hence, in the interval 0 ≤ D ≤ 0.5, output

voltage varies in the range 0 ≤ VO ≤ VS and we get step down or Buck operation. Whereas, in

the interval 0.5 ≤ D ≤ 1, output voltage varies in the range VS ≤ VO ≤ ∞ and we get step up or

Boost operation.

According to direction of output voltage and current Semiconductors devices used in

chopper circuit are unidirectional. But arranging the devices in proper way we can get output

voltage as well as output current from chopper in our required direction. So, on the basis of

this features chopper can be categorized as follows :

.

When output voltage (Vo) follows th

Type A Chopper or First–Quadrant Chopper This type of chopper is shown in the figure. It is known as first-quadrant chopper or type A

chopper. When the chopper is on, v0 = VS as a result and the current flows in the direction of

the load. But when the chopper is off v0 is zero but I0 continues to flow in the same

direction through the freewheeling diode FD, thus average value of voltage and current say

V0 and I0 will be always positive as shown in the graph.

Chopper First Quadrant In type A chopper the power flow will be always from source to the load. As the average

voltage V0 is less than the dc input voltage Vs

Type B Chopper or Second-Quadrant Chopper

Chopper Second Quadrant In type B or second quadrant chopper the load must always contain a dc source E . When the

chopper is on, v0 is zero but the load voltage E drives the current through the inductor L

and the chopper, L stores the energy during the time Ton of the chopper . When the chopper

is off , v0 =( E+ L . di/dt ) will be more than the source voltage Vs . Because of this the

diode D2 will be forward biased and begins conducting and hence the power starts flowing to

the source. No matter the chopper is on or off the current I0 will be flowing out of the load

and is treated negative . Since VO is positive and the current I0 is negative , the direction of

power flow will be from load to source. The load voltage V0 = (E+L .di/dt ) will be more

than the voltage Vs so the type B chopper is also known as a step up chopper .

Type -C chopper or Two-quadrant type-A Chopper Type C chopper is obtained by connecting type –A and type –B choppers in parallel. We will

always get a positive output voltage V0 as the freewheeling diode FD is present across the

load. When the chopper is on the freewheeling diode starts conducting and the output voltage

v0 will be equal to Vs . The direction of the load current i0 will be reversed. The current

i0 will be flowing towards the source and it will be positive regardless the chopper is on

or the FD conducts. The load current will be negative if the chopper is or the diode D2

conducts. We can say the chopper and FD operate together as type-A chopper in first

quadrant. In the second quadrant, the chopper and D2 will operate together as type –B

chopper.

Chopper Two Quadrant The average voltage will be always positive but the average load current might be positive or

negative. The power flow may be life the first quadrant operation ie from source to load or

from load to source like the second quadrant operation. The two choppers should not be

turned on simultaneously as the combined action my cause a short circuit in supply lines. For

regenerative braking and motoring these type of chopper configuration is used.

Type D Chopper or Two-Quadrant Type –B Chopper

Two Quadrant Type B chopper or D Chopper Circuit The circuit diagram of the type D chopper is shown in the above figure. When the two

choppers are on the output voltage v0 will be equal to Vs . When v0 = – Vs the two choppers

will be off but both the diodes D1 and D2 will start conducting. V0 the average output voltage

will be positive when the choppers turn-on the time Ton will be more than the turn off time

Toff its shown in the wave form below. As the diodes and choppers conduct current only in

one direction the direction of load current will be always positive.

Positive First Quadrant Operation and Negative Fourth Quadrant Operation

The power flows from source to load as the average values of both v0 and i0 is

positive. From the wave form it is seen that the average value of V0 is positive thus the

forth quadrant operation of type D chopper is obtained.

From the wave forms the Average value of output voltage is given by

V0= (Vs Ton-VsToff)/T = Vs.(Ton-Toff)/T

Type –E chopper or the Fourth-Quadrant Chopper Type E or the fourth quadrant chopper consists of four semiconductor switches and four

diodes arranged in antiparallel. The 4 choppers are numbered according to which quadrant

they belong. Their operation will be in each quadrant and the corresponding chopper only be

active in its quadrant.

E-type Chopper Circuit diagram with load emf E and E Reversed

First Quadrant During the first quadrant operation the chopper CH4 will be on . Chopper CH3 will be off and

CH1 will be operated. AS the CH1 and CH4 is on the load voltage v0 will be equal to the

source voltage Vs and the load current i0 will begin to flow . v0 and i0 will be positive as the

first quadrant operation is taking place. As soon as the chopper CH1 is turned off, the positive

current freewheels through CH4 and the diode D2 . The type E chopper acts as a step- down

chopper in the first quadrant.

Second Quadrant In this case the chopper CH2 will be operational and the other three are kept off. As CH2 is

on negative current will starts flowing through the inductor L . CH2 ,E and D4. Energy is

stored in the inductor L as the chopper CH2 is on. When CH2 is off the current will be fed

back to the source through the diodes D1 and D4. Here (E+L.di/dt) will be more than the

source voltage Vs . In second quadrant the chopper will act as a step-up chopper as the power

is fed back from load to source

Third Quadrant In third quadrant operation CH1 will be kept off , CH2 will be on and CH3 is operated. For

this quadrant working the polarity of the load should be reversed. As the chopper CH3 is on,

the load gets connected to the source Vs and v0 and i0 will be negative and the third

quadrant operation will takes place. This chopper acts as a step-down chopper

Fourth Quadrant CH4 will be operated and CH1, CH2 and CH3 will be off. When the chopper CH4 is turned

on positive current starts to flow through CH4, D2 ,E and the inductor L will store energy. As

the CH4 is turned off the current is feedback to the source through the diodes D2 and D3 , the

operation will be in fourth quadrant as the load voltage is negative but the load current is

positive. The chopper acts as a step up chopper as the power is fed back from load to source.

Voltage Commutated Chopper

Similar to step down chopper.

T1 = Main thyristor, TA = Auxiliary

thyristor, L,C = commutating components,

Rc = charging resistor

Assume output current is constant.

Close the switch, initially capacitor

short circuited, after 4 - 5 time constants, Vc =

Vs.

At t = 0, T1 is on, load is connected

across the supply Vo = Vs.

Tank circuit starts conduction ( diode forward bias).

After conduction polarities across capacitor are changed.

D is reverse biased polarities across capacitor are changed.

Upto t2 we completed now we have to turn off the main thyristor.

Make TA on, T1 to be off (applying reverse voltage).

To make the conduction continues use free wheeling diode.

In order to make the output continuous, the existing path will be changed as Vs, C,

TA and the load.

Voltage across the capacitor changes.

Now make the voltage across capacitor > Vs.

Free wheeling diode conducts, output voltage becomes zero.

To start next cycle, no need to close switch 's'.

A reverse voltage is applied across conducting SCR due to which current through SCR

becomes zero and it is getting off. Hence it is called voltage commutation.

Other name of this is impulse commutation. It is because a high reverse voltage will

turn off the SCR.

Limitations of voltage commutated chopper:

A starting circuit is required.

load voltage at once rises to 2Vs at the instant commutation of main SCR is initiated.

It can't work at no load. It is because at no load, capacitor would not get charged frm -

Vs to Vs when auxiliary SCR is triggered for commutating the main SCr.

Main thyristor is required to carry current more than load current. So, it is to be over

rated.

The values of commutating components C and L can be obtained.

The values depend upon turn off time of main thyristor T1. during tc capacitor voltage

changes from -Vs to zero linearly.

ic = C dV / dt for a constant load current Io.

Io = C . Vs / tc

C = Io . tc / Vs

The commutation circuit turn off time tc must be greater than thyristor turn off time.

Load current should not be too large.

Current Commutated Chopper:

Current commutated chopper

Capacitor is charged to Vs, main thyristor T1 is fired at t = 0. So that load voltage Vo =

Vs.

At t = t1, auxiliary thyristor is turned on to commutate main thyristor.

With turning on of TA, an oscillatory current ic is set up in the circuit.

At t2, Vc = - Vs and ic tends to

reverse in the auxiliary thyristor TA, it

gets naturally commutated.

As TA is reverse biased and

turned off at t2. Oscillatory current

ic begins to flow through C, L, D2 and

T1.

At t3 ic rises to io so that iT1 =

0. As a result main SCR T1 is turned off

at t3. Since oscillating current through

T1 turns it off it is called current

commutated chopper.

After t3 ic supplies load current

io and the excess current. iD1 = ic -

Io is conducted through diode D1.

Afetr t4, a constant current

equal to Io flows through Vs, C, L,

D2 and load.

Capacitor c is charged linearly

to source voltage Vs at t5, so during

time ( t5 - t4 ) ic= Io.

In this commutation an opposite

current pulse will be injected through

SCR. As a result currents decreases and

finally comes to zero if both the currents would be equal and opposite.

Anti parallel diode is useful to apply the reverse voltage after current through SCR

becomes to zero. The value of reverse voltage is low. So

Turn off time increases.

Turn off power loss increases.

Jones chopper employes the principle of voltage commutation.

Morgan's chopper based on the principle of current commutation.

Introduction to Resonant Converters Classification

• ZCS, zero-current-switching – Switch turns on and off without current

• ZVS, zero-voltage-switching – Switch turns on and off without voltage

• ZVS-CV, zero-voltage-switching, clamped voltage – As before but at least two switches

– Voltage over switch is limited to the supply voltage

ZCS Resonant-Switch Converter

ZVS Resonant-Switch Converter

EE6503 POWER ELECTRONICS L T P C

3 0 0 3

CO1: Ability to understand and analyze about linear and digital electronic circuits.

CO2: Ability to understand and analyze about phase controlled rectifiers.

CO3: Ability to understand and analyze about dc-dc converters.

CO4: Ability to understand and analyze about inverters and PWM.

CO5: Ability to understand and analyze about ac-ac converters

UNIT I POWER SEMI-CONDUCTOR DEVICES Study of switching devices, Diode, SCR, TRIAC, GTO, BJT, MOSFET, IGBT-Static

and Dynamic characteristics - Triggering and commutation circuit for SCR- Design of Driver

and snubber circuit.

UNIT II PHASE-CONTROLLED CONVERTERS 2-pulse,3-pulse and 6-pulseconverters– performance parameters –Effect of source inductance–– Gate

Circuit Schemes for Phase Control–Dual converters.

UNIT III DC TO DC CONVERTER Step-down and step-up chopper-control strategy–Forced commutated chopper–Voltage

commutated, Current commutated, Load commutated, Switched mode regulators- Buck,

boost, buck- boost converter, Introduction to Resonant Converters. UNIT IV INVERTERS

Single phase and three phase voltage source inverters(both1200modeand180

0mode)–Voltage&

harmonic control--PWM techniques: Sinusoidal PWM, modified sinusoidal PW M -

multiple PW M – Introduction to space vector modulation –Current source inverter.

UNIT V AC TO AC CONVERTERS Single phase and Three phase AC voltage controllers–Control strategy- Power Factor

Control – Multistage sequence control -single phase and three phase cyclo converters –

Introduction to Matrix converters.

UNIT IV INVERTERS Inverters can be broadly classified into two types. They are

1. Voltage Source Inverter (VSI)

2. Current Source Inverter (CSI)

When the DC voltage remains constant, then it is called Voltage Source Inverter(VSI) or

Voltage Fed Inverter (VFI).

When input current is maintained constant, then it is called Current Source Inverter (CSI) or

Current Fed Inverter (CFI).

Sometimes, the DC input voltage to the inverter is controlled to adjust the output. Such

inverters are called Variable DC Link Inverters. The inverters can have single phase or three-

phase output.

A voltage source inverter(VSI) is fed by a stiff DC voltage, whereas a current source inverter

is fed by a stiff current source.

A voltage source can be converted to a current source by connecting a series inductance

and then varying the voltage to obtain the desired current.

A VSI can also be operated in current-controlled mode, and similarly a CSI can also be

operated in the voltage control mode.

The inverters are used in variable frequency ac motor drives, uninterrupted power

supplies, induction heating, static VAR compensators, etc.

The following table gives us the comparative study between VSI and CSI

VSI CSI VSI is fed from a DC voltage source

having small or negligible impedance.

CSI is fed with adjustable current from a

DC voltage source of high impedance.

Input voltage is maintained constant The input current is constant but

adjustable.

Output voltage does not dependent on the

load

The amplitude of output current is

independent of the load.

The waveform of the load current as well

as its magnitude depends upon the nature

of load impedance.

The magnitude of output voltage and its

waveform depends upon the nature of the

load impedance.

VSI requires feedback diodes The CSI does not require any feedback

diodes.

The commutation circuit is complicated Commutation circuit is simple as it

contains only capacitors.

Power BJT, Power MOSFET, IGBT,

GTO with self commutation can be used

in the circuit.

They cannot be used as these devices

have to withstand reverse voltage.

Definition: The circuit which convert DC power into AC power at desired output voltage and

frequency are called as Inverters.

Normally the DC source is a battery or output of

the controlled rectifier.

Inverters are widely used in standby power

supplies, UPS, induction heating, induction motor

drives etc.

The inverter circuit's output voltage

waveform can be square wave, quasi-square wave

or low distorted sine wave.

AC output voltage is built by using SCR as switches. So inverter circuit with fewer

components have non sinusoidal output waveform.

By adding complex circuits, it is possible to obtain sinusoidal output voltages.

With the help of drives of the switches(SCRs), the output voltage can be controlled (ie,

adjustable).

To control the output voltage of inverters, the pulse width modulation(PWM) techniques

are generally used. Such inverters are known as PWM inverters.

The output voltage of the inverter contain harmonics whenever it is non sinusoidal.

These harmonics can be reduced by using proper control schemes

Classification of Inverter Circuits:

When input DC voltage remains constant, then it is called voltage source inverter (VSI)

or Voltage Fed Inverter (VFI).

When input supply current is maintained constant, then it is called current source

inverter (CSI) or Current Fed Inverter (CFI).

Sometimes the DC input voltage to the inverter is controlled to adjust the output. Such

inverters are called variable DC link inverters.

The inverters output can be single phase or three phase.

There are four important inverter circuits.

Center-tapped DC supply (or) Half bridge inverter

Inverter with center taped load

Single phase bridge inverter

Three phase bridge inverter

Requirements of a Practical Inverter are listed below

1. Ability to operate into an inductive load

2. Provision for over current protection.

3. Controllable output

4. Close proximity of the output waveform to sinusoidal waveform

5. Ability to work with load disconnected

6. Drive rating should not be exceeded

Inverter Applications:

1. Standby aircraft power supplies

2. High voltage DC Transmission

3. Variable frequency AC drives

4. Induction heating

5. Uninterruptible power supplies for computers

Single phase inverters

The circuit diagram consists of four distinct IGBT such that they are connected as the

bridge circuit.

The input to the circuit is the 220v DC supply from the rectifier unit. The IGBT are

triggered accordingly such that the AC output voltage is obtained at the output.

The operation of the circuit is as follows.

First the IGBT S1 and S2 are turned on by triggering the gate of the IGBT .

During this time the input supply is DC and at the output is applied across the load.

The current starts from the supply positive, S1, S2, load and to the negative of the

supply. The conduction path for the first cycle of operation is shown in figure.

During the next phase or the cycle the IGBT S3 and S4 are turned on by giving trigger

pulse to the gate of the IGBT .

During this period the input voltage is applied at the output but in the negative direction.

The current conduction starts from the supply, S3, load, S4 and to the positive of the

supply.

The current conduction is showed in the figure.

As the two cycles continue the positive and the negative voltage is applied at the load

and the current direction changes in the two cycles.

As the current direction changes the alternative voltage is obtained at the load thus

converting Dc voltage to AC voltage.

Waveform for R load Waveform for RL load

Three Phase Inverter

A three-phase inverter converts a DC

input into a three-phase AC output. Its three arms

are normally delayed by an angle of 120° so as to

generate a three-phase AC supply.

The inverter switches each has a ratio of

50% and switching occurs after every T/6 of the

time T (60° angle interval). The switches S1 and

S4, the switches S2 and S5 and switches S3 and

S6 complement each other.

The figure below shows a circuit for a

three phase inverter. It is nothing but three single

phase inverters put across the same DC source.

The pole voltages in a three phase inverter are equal to the pole voltages in single phase

half bridge inverter.

The two types of inverters above have two modes of conduction − 180° mode of

conduction and 120° mode of conduction.

180° mode of conduction In this mode of conduction, every device is in conduction state for 180° where they are

switched ON at 60° intervals.

The terminals A, B and C are the output terminals of the bridge that are connected to the

three-phase delta or star connection of the load.

The operation of a balanced star connected load is explained in the diagram below. For

the period 0° − 60° the points S1, S5 and S6 are in conduction mode. T

The terminals A and C of the load are connected to the source at its positive point.

The terminal B is connected to the source at its negative point.

In addition, resistances R/2 is between the neutral and the positive end while resistance

R is between the neutral and the negative terminal.

INTERVAL DURATION CONDUCTING PERIOD CONDUCTING SWITCHES

S1 S2 S3 S4 S5 S6

I 60ᵒ 0ᵒ to 60ᵒ 0 to π/3 1 1 1

II 60ᵒ 60ᵒ to 120ᵒ π/3 to 2 π/3 1 1 1

III 60ᵒ 120ᵒ to 180ᵒ 2π/3 to π 1 1 1

IV 60ᵒ 180ᵒ to 240ᵒ Π to 4 π/3 1 1 1

V 60ᵒ 240ᵒ to 300ᵒ 4 π/3 to 5 π/3 1 1 1

VI 60ᵒ 300ᵒ to 360ᵒ 5 π/3 to 2 π 1 1 1

The load voltages are gives as follows;

VAN = V/3,

VBN = −2V/3,

VCN = V/3

The line voltages are given as follows;

VAB = VAN − VBN = V,

VBC = VBN − VCN = −V,

VCA = VCN − VAN = 0

INTERVAL DURATION CONDUCTING

PERIOD

OUTPUT VOLTAGES FOR 180°

VA VB VC VAB VBC VCA

I 60ᵒ 0ᵒ to 60ᵒ V/3 -2V/3 V/3 V -V 0

II 60ᵒ 60ᵒ to 120ᵒ 2V/3 -V/3 -V/3 V 0 -V

III 60ᵒ 120ᵒ to 180ᵒ V/3 V/3 -2V/3 0 V -V

IV 60ᵒ 180ᵒ to 240ᵒ -V/3 2V/3 -V/3 -V V 0

V 60ᵒ 240ᵒ to 300ᵒ -2V/3 V/3 V/3 -V 0 V

VI 60ᵒ 300ᵒ to 360ᵒ -V/3 -V/3 2V/3 0 -V V

120° mode of conduction In this mode of conduction, each electronic device is in a conduction state for 120°. It is

most suitable for a delta connection in a load because it results in a six-step type of

waveform across any of its phases. Therefore, at any instant only two devices are

conducting because each device conducts at only 120°.

The terminal A on the load is connected to the positive end while the terminal B is

connected to the negative end of the source. The terminal C on the load is in a condition

called floating state. Furthermore, the phase voltages are equal to the load voltages as

shown below.

INTERVAL DURATION CONDUCTING PERIOD CONDUCTING SWITCHES

S1 S2 S3 S4 S5 S6

I 60ᵒ 0ᵒ to 60ᵒ 0 to π/3 1 1

II 60ᵒ 60ᵒ to 120ᵒ π/3 to 2 π/3 1 1

III 60ᵒ 120ᵒ to 180ᵒ 2π/3 to π 1 1

IV 60ᵒ 180ᵒ to 240ᵒ Π to 4 π/3 1 1

V 60ᵒ 240ᵒ to 300ᵒ 4 π/3 to 5 π/3 1 1

VI 60ᵒ 300ᵒ to 360ᵒ 5 π/3 to 2 π 1 1

INTERVAL DURATION

CONDUCTING

PERIOD

OUTPUT VOLTAGES FOR 120°

VA VB VC VAB VBC VCA

I 60ᵒ 0ᵒ to 60ᵒ V/2 -V/2 0 V -V/2 -V/2

II 60ᵒ 60ᵒ to 120ᵒ V/2 0 -V/2 V/2 V/2 -V

III 60ᵒ 120ᵒ to 180ᵒ 0 V/2 -V/2 -V/2 V -V/2

IV 60ᵒ 180ᵒ to 240ᵒ -V/2 V/2 0 -V V/2 V/2

V 60ᵒ 240ᵒ to 300ᵒ -V/2 0 V/2 -V/2 -V/2 V

VI 60ᵒ 300ᵒ to 360ᵒ 0 -V/2 V/2 V/2 -V V/2

The load voltages are gives as follows;

VAN =

VBN =

VCN =

The line voltages are given as follows;

VAB = VAN − VBN =

VBC = VBN − VCN =

VCA = VCN − VAN =

Waveforms for 180° mode of

conduction

Waveforms for 120° mode

of conduction

INTERVAL CONDUCTING

PERIOD

CONDUCTING

SWITCHES

FOR 180°

CONDUCTING

SWITCHES

FOR 120°

S1 S2 S3 S4 S5 S6 S1 S2 S3 S4 S5 S6

I 0ᵒ to 60ᵒ 1 1 1 1 1 II 60ᵒ to 120ᵒ 1 1 1 1 1 III 120ᵒ to 180ᵒ 1 1 1 1 1 IV 180ᵒ to 240ᵒ 1 1 1 1 1 V 240ᵒ to 300ᵒ 1 1 1 1 1 VI 300ᵒ to 360ᵒ 1 1 1 1 1

Pulse Width Modulation PWM is a technique that is used to reduce the overall harmonic distortion (THD) in a

load current. It uses a pulse wave in rectangular/square form that results in a variable

average waveform value f(t), after its pulse width has been modulated. The time period

for modulation is given by T. Therefore, waveform average value is given by

y¯=1T∫T0f(t)dty¯=1T∫0Tf(t)dt

Sinusoidal Pulse Width Modulation In a simple source voltage inverter, the switches can be turned ON and OFF as needed.

During each cycle, the switch is turned on or off once. This results in a square

waveform. However, if the switch is turned on for a number of times, a harmonic profile

that is improved waveform is obtained.

The sinusoidal PWM waveform is obtained by comparing the desired modulated

waveform with a triangular waveform of high frequency.

Regardless of whether the voltage of the signal is smaller or larger than that of the

carrier waveform, the resulting output voltage of the DC bus is either negative or

positive.

The sinusoidal amplitude is given as Am and that of the carrier triangle is give as Ac.

For sinusoidal PWM, the modulating index m is given by Am/Ac.

Modified Sinusoidal Waveform PWM A modified sinusoidal PWM waveform is used for power control and optimization of

the power factor.

The main concept is to shift current delayed on the grid to the voltage grid by modifying

the PWM converter.

Consequently, there is an improvement in the efficiency of power as well as

optimization in power factor.

Multiple PWM The multiple PWM has numerous outputs that are not the same in value but the time

period over which they are produced is constant for all outputs.

Inverters with PWM are able to operate at high voltage output.

The waveform below is a sinusoidal wave produced by a multiple PWM

Voltage and Harmonic Control A periodic waveform that has frequency, which is a multiple integral of the fundamental

power with frequency is known as a harmonic. Total harmonic distortion (THD) on the

other hand refers to the total contribution of all the harmonic current frequencies.

Harmonics are characterized by the pulse that represent the number of rectifiers used in

a given circuit. It is calculated as follows −

h=(n×P)+1or−1h=(n×P)+1or−1

Where n − is an integer 1, 2, 3, 4….n

P − Number of rectifiers

It is summarized in the table below −

Harmonic Frequency

1st 60 Hz

2nd 120 Hz

3rd 180Hz

4th 240Hz

5th . .

49th

300Hz . .

2940Hz

Harmonics have an impact on the voltage and current output and can be reduced using

isolation transformers, line reactors, redesign of power systems and harmonic filters.

Voltage and Harmonic Control of Inverters:

In applying Voltage and Harmonic Control of Inverters for motor control

both V andf(keeping V/fconstant) need to be varied. Further, the inverters apply

essentially nonsinusoidal ac voltage to the motor.

External filter circuits cannot be employed due to the difficulty in operating inverters

over a wide range of frequencies. It is therefore necessary to keep down the harmonic

content of the ac output of the inverters. While the inverter frequency is adjusted by

varying the rate of thyristor firing, the voltage can be controlled in the following ways:

1.Control of DC Input Voltage

In this scheme a controlled converter supplies a variable dc voltage to the inverter. This

method has the advantage of fixed harmonic voltage content in the inverter output. It

presents the difficulty of doubtful reliability of the commutation circuitry at low values

of the dc input as the commutation voltage in many inverter circuits is proportional to

the dc input voltage.

2.Chopper Control of DC Input Voltage

Here the fixed dc output voltage of an uncontrolled 3-phase full-wave bridge rectifier is

controlled by a chopper circuit as shown in Fig. 11.53. This scheme has the advantage of

good power factor at the ac input of the bridge rectifier and a faster voltage response owing to

a smaller time-constant of the LC-filter on the output side of a high-frequency chopper. This

scheme also suffers from the disadvantages of poor commutation (of inverter) at low input dc

voltages and the fact that two power controllers have to be used in series.

3.Use of Inverters with Independent Voltage Control

Inverter circuits (to be discussed later in this section) have been devised which permit

independent control of both the output voltage and frequency. This method is illustrated

schematically in Fig. 11.54.

Series Resonant Inverter A resonant inverter is an electrical inverter whose operation is based on oscillation of

resonant current. Here, the switching device and the resonanting component are

connected in series to each other. As a result of the natural features of the circuit, the

current passing through the switching device drops to zero.

This type of inverter yields a sinusoidal waveform at very high frequencies in the range

of 20kHz-100kHz. It is therefore, most suitable for applications that demand a fixed

output such as induction heating and flourescent lighting. It is usually small in size

because its switching frequency is high.

A resonant inverter has numerous configurations and thus it is categorized into two

groups −

Those with unidirectional switches

Those with bidirectional switches

A single phase half bridge inverter has a resistance of 2.5Ω and

input DC voltage of 50V. Calculate the following −

Solution −

a. The RMS voltage occurring at the fundamental frequency

E1RMS=0.9×50V=45VE1RMS=0.9×50V=45V

b. The power Output

RMS output voltage EORMS=E=50VEORMS=E=50V

Output power =E2/R=(50)2/2.5=1000W=E2/R=(50)2/2.5=1000W

c. Peak current and average current

Peak current Ip=E0/R=50/2.5=20AIp=E0/R=50/2.5=20A

Average current=Ip/2=20/2=10A=Ip/2=20/2=10A

d. Harmonic RMS voltage

En={(EORMS)2−(E1RMS)2}0.5=[502−452]0.5=21.8VEn={(EORMS)2−(E1RMS)2}0.

5=[502−452]0.5=21.8V

e. Total harmonic distortion

En/E1RMS=21.8/45=0.48×100%=48%

Space vector modulation (SVM) Space vector modulation (SVM) is an algorithm for the control of pulse width

modulation (PWM).

It is used for the creation of alternating current (AC) waveforms; most commonly to

drive 3 phase AC powered motors at varying speeds from DC using multiple class-D

amplifiers.

There are variations of SVM that result in different quality and computational

requirements.

One active area of development is in the reduction of total harmonic distortion (THD)

created by the rapid switching inherent to these algorithms.

To implement space vector modulation, a reference signal Vref is sampled with a

frequency fs (Ts = 1/fs).

The reference signal may be generated from three separate phase references using

the {αβγ } transform.

The reference vector is then synthesized using a combination of the two adjacent active

switching vectors and one or both of the zero vectors.

Various strategies of selecting the order of the vectors and which zero vector(s) to use

exist.

Strategy selection will affect the harmonic content and the switching losses.

The alpha-beta (αβγ ) transformation (also known as the Clarke transformation) is a

mathematical transformation employed to simplify the analysis of three-phase circuits.

Conceptually it is similar to the dq0 transformation.

One very useful application of the alpha-beta transformation is the generation of the

reference signal used for space vector modulation control of three-phase inverters.

Space Vector Switching State

(Three Phases) On-state Switch

Vector

Definition

Zero

Vector 0V

[PPP] 531 ,, SSS

00 V

[OOO] 264 ,, SSS

Active

Vector

1V

[POO] 261 ,, SSS 01

3

2 jd eVV

2V

[PPO] 231 ,, SSS 32

3

2

j

d eVV

3V

[OPO] 234 ,, SSS 3

2

33

2

j

d eVV

4V

[OPP] 534 ,, SSS 3

3

43

2

j

d eVV

5V

[OOP] 564 ,, SSS 3

4

53

2

j

d eVV

6V

[POP] 561 ,, SSS 3

5

63

2

j

d eVV

Current source inverter

In the introductory remarks, one merit of CSI has been stated, i.e. it can be used for the

speed control of ac, specially induction, motors subject to variation in load torque.

In recent years, self-commutated power switching devices, such as power transistors

etc., are being used in VSI, but not costly inverter-grade thyristors (having low turn-off

time), along with bulky commutation circuits.

These circuits also need additional diodes for feeding the reactive power back to the

supply, when used with heavily inductive loads.

Advantages The circuit for CSI, using only converter grade thyristor, which should have reverse

blocking capability, and also able to withstand high voltage spikes during commutation,

is simple.

An output short circuit or simultaneous conduction in an inverter arm is controlled by

the ‘controlled current source’ used here, i.e., a current limited voltage source in series

with a large inductance.

The converter-inverter combined configuration has inherent four-quadrant operation

capability without any extra power component.

Disadvantages A minimum load at the output is required, and the commutation capability is dependent

upon load current. This limits the operating frequency, and also puts a limitation on its

use for UPS systems.

At light loads, and high frequency, these inverters have sluggish performance and

stability problems.

EE6503 POWER ELECTRONICS L T P C

3 0 0 3

CO1: Ability to understand and analyze about linear and digital electronic circuits.

CO2: Ability to understand and analyze about phase controlled rectifiers.

CO3: Ability to understand and analyze about dc-dc converters.

CO4: Ability to understand and analyze about inverters and PWM.

CO5: Ability to understand and analyze about ac-ac converters

UNIT I POWER SEMI-CONDUCTOR DEVICES Study of switching devices, Diode, SCR, TRIAC, GTO, BJT, MOSFET, IGBT-Static

and Dynamic characteristics - Triggering and commutation circuit for SCR- Design of Driver

and snubber circuit.

UNIT II PHASE-CONTROLLED CONVERTERS 2-pulse,3-pulse and 6-pulseconverters– performance parameters –Effect of source inductance–– Gate

Circuit Schemes for Phase Control–Dual converters.

UNIT III DC TO DC CONVERTER Step-down and step-up chopper-control strategy–Forced commutated chopper–Voltage

commutated, Current commutated, Load commutated, Switched mode regulators- Buck,

boost, buck- boost converter, Introduction to Resonant Converters. UNIT IV INVERTERS

Single phase and three phase voltage source inverters(both1200modeand180

0mode)–Voltage&

harmonic control--PW M techniques: Sinusoidal PW M, modified sinusoidal PW M -

multiple PW M – Introduction to space vector modulation –Current source inverter.

UNIT V AC TO AC CONVERTERS Single phase and Three phase AC voltage controllers–Control strategy- Power Factor

Control – Multistage sequence control -single phase and three phase cyclo converters –

Introduction to Matrix converters.

UNIT V AC TO AC CONVERTERS

A voltage controller, also called an AC voltage controller or AC

regulator is an electronic module based on

either thyristors, TRIACs, SCRsor IGBTs, which converts a fixed

voltage, fixed frequency alternating current (AC) electrical input

supply to obtain variable voltage in output delivered to a

resistive load. This varied voltage output is used for dimming street

lights, varying heating temperatures in homes or industry, speed

control of fans and winding machines and many other applications,

in a similar fashion to an autotransformer.[1][2] Voltage controller

modules come under the purview of power electronics. Because

they are low-maintenance and very efficient, voltage controllers

have largely replaced such modules as magnetic

amplifiers and saturable reactors in industrial use

Modes of operation

Voltage controllers work in two different ways; either through "on-and-off control" or through "phase control".[3][4] [5]

On-and-off control[edit]

In an on-and-off controller, thyristors are used to switch on the circuits for a few cycles of voltage and off for certain cycles, thus altering the total RMS voltage value of the output and acting as a high speed AC switch. The rapid switching results in high frequency distortion artifacts which can cause a rise in temperature, and may lead to interference in nearby electronics.[2][4] Such designs are not practical except in low power applications

Phase angle control[edit]

In phase angle control, thyristors are used to halve the voltage cycle during input. By controlling the phase angle or trigger angle, the output RMS voltage of the load can be varied. The thyristor is turned on for every half-cycle and switched off for each remaining half-cycle. The phase angle is the position at which the thyristor is switched on. TRIACs are often used instead of thyristors to perform the same function for better efficiency.[7] If the load is a combination

of resistance and inductance, the current cycle lags the voltage cycle, decreasing overall power output.

Types of voltage controllers[edit]

There are essentially two types of voltage controllers: single-phase voltage controllers which control voltage of 230 rms, 50–60 Hz power supply, and three-phase voltage controllers which control 400 rms voltage, 50–60 Hz power supply (depending on the country).

Applications

Light dimming circuits for street lights

Industrial & domestic heating

Induction heating

transformer tap changing

Speed control of Motors (variable torque)

speed control of winding machines,fans

AC magnet controls

Single Phase Full Wave Ac

Voltage Controller

Operation with resistive loads

The device(s) is triggered at a phase-angle 'α' in each cycle. The current follows the voltage wave

shape in each half and extinguishes itself at the zero crossings of the supply voltage. In the two-SCR

topology, one SCR is positively biased in each half of the supply voltage. There is no scope for

conduction overlap of the devices. A single pulse is sufficient to trigger the controlled devices with a

resistive load. In the diode-SCR topology, two diodes are forward biased in each half. The SCR always

receives a DC voltage and does not distinguish the polarity of the supply. It is thus always forward

biased. The bi-directional TRIAC is also forward biased for both polarities of the supply voltage.

Power Factor

The fundamental load/supply current lags the supply voltage by the φ1, 'Fundamental Power Factor'

angle. Cosφ1 is also called the 'Displacement Factor'. However this does not account for the total

reactive power drawn by the system. This power factor is inspite of the actual load being resistive!

The reactive power is drawn also y the trigger-angle dependent harmonics. Now

Operation with inductive loads

The current builds up from zero in each cycle. It quenches not at the zero crossing of the applied

voltage as with the resistive load but after that instant. The supply voltage thus continues to be

impressed on the load till the load current returns to zero. A single-pulse trigger for the TRIAC 26.1

(c) or the antiparallel SCR (b) has no effect on the devices if it (or the anti-parallel device) is already

in conduction in the reverse direction. The devices would fail to conduct when they are intended to,

as they do not have the supply voltage forward biasing them when the trigger pulse arrives. A single

pulse trigger will work till the trigger angle α > φ, where φ is the power factor angle of the inductive

load. A train of pulses is required here. The output voltage is controllable only between triggering

angles φ and 180o . With an inductance in the load the distinguishing feature of the load current is

that it must always start from zero. However, if the switch could have permanently kept the load

connected to the supply the current would have become a sinusoidal one phase shifted from the

voltage by the phase angle of the load, φ. This current restricted to the half periods of conduction is

called the 'steady-state component' of load current iss. The 'transient component' of load current itr,

again in each half cycle, must add up to zero with this iss to start from zero.

(2-pulse converter)

Single phase fully controlled bridge converter

The single phase fully controlled rectifier allows conversion of single phase AC

into DC.

Used in various applications such as battery charging, speed control of DC

motors and front end of UPS (Uninterruptible Power Supply) and SMPS ∴ is=-

io

Single phase and Three phase AC voltage controllers–Control strategy- Power

Factor Control – Multistage sequence control -single phase and three phase cyclo

converters –Introduction to Matrix converters.