Power Electronics Chopper (dc – dc converter)

Chopper (DC – DC Converter)

ER. FARUK BIN POYEN

ASST. PROFESSOR

DEPT. OF AEIE

[email protected]

mailto:[email protected]

Contents:

Features

Elementary Principles

Classification

Principle of Chopper Action

Buck Chopper

Boost Chopper

Buck Boost Chopper

Different Quadrant Operations of Chopper

2

Features of Choppers

Chopper is a static device that converts fixed DC input voltage to a variable DC output

voltage directly.

DC equivalent of an AC transformer as it behaves in an identical manner.

More efficient than AC transformers as they involve in one stage conversion.

They offer smooth control, high – efficiency, fast response and regeneration.

A chopper can act as a step – up or step down DC device.

Chopper can be operated in either a continuous or continuous current conduction mode.

Choppers can be built with and without electrical isolation.

They find application in trolley cars, marine hoists, forklift trucks and mine haulers.

3

Elementary Principle

High speed on/off semiconductor switch.

It connects source to load and disconnects the load from source at high speed.

Continuous triggering ON and triggering OFF at rapid switching speed.

By varying ON and OFF time of the SCR, the average voltage across the load can be

varied.

4

Classification of Choppers

Depending upon the direction of the output current and voltage, choppers are classified

into five classes viz.

Class A (One Quadrant Operation)

Class B (One Quadrant Operation)

Class C (Two Quadrant Operation)

Class D (Two Quadrant Operation)

Class E (Four Quadrant Operation)

Based on the output voltage, choppers are classified into

Step Up (Boost) Chopper

Step Down (Buck) Chopper

Buck – Boost Chopper

5

Further Classification of Choppers

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

the choppers are classified into two categories viz.

1. 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.

2. 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.

6

DC/DC Converter Technology

Linear Regulators

Switching Regulators

Charge Pumps

7

DC/DC Converter Technology Comparison

Parameter Linear regulatorSwitching regulator

Charge pump

Efficiency Low High Medium

EMI Noise Low High Medium

Output current Low to medium Low to High Low

Boost (step-up) No Yes Yes

Buck (step-down) Yes Yes Yes

Solution size Small Large Medium

Chopper: Methods of Control

The output dc voltage can be verified by the following methods.

1. Constant Frequency Control or Pulse Width Modulation (PWM) Control

2. Variable Frequency Control

PULSE WIDTH MODULATION

In PWM, the pulse width ton of the output waveform is varied keeping chopping frequency

’f’ and hence chopping period ‘T’ constant. Therefore output voltage is varied by varying the

ON time, ton . Figure shows the output voltage waveform for different ON times.

VARIABLE FREQUENCY CONTROL

In this method of control, chopping frequency f is varied keeping either ton or t off constant.

This method is also known as frequency modulation.

In frequency modulation to obtain full output voltage, range of frequency has to be varied

over a wide range. This method produces harmonics in the output and for large t off load

current may be discontinuous.

8

Chopper: Methods of Control 9

V0

V

V

V0

t

ttON

tON tOFF

tOFF

T

v0

V

V

v0

t

t

tON

tON

T

T

tOFF

tOFF

PWM Control Variable Frequency Control

Principle of Chopper Operation:

A switch is connected in series with a DC voltage source and load.

The switch can be a power transistor, an SCR or a GTO.

Ideal switch will offer zero resistance (ON), infinite resistance (OFF) and instantaneous

switching (no lag).

The switch can be turned ON and turned OFF with the help of triggering circuit and

commutating circuit respectively.

It can be turned on or turned off as desired at very high frequency.

During the TON period, chopper is in ON condition and load voltage is equal to source

voltage Vs.

During the TOFF period, chopper is OFF condition, load current flows through the

freewheeling diode FD.

10


As a result, load terminals are short circuited by freewheeling diode FD.

Thus the load voltage is zero during TOFF period.

In this manner, a chopped DC voltage is produced across the load terminals.

From the T=TON +TOFF equation, it is clear that the load voltage depends on two factors.

a) The supply voltage.

b) The duty cycle of the chopper

Since the supply voltage is constant, load voltage is governed by the duty-cycle of the

chopper.

In other words, the load voltage is dependent on two factors TON and TOFF.

11


Hence it is concluded that the average load voltage can be controlled by varying the value

of TON and/of TOFF in the following two ways.

a) Varying TON and keeping the periodic time T constant. This is called constant frequency

system.

b) Variable frequency system. i.e. keeping either TON constant and varying TOFF or keeping

TOFF constant and varying TON.

VO = (TON/[TON+TOFF] )VS = (TON/T )VS = α. VS

Here α = Duty cycle = (TON/T); where T = TON +TOFF ;Chopper frequency f = 1/T

𝐸𝑓𝑓𝑒𝑐𝑡𝑖𝑣𝑒 𝑅𝑀𝑆 𝑂𝑢𝑝𝑢𝑡 𝑉𝑜𝑙𝑡𝑎𝑔𝑒 𝑉𝑂(𝑅𝑀𝑆) =𝑉𝑖2𝑇𝑂𝑁𝑇

= 𝑉𝑖𝑇𝑂𝑁𝑇

= 𝑉𝑖 𝑑

12


The representations of the chopper circuit during different phases are shown below.

13


Output Voltage is less than input voltage.

The thyristor in the circuit acts as a switch.

When thyristor is ON, supply voltage appears across the load

When thyristor is OFF, the voltage across the load will be zero.

Practical arrangement includes an inductor (L) and a diode which are used to eliminate

current pulsations providing a smooth DC current.

With S closed, D is Off and it remains Off as long as S in On.

The i/p current builds up exponentially and flows through L and load.

V O equals V I.

With S OFF or open, the current through L decays to zero.

This causes an inductive voltage with opposite polarity across L.

14


V L forward biases diode D.

Current flows through L, Load and D.

This arrangement permits the use of simple filter inductance L to provide a satisfactorily

smooth DC load current.

With higher switching frequency, smaller inductance is sufficient to get desired O/P.

15


The o/p voltage is equal to the i/p voltage when the switch is ON and D is reverse biased.

Diode current is same as the load current during T OFF.

During T ON, I O is same as I I.

16

Step Down (Buck) Chopper – Continuous Current

The average value of inductor current is:

𝐼𝐿 =𝐼𝑚𝑎𝑥 + 𝐼𝑚𝑖𝑛

2= 𝐼𝑂 =

𝑉𝑂𝑅

Again

𝑉𝐿 = 𝑉𝑂 = 𝐿𝑑𝑖𝑂𝑑𝑡

→𝑑𝑖𝑂𝑑𝑡

=𝑉𝑂𝐿→ ∆𝑖𝑂 =

𝑉𝑂𝐿∆𝑡

With Switch Open,

𝑃𝑒𝑎𝑘 𝑡𝑜 𝑃𝑒𝑎𝑘 𝑅𝑖𝑝𝑝𝑙𝑒 𝐶𝑢𝑟𝑟𝑒𝑛𝑡 𝐼𝑃−𝑃 = ∆𝑖𝑂 = 𝐼𝑚𝑎𝑥 − 𝐼𝑚𝑖𝑛 =𝑉𝑂𝐿𝑇𝑂𝐹𝐹

Hence,

𝐼𝑚𝑎𝑥 =𝑉𝑂𝑅+𝑉𝑂2𝐿

𝑇𝑂𝐹𝐹; 𝐼𝑚𝑖𝑛 =𝑉𝑂𝑅−𝑉𝑂2𝐿

𝑇𝑂𝐹𝐹

𝐼𝐷 =𝐼𝑂𝐹𝐹 . 𝑇𝑂

𝑇; 𝐼𝑂 =

𝐼𝑖𝑑; 𝑑 = 𝑑𝑢𝑡𝑦 𝑐𝑦𝑐𝑙𝑒

17

Step Down (Buck) Chopper – Continuous Current

As elements are ideal, DC power drawn from source must equal the DC power absorbed

by load.

𝑃𝑂 = 𝑃𝑖 → 𝑉𝑜𝐼𝑜 = 𝑉𝑖𝐼𝑖

18

Step Down (Buck) Chopper – Discontinuous Current

For low value of “d” with low L, I L decreases and may fall to zero during T OFF.

It again builds up with T ON, and hence it is called discontinuous current.

19

Step Down (Buck) Chopper – Discontinuous Current

This mode is undesirable and is avoided by proper selection of chopping frequency and L.

The minimum value of L for continuous current mode is ensured by setting I min = 0.

𝐼𝑚𝑖𝑛 = 0 =𝑉𝑂𝑅−𝑉𝑂𝑅𝑇𝑂𝐹𝐹

𝑉𝑂𝑅

= 𝑇𝑂𝐹𝐹 .𝑉𝑂2𝐿

2𝐿 = 𝑇𝑂𝐹𝐹𝑅

𝐿 =𝑇𝑂𝐹𝐹2

𝑅

20

Types of Choppers:

Type A Chopper or First–Quadrant Chopper

Type B Chopper or Second-Quadrant Chopper

Type-C chopper or Two-quadrant type-A Chopper

Type-D Chopper or Two-Quadrant Type–B Chopper

Type-E chopper or the Fourth-Quadrant Chopper

21

Step Up (Boost) Chopper:

The output voltage is more than the input voltage by several times.

L is used to provide a smooth i/p current.

The SCR (S) acts as the switch which works in the PWM mode.

With S On, the L is connected to the supply.

Load voltage V L jumps instantaneously to V I, but current through L increases linearly &

stores energy.

When S is Open, the current collapses and energy stored in L is transferred to C through

D.

The induced voltage across the inductor reverses and adds to the source voltage increasing

the O/P voltage.

The current that was flowing through S now flows through L, D and C to the load.

Energy stored in the inductor is released to the load.

22


With S closed again, D becomes reverse biased, the capacitor energy supplies the load

voltage and the cycle repeats.

𝑉𝑂 = 𝑉𝑖 + 𝑉𝐿

V O is always higher than V I as polarity of inductor voltage V L is same as V I.

If inductor L is very large, source current I I is ripple free and is considered constant.

𝑊𝑂𝑁 = 𝑉𝑖𝐼𝑖𝑇𝑂𝑁

Assuming C to be large enough to neglect the voltage ripples, V O is considered constant.

𝑊𝑂𝐹𝐹 = 𝑉𝑜 − 𝑉𝑖 ∗ 𝐼𝑖 ∗ 𝑇𝑂𝐹𝐹

Since losses are neglected, the energy transferred during T OFF by L mist be equal to

energy gained during T ON.

𝑊𝑂𝑁 = 𝑊𝑂𝐹𝐹 = 𝑉𝑖𝐼𝑖𝑇𝑂𝑁 = 𝑉𝑜 − 𝑉𝑖 ∗ 𝐼𝑖 ∗ 𝑇𝑂𝐹𝐹

𝑉𝑂 = 𝑉𝐼 1 +𝑇𝑂𝑁𝑇𝑂𝐹𝐹

= 𝑉𝐼𝑇

𝑇 − 𝑇𝑂𝑁= 𝑉𝑖

1

1 −𝑇𝑂𝑁𝑇

= 𝑉𝐼1

1 − 𝑑

Thus V O is always greater than V I.

23


𝑃𝐼 = 𝑃𝑂 → 𝑉𝐼𝐼𝐼 =𝑉𝑂2

𝑅→ 𝐼𝐼 =

𝑉𝑂2

𝑉𝐼∗1

𝑅

𝐼𝑂 = 𝐼𝐼 ∗𝑇𝑂𝐹𝐹

𝑇=> 𝐼𝑂 = 𝐼𝐼 1 − 𝑑

𝑃𝑂 = 𝑃𝐼 => 𝑉𝐼𝐼𝐼 =𝑉𝑂2

𝑅=

𝑉𝐼2

1−𝑑 2 ∗1

𝑅

𝐼𝐼 =𝑉𝐼

1−𝑑 2 ∗1

𝑅

𝐼𝐿 =𝐼𝑚𝑎𝑥+𝐼𝑚𝑖𝑛

2= 𝐼𝐼

∴ 𝐼𝑚𝑎𝑥 + 𝐼𝑚𝑖𝑛 = 2 ∗ 𝐼𝐼

24


Voltage across L is

𝑉𝐿 = 𝑉𝐼 = 𝐿 ∗𝑑𝑖𝐼

𝑑𝑡𝑜𝑟

𝑑𝑖𝐼

𝑑𝑡=

𝑉𝐼

𝐿

∆𝐼𝐼 =𝑉𝐼

𝐿𝑇𝑂𝑁 𝑜𝑟 𝐼𝑚𝑎𝑥 − 𝐼𝑚𝑖𝑛 =

𝑉𝐼

𝐿𝑇𝑂𝑁; 𝐴𝑔𝑎𝑖𝑛 𝐼𝑚𝑎𝑥 + 𝐼𝑚𝑖𝑛 = 2 ∗ 𝐼𝐼

Solving

𝐼𝑚𝑎𝑥 = 𝑉𝐼1

𝑅 1 − 𝑑 2+𝑇𝑂𝑁2𝐿

; 𝐼𝑚𝑖𝑛 = 𝑉𝐼1

𝑅 1 − 𝑑 2−𝑇𝑂𝑁2𝐿

𝐼𝑝−𝑝 = 𝐼𝑚𝑎𝑥 − 𝐼𝑚𝑖𝑛 =𝑉𝐼𝑇𝑂𝑁

𝐿

For continuous current mode: 𝐼𝑚𝑖𝑛 = 0 = 𝑉𝐼1

𝑅 1−𝑑 2 −𝑇𝑂𝑁

2𝐿

=>1

𝑅 1−𝑑 2 =𝑇𝑂𝑁

2𝐿

=> 𝐿 =𝑅𝑇𝑂𝑁2

1 − 𝑑 2

25

Buck – Boost Chopper:

It combines the concept of both the step – up and step – down choppers.

The output voltage is either higher or lower than the input voltage.

The output voltage polarity can also be reversed.

The switch is either an SCR or GTO or IGBT.

When S in ON, D is reverse biased & I D is zero.

The voltage across L is equal to the i/p voltage.

The capacitor supplies energy to the output load

The current through the inductor increases linearly with time.

With S OFF, the source is disconnected.

The current through inductor does not change instantaneously and it forward biases the

diode, providing a path for the load current.

O/P voltage becomes equal to the inductor voltage.

26


With S = ON (T ON); W ON = VI*II*TON

With S = OFF (T OFF); W ON = VI*II*TOFF

Ignoring losses, W ON = W OFF => VI*II*TON = VI*II*TOFF

𝑉𝑂 = 𝑉𝐼𝑑𝑇

𝑖−𝑑 𝑇= 𝑉𝐼

𝑑

𝑖−𝑑(O/P voltage is controlled by changing duty cycle d)

𝐼𝐿 =𝐼𝑚𝑎𝑥+𝐼𝑚𝑖𝑛

2; 𝐼𝐼 = 𝐼𝐿𝑑 =

𝐼𝑚𝑎𝑥+𝐼𝑚𝑖𝑛

2𝑑

27


The average O/P power is given as 𝑃𝐼 = 𝑉𝐼𝐼𝐼 =𝐼𝑚𝑎𝑥+𝐼𝑚𝑖𝑛

2𝑑𝑉𝐼 = 𝑃𝑂 =

𝑉𝑂2

𝑅

𝐼𝑚𝑎𝑥 + 𝐼𝑚𝑖𝑛 =2𝑑𝑉𝐼

𝑅 1−𝑑 2 ; 𝐼𝑚𝑎𝑥 − 𝐼𝑚𝑖𝑛 =𝑉𝐼

𝐿𝑑𝑇

𝐼𝑚𝑎𝑥 = 𝑉𝐼1

𝑅 1−𝑑 2 +𝑇

2𝐿𝑑; 𝐼𝑚𝑖𝑛 = 𝑉𝐼

1

𝑅 1−𝑑 2 −𝑇

2𝐿𝑑; 𝐼𝑝−𝑝 =

𝑉𝐼𝑇𝑑

𝐿

For continuous current condition,

𝐼𝑚𝑖𝑛 = 0 = 𝑉𝐼1

𝑅 1 − 𝑑 2−

𝑇

2𝐿𝑑 => 𝐿 =

𝑅𝑇𝑑

21 − 𝑑 2

28

Type A Chopper or First–Quadrant Chopper

When chopper is ON, supply voltage V is connected across the load.

When chopper is OFF, V 0 = 0 and the load current continues to flow in the same

direction through the FWD.

The average values of output voltage and current are always positive.

Class A Chopper is a step-down chopper in which power always flows form source to

load.

It is used to control the speed of dc motor.

The output current equations obtained in step down chopper

with R-L load can be used to study the performance of Class A Chopper

29

Type A Chopper or First–Quadrant Chopper 30

Type B Chopper or Second-Quadrant Chopper

Load contains a DC source E like a battery.

When chopper is ON, E drives a current through L and R in a direction opposite to that

shown in figure.

During the ON period of the chopper, the inductance L stores energy.

When Chopper is OFF, diode D conducts, and part of the energy stored in inductor L is

returned to the supply.

Average output voltage is positive and average output current is negative.

In this chopper, power flows from load to source. O/P current is –ve.

Class B Chopper is used for regenerative braking of dc motor.

Class B Chopper is a step-up chopper.

𝑉0 = 𝐸 + 𝐿𝑑𝑖

𝑑𝑡> 𝑉𝑠

31

Type B Chopper or Second-Quadrant Chopper 32

Type – C or Two – Quadrant Type – A Chopper

Class C Chopper is a combination of Class A and Class B Choppers in parallel.

CH1 (ON) and FD (conducting) operate together as Type A chopper (1st quadrant).

CH2 (ON) and D2 (conducting) operate together as Type B chopper (2nd quadrant).

When CH1 is ON, the load current is positive.

The output voltage is equal to ‘V’ & the load receives power from the source.

When CH1 is turned OFF, energy stored in inductance L forces current to flow through

the diode D2 and the output voltage is zero.

Current continues to flow in positive direction.

When CH2 is triggered, the voltage E forces current to flow in opposite direction through

L and CH2

The output voltage is zero

33

Type – C or Two – Quadrant Type – A Chopper

On turning OFF CH2, the energy stored in the inductance drives current through diode D1

and the supply Output voltage is V, the input current becomes negative and power flows

from load to source.

Average output voltage is positive because of the presence of FD diode.

Average output current can take both positive and negative values.

Choppers CH1 & CH2 should not be turned ON simultaneously as it would result in short

circuiting the supply.

Class C Chopper can be used both for dc motor control and regenerative braking of dc

motor.

Class C Chopper can be used as a step-up or step-down chopper.

34

Type – C or Two – Quadrant Type – A Chopper 35

Type-D or Two-Quadrant Type –B Chopper

Class D is a two quadrant chopper.

When both CH1 and CH2 are triggered simultaneously, the output voltage V O = V S and

output current flows through the load.

When CH1 and CH2 are turned OFF, the load current continues to flow in the same

direction through load, D1 and D2, due to the energy stored in the inductor L.

Output voltage V O = - V S.

Average load voltage is positive if chopper ON time is more than the OFF time

Average output voltage becomes negative if t ON < t OFF .

Hence the direction of load current is always positive but load voltage can be positive or

negative.

36

Type-D or Two-Quadrant Type –B Chopper

T ON > 0.5, average value of V 0 and I 0 are positive, power flows from source to load.

T OFF > 0.5, average value of V 0 is –ve and I 0 is +ve, power flows from load to source.

𝑉𝑜 =𝑉𝑆𝑇𝑂𝑁 − 𝑉𝑆𝑇𝑂𝐹𝐹

𝑇= 𝑉𝑆 .

𝑇𝑂𝑁 − 𝑇𝑂𝐹𝐹𝑇

V 0 is +ve, for 𝑇𝑂𝑁 > 𝑇𝑂𝐹𝐹

V 0 is –ve, for 𝑇𝑂𝑁 < 𝑇𝑂𝐹𝐹

V 0 = 0, 𝑇𝑂𝑁 = 𝑇𝑂𝐹𝐹

37

Type-D or Two-Quadrant Type –B Chopper 38


Forward Motoring Operation (Rectifying operation):

It is the first quadrant operation.

When switches T1 and T2 are conducting, both the output current (I0) and output voltage

(Vo) are positive.

Now the circuit path will be, Vs - T1 - Load - T2 - Vs.

The power is flows from the source to load.

This is known as rectifying operation.

This operation occurs in first quadrant.(as the current and voltage are positive)


Reverse Braking Mode Operation (Inverting operation):

It is the fourth quadrant operation.

When switches T1 and T2 are switched OFF, the load inductance generates a huge

amount voltage to maintain the current in the same direction.

The inductance voltage forward biases both diodes D1 and D2.

Now the circuit path will be Load - D1 - Vs - D2 - Load.

Here the output current is positive but the output voltage is negative. i.e., the power

flows from load to source.

Thus we can achieve reverse breaking operation.

Here the motor speed can be controlled by changing the duty cycle of the switches.

Type – E or the Four – Quadrant Chopper

It consists of four switches and four diodes in anti – parallel.

Numbering of choppers is done corresponding to their quadrant operation.

When CH1 and CH4 are triggered, output current I 0 flows in positive direction through

CH1 and CH4 , and with output voltage V O = V S.

This gives the first quadrant operation.

When both CH1 and CH4 are OFF, the energy stored in the inductor L drives I O through

D2 and D3 in the same direction, but output voltage V O = - V.

Therefore the chopper operates in the fourth quadrant.

41


When CH2 and CH3 are triggered, the load current I O flows in opposite direction &

output voltage V O = -V.

Since both I O and V O are negative, the chopper operates in third quadrant.

When both CH2 and CH3 are OFF, the load current I O continues to flow in the same

direction D1 andD4 and the output voltage V O = V.

Therefore the chopper operates in second quadrant as V O is positive but I O is negative.

42

Type – E or the Four – Quadrant Chopper 43


1st Quadrant:

CH2 and CH3 are OFF.

CH4 is ON, CH1 is operated.

CH1 and CH4 being ON, V O = V S, I O starts flowing.

Both V O and I O are +ve.

With CH1 = OFF, +ve current freewheels through CH4 and D2.

Both V O and I O are controlled in the 1st quadrant.

Acts as Step – Down chopper.

44


2nd Quadrant:

CH2 is operated and CH1, CH3 and Ch4 are kept OFF.

With CH2 ON, reverse current flows through L, CH2, D4 and E.

L stores energy during the ON state on CH2.

CH2 = OFF, current is fed to the source through D1 and D4.

V O > V S resulting –ve I O.

Chopper is in Step Up mode and load must contain E.

45


3rd Quadrant:

CH1 is OFF, CH2 is ON.

CH3 is operating.

Polarity of E is reversed.

With CH3 ON, load gets connected to source V S, hence both V O and I O are –ve.

With CH3 OFF, -ve current freewheels through CH2 and D4.

V O and I O are controlled in 3rd quadrant.

Chopper operates as a Step Down chopper.

46


4th Quadrant:

CH4 is operated here while other devices are OFF.

Load E has its polarity reversed to that of the source.

With CH4 ON, +ve current flows through CH4, D2, L and E.

L stores energy when CH4 is ON.

With CH4 OFF, current is fed back to source through D2 and D3.

Load voltage is –ve but load current is +ve leading the chopper to operate in the 4th

quadrant.

Power is fed back from load to source.

Here chopper operates as Step – Up chopper.

47

Thyrsitor Chopper Circuits:

In chopper circuit with thyristors, the process of turning off is termed as Commutation.

These commutation techniques are broadly classified into the following.

1. Forced Commutation: External elements (L & C) which do not carry load current are

used.

a) Voltage Commutation: Application of large pulse reverse voltage usually by

switching a previously charged capacitor.

b) Current Commutation: External pulse current > load current is applied in reversed

direction.

2. Load Commutation: Thyrsitor is turned off when load current flowing through either

a) Becomes zero due to the nature of the load circuit.

b) Is transferred to another device from the conducting thyristor.

48

Voltage Commutated Chopper

It generally finds application in high power circuits where load fluctuations are not very

large.

This chopper is also known as “parallel capacitor turn – off chopper or impulse –

commutated chopper or classical chopper.

Thyristor T1 is the main power switch.

Commutation circuitry is made up of an auxiliary thyristor TA, capacitor C, diode D and

inductor L.

FD is connected across RLE type load.

Working of this chopper can start only of the C is charged with polarities as marked in

figure.

Jones chopper employes the principle of voltage commutation.

49


This is achieved in one of the following two ways.

1. Close switch S so that the capacitor s gets charged to voltage V S through V S, C S and R

C. Switch S is then opened.

2. Auxiliary thyristor TA is triggered so that C gets charged through source voltage V S, C,

TA and the load. The charging current through C decays and as it reaches to zero, V C =

V S and TA is turned OFF.

50


This is achieved in one of the following two ways.

1. Close switch S so that the capacitor s gets charged to voltage V S through V S, C, S and

R C. Switch S is then opened.

2. Auxiliary thyristor TA is triggered so that C gets charged through source voltage V S, C,

TA and the load. The charging current through C decays and as it reaches to zero, V C =

V S and TA is turned OFF.

51

Voltage Commutated Chopper - Operation

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.

52

Voltage Commutated Chopper - Operation

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.

53

Voltage Commutated Chopper - Operation 54

Voltage Commutated (VC) Chopper

It is simple however it has the following disadvantages.

1. A starting circuit is required.

2. Load voltage at once rises to 2* V S at the instant commutation of main thyristor is

initiated. FD is therefore subject to twice the supply voltage.

3. It can not work at no load because C would not get charged from – V S to V S when

auxiliary SCR is triggered for commutating the main SCR.

55

V C Chopper – Design Consideration

Commutating Capacitor C: Its value depends on the turn off time t OFF of T1. during turn

off, V C linearly changes from (- V S) to zero.

𝐼𝐶 = 𝐶𝑑𝑉

𝑑𝑡; 𝑓𝑜𝑟 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 𝐼𝑂, 𝐼𝐶 = 𝐶

𝑉𝑆

𝑡𝑂𝐹𝐹

=> 𝑪 =𝒕𝑶𝑭𝑭𝑰𝑶𝑽𝑺

Commutating Inductor L: it is designed with the consideration that when T1 is On, an

oscillatory current is established. The current I C flows through the ringing circuit forced

by C, T1, L, D and is given by

𝐼𝐶 =𝑉𝑆𝜔0𝐿

sin𝜔0𝑡 𝑤ℎ𝑒𝑟𝑒 𝜔0 =1

𝐿𝐶

𝑃𝑒𝑎𝑘 𝐶𝑎𝑝𝑎𝑐𝑖𝑡𝑜𝑟 𝐶𝑢𝑟𝑟𝑒𝑛𝑡 𝐼𝐶𝑃 =𝑉𝑆

𝜔0𝐿= 𝑉𝑆

𝐶

𝐿

𝐼𝐶𝑃 ≤ 𝐼0 𝑜𝑟 𝑉𝑆𝐶

𝐿≤ 𝐼0 => 𝑳 ≥

𝑽𝑺

𝑰𝑶

𝟐𝑪

56

Current Commutating (C C) Chopper:

The circuit for Current Commutating Chopper is shown below.

T1 is the main thyristor.

The commutating components comprise TA, C, L, D1 and D2.

FD is the freewheeling and R C is the charging resistor.

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

57

Current Commutating Chopper:

Simplified assumptions for this chopper are as follows.

1. Load current is constant.

2. SCRs and diodes are ideal switches.

3. Charging capacitor is so large that it can be treated as open circuit during the

commutation interval.

The merits of this chopper are given as below.

1. Commutation is reliable as long as the load current is less than the peak commutating

current I CP.

2. Capacitor is always charged with the correct polarity.

3. TA is naturally commutated as its commutating current passes through zero value in the

ringing circuit formed by L and C.

58

Current Commutating Chopper: Operation

Capacitor is charged to V S through V S, C and R C, main thyristor T1 is fired at t = 0.

So that load voltage V O = V S.

At t = t1, auxiliary thyristor TA is turned ON to commutate the main thyristor.

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

At t2, V C = - V S and I C tends to reverse in the auxiliary thyristor TA, it gets naturally

commutated.

As TA is reverse biased and turned off at t2, oscillatory current I C begins to flow through

C, L, D2 and T1.

At t3 I C rises to I O 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 I C supplies load current I O and the excess current. ID1 = I C - I O is conducted

through diode D1.

After t4, a constant current equal to Io flows through V S, C, L, D2 and load.

59

Current Commutating Chopper: Operation

Capacitor C is charged linearly to source voltage V S at t5,

So during time ( t5 - t4 ) I C = I O.

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

1. Turn off time increases.

2. Turn off power loss increases.

Current I C, I T1, I FD and I O are treated as positive.

Voltages V C, V T1, V TA and V O are taken as positive with polarities as marked.

60

C C Chopper – Design Consideration

L and C values should be calculated for reliable commutation for this chopper.

The governing conditions for the values of L and C are provided as below.

1. Peak Commutating Current I CP must be more than the maximum possible load current I

O. This is essential for reliable commutation of main SCR.

The oscillating current in the commutation circuit is given by

𝐼𝐶 = 𝑉𝑆𝐶

𝐿sin𝜔0𝑡 = 𝐼𝐶𝑃 sin𝜔0𝑡

As per design requirement, 𝐼𝐶𝑃 = 𝑉𝑆𝐶

𝐿> 𝐼0 => 𝑉𝑆

𝐶

𝐿= 𝑥𝐼0; 𝑥 =

𝐼𝐶𝑃

𝐼0

𝑥 varies between 1.4 and 3.

61


Circuit Turn Off time T C must be grater than thyristor Turn Off time T OFF.

T C = T OFF + Δt.

𝜔0𝑇𝐶 = 𝜋 − 2𝜃1; 𝐴𝑙𝑠𝑜 𝐼𝐶𝑃 sin 𝜃1 = 𝐼0 => 𝜃1 = 𝑠𝑖𝑛−1𝐼𝑂

𝐼𝐶𝑃= 𝑠𝑖𝑛−1

1

𝑥

Circuit Turn Off time for main SCR 𝑇𝐶 =1

𝜔0𝜋 − 2𝜃1 =

1

𝜔0𝜋 − 2𝑠𝑖𝑛−1

𝐼𝑂

𝐼𝐶𝑃

As load current increases, T OFF decreases.

𝑇𝐶 = 𝜋 − 2𝑠𝑖𝑛−1𝐼𝑂

𝐼𝐶𝑃𝐿𝐶

∴ 𝐶 =𝑇𝐶

𝜋−2𝑠𝑖𝑛−1𝐼𝑂𝐼𝐶𝑃

𝐿

62


Substituting the value of 𝐶 in 𝑉𝑆𝐶

𝐿= 𝑥𝐼0, we get

𝑉𝑆

𝐿.

𝑇𝐶

[𝜋−2𝑠𝑖𝑛−1 1 𝑥 ]= 𝑥. 𝐼0

∴ 𝑳 =𝑽𝑺𝑻𝑪

𝒙. 𝑰𝟎[𝝅 − 𝟐𝒔𝒊𝒏−𝟏 𝟏 𝒙 ]

𝐴𝑔𝑎𝑖𝑛 𝐿 =1

𝑇𝐶[𝜋 − 2𝑠𝑖𝑛−1 1 𝑥 ] 𝐶

𝑪 =𝒙. 𝑰𝟎𝑻𝑪

𝑽𝑺[𝝅 − 𝟐𝒔𝒊𝒏−𝟏 𝟏 𝒙 ]

63

Load Commutated Capacitor:

It consists of four SCRs T1 – T4 and one commutating capacitor C.

T1, T2 act together as one pair and T3 and T4 act as the second pair.

FD is the freewheeling diode across load.

With T1, T2 conducting, they act as the main pair and T3, T4 along with C act as

commutating components and likewise when T3, T4 are conducting.

The capacitor is initially charged to V S with upper –ve and lower +ve plates.

64

Load Commutated Capacitor: Mode I & II

Mode I:

With C having lower plate as +ve and charged, the load commutated chopper is ready to

operate.

When T1, T2 pair is triggered, load voltage passes through V S, T1, C, T2 and load.

V O = V S + V C = 2 V S.

Capacitor C is charged linearly by constant load current I O from V S to – V S.

When capacitor voltage becomes – V S, V O = 0, T3, T4 pair gets triggered.

Mode II:

Capacitor C is slightly overcharged resulting FD getting forward biased and load current

is transferred through T1, T2 to FD.

Load current freewheels through FD.

In the next time span, V C = - V S, V O = 0, I C = 0, I FD = 0, I T1 = I T2 = 0.

V T3 = V T4 = V S and V T1 = V T2 = - ΔV S as capacitor is overcharged by a small voltage

ΔV S.

65

Load Commutated Capacitor: Mode III

Thyrisotr pair T3, T4 is triggered and voltage becomes V O = V S + V C = 2 V S.

T1, T2 pair is reverse biased and therefore OFF.

Load current flows through V S, T4, C, T3 and load charging the capacitor linearly form –

V S to V S.

Load voltage falls form 2 V S to zero.

I C = - I O, I T3 = I T4 = I O.

V T1 = V T2 = - V S.

In the next time span, C is overcharged and FD gets forward biased and load current

freewheels through FD and load.

66

Design of Load Commutating Capacitor:

For constant load current I O, capacitor voltage changes from – V S to V S in time T ON.

𝐼𝑂 = 𝐶2𝑉𝑆

𝑇𝑂𝑁=> 𝐶 =

𝐼𝑂.𝑇𝑂𝑁

2𝑉𝑆

𝑉𝑂 =1

22𝑉𝑆 𝑇𝑂𝑁.

1

𝑇= 𝑉𝑆. 𝑇𝑂𝑁. 𝑓

𝑆𝑢𝑏𝑠𝑡𝑖𝑡𝑢𝑡𝑖𝑛𝑔 𝑇𝑂𝑁 =2.𝑉𝑆.𝐶

𝐼𝑂𝑔𝑖𝑣𝑒𝑠 𝑉𝑂 = 𝑉𝑆. 𝑓.

2𝑉𝑆𝐶

𝐼𝑂=

2.𝑉𝑆2.𝐶.𝑓

𝐼𝑂

Minimum chopper time period T min = T ON. Max chopping frequency f max = 1/T ON.

𝐶 =𝐼𝑂2𝑉𝑆

.1

𝑓𝑚𝑎𝑥

Circuit Turn Off time for each thyristor is

𝑡𝐶 =1

2𝑇𝑂𝑁 =

1

2𝐶2. 𝑉𝑆𝐼𝑂

=𝐶. 𝑉𝑆𝐼𝑂

𝑇𝑜𝑡𝑎𝑙 𝐶𝑜𝑚𝑚𝑢𝑡𝑎𝑡𝑖𝑜𝑛 𝐼𝑛𝑡𝑒𝑟𝑣𝑎𝑙 𝑇𝑂𝑁 =2𝐶.𝑉𝑆

𝐼𝑂

67

Chopper Merits and Demerits:

Merits:

1. Capable of commutating any amount of load current.

2. No commutating inductor is required.

3. Filtering requirements are minimum as it can work at high frequencies (KHz).

Demerits:

1. Peak load voltage is twice the supply.

2. Higher switching losses hence lesser efficiency.

3. FD is subject to twice the supply.

4. Commutating capacitor has to carry full load current at a frequency that is half of the

chopping frequency.

68

Chopper Applications:

Chopper are used for DC motor control( battery - supplied vehicles),

Solar and Wind energy conservation

It’s also used in electric cars.

Airplane and spaceships ,where onboard-regulated DC power supplies are required.

Chopper circuits are used as power supplies in computers, commercial electronics,

electronics instruments.

69

References:

www.circuitstoday.com

Choppers, Aniruddha Kr. Gautam, Slideshare.

www.completepowerelectronics.com

https://engineeringwrittennotes.blogspot.in/2016/07/voltage-commutated-chopper-

voltage.html

https://engineeringwrittennotes.blogspot.in/2016/07/current-commutated-chopper-

engineering.html

70

http://www.circuitstoday.com/

http://www.completepowerelectronics.com/

https://engineeringwrittennotes.blogspot.in/2016/07/voltage-commutated-chopper-voltage.html

https://engineeringwrittennotes.blogspot.in/2016/07/current-commutated-chopper-engineering.html

Engineering

Power Electronics Chopper (dc – dc converter)