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EE420 POWER
ELECTRONICSChapter 07 ThyristorsLecture 14-15
Dr. Tauseef Tauqeer
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Thyristors
2
Learning objectives
To learn about different type of thyristors,
Limitation of thyristors as switches,
Understand the gate characteristics and gate control
requirement of different type of thyristors.
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Thyristors are family of power semiconductor devices and are usedextensively in the power electronics circuits.
Conventional thyristors are designed with gate controlled turned on
capability but without gate controlled turn-off capability, in which case
the thyristor can recover from its non-conducting state only when the
current is brought to zero by some other means. Gate turn-off thyristors (GTOs) are designed to have both controlled
turn-on and turn-off capability.
Compared to the transistors, thyristors have lower on-state conduction
losses and higher power handling capability.
On the other hand transistors generally have superior switchingperformances in terms of faster switching speed and lower switching
losses.
Advances are being continuously made to achieve devices with the
best of both (i.e. low on-state and switching losses, while increasing
their power handling capability.
7.1: Introduction
3Dr. Tauseef Tauqeer
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7.2: Thyristor Characteristics
4
Figure 7.1: Thyristor symbol and three pn junctions
A thyristor is a four layer semiconductor device of pnpnstructure with three pn junctions. It has three terminals:anode, cathode, and gate.
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7.2: Thyristor Characteristics
5
Figure 23-1(Mohan): Structural details of generic thyristor, vertical cross section
When the anode voltage is made positive with respect to cathode, the
junction J1 and J3 are forward biased. The junction J2 is reversebiased, and only a small leakage current flows from anode tocathode.
Once a gate signal is applied it triggers the conduction. With higherconcentration of holes in the p-type layer it becomes a +ve feedbackprocess that can sustain even if the gate signal is removed.
(appliedelectric field)
holes
electronselectrons
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7.2: Thyristor Characteristics
6
When the anode voltage is
made positive with respect tocathode, the junction J1 and
J3 are forward biased. The
junction J2 is reverse biased,
and only a small leakage
current flows from anode tocathode. The thyristor is then
said to be in the Forward
Blocking, or Off State
Condition and the leakage
current is known as off-state
current ID.
If the anode-to-cathode voltage VAKis increased to a sufficiently large
value, the reverse biased junction J2 breaks. This is known as
Avalanche Breakdown and the corresponding voltage is called forward
breakdown voltage VBO.
Figure 7.3: Thyristor circuit and vicharacteristics.
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7.2: Thyristor Characteristics
7
Because the other junctionsJ1and J3are already forward
biased, there is free
movement of carriers across
all three junctions, resulting
in a large forward anode
current. The device is then in
conducting state, or on state.
The voltage drop would be
due to the ohmic drop in the
four layers and it is small,typically, 1 V. Thyristor circuit and vicharacteristics.
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7.2: Thyristor Characteristics
8
In the On state, the anode
current is limited by an external
impedance or a resistance RL
as shown in figure.
Thyristor circuit and vicharacteristics.
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7.2: Thyristor Characteristics
9
Thus the thyristor is alatching device, it latchesinto full conduction in itsforward direction.
Thus as described above, athyristor can be turned on
by increasing the forwardvoltage VAKbeyond VBO, butsuch a turn on could bedestructive.
In practice, the forward
voltage VAK is maintainedbelow VBOand the thyristoris turned on by applying apositive voltage between itsgate and cathode.
Thyristor circuit and vicharacteristics.
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7.2: Thyristor Characteristics
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Latching Current IL is the
minimum anode current inthe on-state immediatelyafter the thyristor hasbeen turned on and thegate signal has been
removed.
Once the thyristor isturned on by a gatingsignal and its anodecurrent is greater than the
holding current, the devicecontinues to conduct dueto positive feedback, evenif gating signal is removed. Thyristor circuit and vi
characteristics.
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7.2: Thyristor Characteristics
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Once a thyristor conducts, it behaves like a conducting
diode and there is no control over the device. The devicecontinues to conduct because there is no depletion layer onthe junction J2due to free movement of carriers.
However, if the forward anode current is reduced below alevel known as the holding current IH, a depletion region
develops around junction J2 due to reduced number ofcarriers and the thyristor is in blocking state. HoldingCurrent (IH
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7.2: Thyristor Characteristics
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Forward voltage drop from 0.5 to 2V Triggered by a gate pulse of few mA.
Switching time 10 s to 400 s
After turn-on, the gate has no effect in turning off.
Have di/dt (up to 1000A/s) and dv/dt (1000V/s) limitations.
Thyristors available up to 6000 V, 6000 A.
Low cost, high efficiency, and high voltage and current capability.
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There are Various methods toturn on the Thyristor. Gate Current
is the most practical one.
If the thyristor is forward biased,
the injection of gate current by
applying positive gate voltage
between the gate and the cathode
terminals turns on the thyristor. As the gate current is
increased, the forward blocking
voltage is decreased.
Figure 7.7: Effect of Gate currenton the forward blocking voltage
7.4: Thyristor Turn ON
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The time delay known as turn on time, ton
is defined as the time
interval between 10% of steady state gate current (0.1 IG) and
90% of the steady state thyristor on-state current (0.9 IT).
Figure 7.8: Turn ON
characteristics
ton= td+ tr,
=delay time + rise time
td is defined as the timeinterval between 10% of gatecurrent and 10% of thyristor
on-state current. tris the time required for the
anode current to rise from10% on-state current to 90%of on-state current.
7.4: Thyristor Turn ON
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7.4: Thyristor Turn ON
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The following should be considered in designing the gate
control circuit1. The gate signal should be removed after the thyristor
is turned on. A continuous gating signal wouldincrease the power loss in the gate junction.
2. There should be no gate signal when the thyristor isreverse biased; otherwise the thyristor may fail dueto an increased leakage current.
3. The width of the gate pulse tGmust be longer thanthe time required for the anode current to rise to the
Latching current value IL. In practice, the pulse widthtG is normally made more than the turn-on time ofthe thyristor.
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7.5: Thyristor Turn OFF
16
A Thyristor that is in the on state, can be
turned off by reducing the forward current toa level below the holding current IH. There
are various techniques for turning off
(commutation techniques) a thyristor.
In all the commutation techniques, the anode current is maintainedbelow the holding current for a sufficient long time so that all the
excess carriers in the four layers are swept out or recombined.
Due to two outer pn-junctions, J1and J3, the turn-off characteristics
would be similar to that of a diode, exhibiting reverse recovery time
trr, and peak reverse recovery current IRR.
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7.5: Thyristor Turn OFF
timeionrecombinat
currentrecoveryreverse
rc
rr
t
t
17
rcrrq ttt
Turn-off time,
In line-commutated converter circuit where the input voltage isalternating, a reverse voltage appears across the thyristor
immediately after the forward current goes through the zero value.
This reverse voltage accelerates the turn-off process by sweeping
out the excess carriers from the pn-junctions J1and J3.
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D T f T
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7.5: Thyristor Turn OFF
rcrrq ttt
18
Turn-off time,
The inner pn-junction J2requires a time known as recombination time trcto
recombine excess carriers. A negative reverse voltage would reduce thisrecombination time. trc is dependent on the magnitude of the reverse
voltage.
The turn off time tqis the sum of reverse recovery time trrand recombination
time trc.At the end of turn-off, a depletion layer develops across junction J2
and the thyristor recovers its ability to withstands forward voltage. In all thecommutation techniques, a reverse voltage is applied across the thyristor
during the turn-off process.
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7.5: Thyristor Turn OFF
19
The turn off time tq is the minimum value of time interval between
the instant when the on-state current has decreased to zero and the
instant when the thyristor is capable of withstanding forward voltage
without turning on. tqdepends on the peak value of on-state current
and instantaneous on state voltage.
Reverse recovery charge QRRis the amount of charge that has to be
recovered during turn-off process. Its value is determined from the
area enclosed by the path of reverse recovery current. The value of
QRR depends on the rate of fall of on-state current and the peak
value of on-state current before turn-off. QRR causes corresponding
energy loss within the device.
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The manufacturer use various gate structures to
control di/dt, turn on time, and turn off time.
Thyristor can be easily turned on with a short pulse.For turning off they require special drive circuitry orspecial internal structure to aid in the turn-off process.
There are several versions of thyristors with turn-offcapability and the goal of any new device is to improvethe turn-off capability.
With the emergence of new devices with both turn-onand turn-off capability, the device with just the turn-oncapability is referred to as Conventional Thyristor orjus the Thyristor.
Other members of the thyristor or silicon-controlledrectifier (SCR) family have other names based onacronyms.
7.6: Thyristor Types
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7.6: Thyristor Types
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1) Phase-controlled thyristor (or SCR)
2) Bidirectional phase-controlled thyristor (BCT)
3) Fast switching thyristor (or SCR)
4) Light-activated silicon-controlled rectifier (LASCR)
5) Bidirectional triode thyristor (TRIAC)
6) Reverse-conducting thyristor (RCT)
7) Gate turn-off thyristor (GTO)
8) FET-controlled thyristor (FET-CTH)
9) MOS turn-off thyristor (MTO)
10)Emitter turn-off (control) thyristor (ETO)11)Integrated gate-commutated thyristor (IGCT)
12)MOS-controlled thyristor (MCT)
13)Static Induction Thyristor (SITH)
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Controlled Rectifier
In diode rectifier, the DC voltage at the output is fixed and cannot be controlled and thus called non-controlled rectifier
In Controlled Rectifier, the output DC voltage is adjustable
Phase controlled thyristors are used
The output voltage of the thyristor rectifier is varied bycontrolling the delay or firing angle of thyristors
A phased control rectifier can be turned on by applying a shortpulse to the gate and turned off by natural line commutation
In case of highly inductive load, it is turned off by firinganother thyristor of the rectifier during the negative cycle ofthe input voltage
Examples: DC Welders, DC Motor Drives, Battery Charging,DC Power Supply etc.
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( )
( )
( )
2 2
( )
1sin
2
1 cos2
1sin
2
1 sin 2
2 2
o dc m
m
o dc
o dc
o rms m
m
V V d
V
VI
R
V V d
V
Single Phase Half Wave Rectifier (1/3)
Load is purely resistive, R
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Single Phase Half Wave Rectifier (2/3)
Load is purely / highly inductive, L
Average voltage across inductor is zero
From to , the energy will be stored in inductor
Beyond , Ldi/dt becomes negative and forcing thethyristor to remain on
-ev Ldi/dt will be equal to +ve Ldi/dt
The duration during the positive and negative cycle will e
the same i.e. -for +ve cycle and 2--for ev cycle So the thyristor conducts from to 2-
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Freq=50 Hz Cycle
11.78msec
Angel Calculations10msec=180deg
11.78msec=212deg
212-180=>32.14deg
Maths
= tan1
= 32.14deg
= 2 + ()2
Z=5.904 Ohm
=
with lag
311/5.904=52.7A
VL
Vs VR
IR,L
V1
VSINE R1
5
D1
DIODE
VRload
Current
V1(+)
L1
10mHL1(1)
Single Phase Half Wave Rectifier
RL load (3/3)
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Single Phase Full Wave Rectifier
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RL load
( )
( )
( )
2 2
( )
( )( )
2sin2
2cos
2sin
2
2
o dc m
m
o dc
o dc
o rms m
m
s
o rms
o rms
V V d
V
VI
R
V V d
VV
VI
If =0 thenVdc=2Vm/
If =/2 thenVdc=0
If = thenVdc=-2Vm/
So, Vdc=+ve for 0< < /2
And, Vdc=-ve for /2 <
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