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1. 1

Electronic Science
Power Electronics

10. SCR

Module 10
SCR

1. Introduction
2. SCR characteristics
3. Two transistor static and transient models
4. SCR turn-on methods
5. SCR turn-off methods
6. SCR protection and Triggering circuits
7. Gate protection circuits
8. Summary



Learning Objectives
1. To study the SCR Characteristics
2. To understand two Transistor Static and Transient Models
3. To learn the SCR Turn-on and Turn-off methods
4. To study SCR Protections and Gate Triggering Circuits
5. To understand the Gate Protection Circuits

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Electronic Science
Power Electronics

10. SCR

1. Introduction
Thyristor is power semiconductor device and is widely used in power electronic circuits. Thyristors are
operated as bi-stable switches, operating from non-conducting to conducting state. There are different
types of thyristors and will be discussed in subsequent section. Silicon controlled rectifier (SCR) is the
most widely used thyristor. SCR can be assumed as ideal unidirectional switch for many applications, but
practical thyristor has certain limitations. A SCR basically is a four layer semiconductor device. It has
pnpn structure with three junctions, J1, J2 and J3. It has three terminals anode, cathode and gate.

Figure 1 Thyristor symbol and three pn-junctions
2. SCR characteristics
When the anode voltage is made positive with respect to cathode, junctions J1 and J3 are forward biased
and J2 is reverse biased. Hence only small leakage current flows from anode to cathode. The thyristor then
said to be in off state condition. If anode to cathode voltage VAK is increased to a sufficiently large value,
the reverse biased junction J2 will break. This known as avalanche breakdown and corresponding voltage
is called as forward break over voltage. Since J1 and J3 are already in forward biased, resulting in large
forward current. The device is then in conduction state or ON state. The voltage drop will be due to ohmic
drop in the layers, and it is small about 1V. In ON state the anode current is limited by external resistance
RL. The anode must be more than a value known as latching current IL, in order to maintain the required
amount of carrier flow across the junction. IL is the minimum anode current required to maintain the
thyristor in the ON state immediately thyristor is turned ON. The typical v-i characteristics of the thyristor
is as shown in the Figure 2. Once the thyristor conducts, it behaves like a conducting diode and there is no
control over the device. The device will continue to conduct because there is no depletion layer on J2 due
to free movement of carriers. If the forward current is reduced below the level known as holding current
IH, a depletion region will develop around J2 due to reduced number of carriers, and thyristor goes into
blocking state. Holding current is always less than latching current.


A
K
G
A K
G
J
1 J
2 J
3
p n p n

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10. SCR


Figure 2 Thyristor (a) Circuit, (b) v-i characteristics

When cathode voltage is larger than anode voltage J1 and J3 are reverse biased and J2 will be forward
biased. So thyristor will be in OFF state and only current flow will be due to leakage current.
3. Two Transistor model of Thyristor:
The regenerative (latching) action of thyristor due to positive feedback can be explained using the two
transistor model of the thyristor. Thyristor can be considered as two complementary transistors, one NPN
(Q2) and other PNP (Q1) as shown in figure. The collector current IC, current gain α, emitter current IE and
leakage current are related as: CBOEC
III  . The common base collector current is defined by:EC
II
. For transistor Q1, 222 CBOKC
III  . Using those equations the total anode current can be
written as:  
21
212
1 




CBOCBOG
A
III
I

The above equation indicates that as if the IG suddenly increased, say from 0 to 1mA, this will
immediately increase α1 and α2. α2 depend on IA and IG. the increase in the values of α1 and α2 would
further increase IA. Therefore there will be regenerative or positive feedback effect. If α1+ α2 tends to
unity, the denominator of above equation approaches to zero, resulting in large IA and thyristor will turn
ON with small gate current.


(a)
RL
VS
+
-
iT
VAK
A
K G
(b)
VAK
VBO
iT
I
L
IH
Reverse
Breakdown
voltage
Reverse
leakage
current
Forward leakage
current
VBR

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10. SCR



Figure 3 – Two transistor Model of Thyristor – (a) Basic Structure, (b) Equivalent Circuit
Under transient conditions SCR may turn on without gate trigerring. Such turn on can not be explained
using static two transistor model. In transient model, one has to consider the junction capacitances of
SCR. Figure 4 shows the two transient model of thyristor.

Figure 4 Two Transistor Transient Model of Thyristor dt
dV
C
dt
dC
V
dt
VCd
dt
qd
i
j
j
j
j
jjj
j
2
2
2
2
222
2
)()(


A

I
A

Q1

Q2

G

K

I
K

I
G

V
j2

C
j1

+

α
1

α
2

C
j2

C
j3

-

(b)
A

I
T = I
A

Q1

Q2

G

K

I
K

I
G

I
B2

I
C1

I
B1= I
C2

α
1

α
2

(a)
p
n
P
p
P
n
IT
IK
IG
J
1
J
2
J
3
Q1
Q2
K
A
G

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10. SCR

The above equation shows the relation between junction current (ij2), junction capacitance (Cj2) and
voltage (Vj2) across the junction J2.
Turn on characteristics of SCR is shown in Figure 5. It consists of delay time, td and rise time tr. When
SCR is triggered the gate current rises, but thyristor does not turn on immediately. It starts turning on with
delay, td. Thus the turn on time is ton = td + tr. The width of triggering pulse must be greater than ton of the
given SCR i.e Gating pulse width, TW > ton.

Figure 5 Turn on characteristics of Thyristor
Turn off characteristics of SCR is complicated than turn on time and is shown in Figure 6. SCR can be
brought back to the blocking state from on state only by reducing the forward current to a level below that
of the holding current. The forward current is reduced by applying a reverse voltage across anode and
cathode and thus forcing the current through the SCR to zero. The SCR has a reverse recovery time trr
which is due to charge storage in the junctions of the SCR. These excess carriers take some time for
recombination resulting in the gate recovery time or reverse recombination time tgr. Thus, the turn-off
time tq is the sum of the durations for which reverse recovery current flows after the application of reverse
voltage and the time required for the recombination of all excess carriers present. At the end of the turn
off time, a depletion layer develops across and the junction can now withstand the forward voltage. The
turn off time is dependent on the anode current, the magnitude of reverse VG applied and the magnitude
and rate of application of the forward voltage. The turn off time for converter grade SCRs is 50 to 100 μs
t
it
IT
0.9 IT
0
0.1 IT
IG
td tr
iG
0
0.1 IG
t

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10. SCR

and that for inverter grade SCR’s is less than 20 μs. To ensure that SCR has successfully turned off, the
circuit off time tc be greater than SCR turn off time tq.

Figure 6 Turn off characteristics of Thyristor

4. SCR Turn ON Methods
Turn ON method also known as triggering. With anode positive with respect to cathode, a thyristor can be
turned ON by following techniques: Forward voltage triggering, Gate triggering, dv/dt triggering,
temperature triggering and Light triggering.
1. Thermal: During forward blocking, most of the applied voltage is applied across reverse biased
junction J2. This voltage across junction J2 associated with leakage current may raise the temperature
of this junction. With increase in temperature, leakage current through J2 further increases. This
cumulative process may turn ON the thyristor at some high temperature. High temperature triggering
may cause thermal runaway and this is generally avoided.
2. High Voltage Triggering: When breakover voltage VBO across thyristor exceeds than the rated
maximum voltage of the device, thyristor turns ON. At the breakover voltage thyristor anode current
is called as Latching current (IL). Breakover voltage is not normally used as a triggering method,
because it may damage the device. It is destructive turn ON method and should be avoided.
t
t
t1 t2 t3 t4 t5
trr tgr
tq
tc
ia
va
Commutation di/dt
Reverse voltage
due to power circuit
On state voltage
drop across SCR

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3. dv/dt triggering: With forward voltage across anode and cathode of a thyristor, J1, J3 are forward
biased and J2 is reverse biased. This junction J2 acts as capacitor because of the presence of space
charge. As pn junction has capacitance, so larger the junction area larger is the capacitance. If the
charging current is becomes large enough, density of moving current carriers in the device induces
switch ON. This method of triggering is not desirable because high charging current may damage the
thyristor.
4. Gate current triggering: It is simple and efficient method to turn ON thyristor. Refer Figure 7. In
gate triggering method VAK is applied less than VBO. So thyristor is forward biased but not
conducting. When small gate current is applied, thyristor turns on and remain in ON state though gate
voltage is removed. Higher the gate current lower is the forward breakover voltage.

Figure 7 SCR characteristics for different gate trigger currents.

5. Light triggering: In this method, the light particles (photon) are made to strike the reverse biased
junction, which causes an increase in the number of electron-hole pairs and triggering of the thyristor.
For light triggered thyristors, a slot is made in the inner P layer. The light of particular wavelength is
irradiated to this P region through the optical fiber. If the intensity of the light is greater than certain
critical value, the thyristor will turn on. Such thyristor is called as LASCR.
Figure 8 shows the LASCR connection with the load, its structure and equivalent electrical circuit.
The light can be exposed to the junction J2, which is light activated. In place of electrical signal at the
gate, light is used as a source and the device is worked as a light activated SCR (Photo-SCR). Figure
8(B) shows its equivalent circuit with two transistor model and photodiode in parallel with junction
capacitance.

VAK
V
BO
iT
I
L
IH
VBR
V
1 V
2 V
3
V
BO > V
1 > V
2 > V
3
I
G3 > I
G2 > I
G1 > I
G0

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10. SCR




Figure 8 LASCR connected to load (a) showing structure and (b) equivalent electrical circuit.
5. SCR Turn OFF Methods
The process of turning off thyristor is called as Commutation. The basic methods are current commutation
and voltage commutation. A thyristor can be turned on by applying a positive voltage about a volt and or
a current of a few tens of milliamps at the gate cathode terminals. But SCR cannot be turned off via gate
terminal. It will be turned off only after the anode current reduced below holding current naturally or by
forced commutation techniques. Thus thyristor commutation can be classified as Natural or Line
Commutation and Forced Commutation.
P
P
N
N
Light
Source
Anode
Cathode
Gate
Silicon
Pellet
Case and heat sink
R
Load
VDC
(a)
Anode

C1

Q2

Photo
Diode

R

Q1

D1

Cathode

Gate

Gate trigger
input

Load

(b)

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10. SCR

 Natural Commutation: or Line Commutation occurs only in AC circuits. Natural commutation of
thyristor takes place in AC voltage regulators, Phase controlled rectifiers, cycloconverters etc.
 Forced Commutation: This type of commutation is applied to DC circuits.
Line Commutation
In thyristor circuit if the source input voltage is ac, the SCR current goes through a natural zero and
reverse voltage appears across SCR. The device then automatically turns off due to natural behavior of
source voltage. This is known as natural commutation or line commutation. This type of commutation is
applied in ac voltage controllers, phase control rectifiers, and cycloconverters. The timing waveforms of
current and voltage shows SCR for different delay or phase angle are shown in Figure 9.The waveform
for the delay angle α = 0. The delay angle α is defined as the angle between the zero crossing of the input
voltage and instant the thyristor is fired.

Figure 9 – Line Commutated of thyristor – (a) circuit, (b) timing waveforms
R
L V
S
+
-
i
T
VAK
T
1
+ -
(a)
π
(c)
v
iT
vAK
0
0
0
ωt
Vm
IRR
2π

Leakage
Current

trr
π
(b)
v
iT
vAK
0
0
0
ωt
Vm
IRR
2π
Leakage
Current
trr

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Electronic Science
Power Electronics

10. SCR

Forced Commutation
In some thyristor circuits input source voltage is dc. So these thyristors can’t be turn off their own. Some
external circuitry is required to reduce the forward current to zero and turn off thyristor whenever
required. Such external circuits are said to be forced commutated circuits. These forced commutation
techniques are generally used in dc-dc applications and dc-ac applications. The different ways of forced
commutation techniques are Load Side Commutation and Line Side Commutation.
A. Load Side Commutation is further classified as
 Self Commutation.
 Impulse Commutation.
 Resonant Pulse Commutation.
 Complementary Commutation.
 External Pulse Commutation.
Self Commutation:

Figure 10 Thyristor Self Commutation (a) Circuit, (b) Waveforms
In this type of commutation, a thyristor is turned off due to the natural characteristics of the circuit. The
assumption is the capacitor is initially uncharged. When thyristor T1 is switched on, the capacitor charges
through T1 and L. As the capacitor charges in time LCtt
0 , the charging current becomes zero
and thyristor T1 is switched off itself. Once thyristor is fired, there is a delay of t0 seconds before T1 is
turned off and t0 is called the commutation time of the circuit. This method of turning off of thyristor is
called as self commutation and the thyristor is said to be self commutated.
L
C
T1 +
+
-
-
vc
vL
+
-
vs LCV
s
i(t)
ωmt 0
π
2Vs
ωmt 0
vc(t)
π

t0

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10. SCR

Impulse Commutation
The impulse commutated thyristor consists of main thyristor T1 and thyristors T2 and T3 are used in forced
communication circuit. It is assumed that capacitor C is charged up to –VS with polarity shown.

Figure 11- Impulse Commutation (a) Basic circuit, (b) Timing waveforms, and (c) diode in series with
inductor for accelerated recharging of capacitor.


Resonant Pulse Commutation
vc
t 0
Vs
-V0
vT1
t 0
Vs
-V0
tOFF
vS
LOAD

-
v0
Lr
C
T1
+
vc
-
+
Im
T2
T3
Dm
-
+
(a) (b)
-
v0
Lr
C
T1
+
vc
-
+
Im
T2
T3
Dm
vS
-
+
L1
D1
LOAD

(c)

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10. SCR


Figure 12 Thyristor Resonant Pulse Commutation (a) Circuit, (b) Waveforms

C
vc
- +
-
L
T1
+
Im
T2
T3
Dm
vS
i(t) LOAD

(a) i(t)
vc (t)
t1
t1 t2
toff
tc t0
Dm on
t
t
-V0
-V1
Im
0
0
VC
VS
(b)

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10. SCR

Complementary Commutation

Figure 13 Complementary Commutation (a) Circuit, (b) Waveforms

External Pulse Commutation

Figure 14 External Pulse Commutation

-
L
C
T1
+
LOAD

+
-
Im
T2 T3
Dm
VS
2V
-
+
V
-
C
T1
+
+ -
ic
T2
vS
R2 R1
vS
(a)
2V
S/R
-VS
-VS
ic
t 0
0
T/2
vc
vt1
vt2
0
0
-2V
S/R
VS
VS
VS
-VS
(b)

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10. SCR

B. Line Side Commutation


Figure 15 Line Side Commutation

The most important parameters:
Break over voltage
• Holding current
• Turn ON time
• Turn OFF time
• Maximal forward current
• Maximal reverse voltage
• Maximal frequency

6. SCR Protection and Triggering Circuits
A thyristor require a minimum time to spread the current conduction uniformly throughout the junctions.
If the rate of rise of anode current is very fast compared to the spreading velocity of a turn on process, a
localized hot-spot heating will occur due to high current density and the device may fail, as a result of
excessive temperature.

-
C
T1
+
+
-
Im
T2
vS
Dm
T3
L
Lr
LOAD

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10. SCR


di/dt Protection


Figure 16 Thyristor Switching circuit with di/dt limiting inductors

The practical device must be protected against high di/dt. As an example consider the following circuit.
Under steady state operation, Dm conducts when T1 is off. If T1 is fired when Dm is still conducting, di/dt
can be very high and limited by the stray inductance of the circuit.
dv/dt Protection
If switch S1 in fig (a) is closed at t=0, a step voltage will be applied to thyristor T1 and dv/dt may be high
enough to turn on the device. The dv/dt can be limited by connecting capacitor Cs in as shown in fig a.
when T1 is turned on, the discharge current of capacitor is limited by RS as shown in fig b. With RC
circuit known as a snubber circuit, the voltage across thyristor will rise exponentially and the thyristor can
be protected.


Figure 17 dv/dt Protection circuit
VAK
(b)
CS
V
S
+
-
A
K G
S1
RS
(a)
C
S
V
S
+
-
VAK
A
K G
S1
R
Dm
VS
C
T1
i Im
LS
+
-
Load

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10. SCR

Thyristor triggering circuits:
Thyristors are used for high voltage and high power circuits. To control the thyristors various control and
trigger circuits are employed which operates at low power. An isolator circuit is required between an
individual thyristor and gate pulse generator circuit. The isolation can be achieved by using isolation
transformer or optocouplers. Optocoupler could be phototransistor or photo SCR. A typical photo SCR is
shown in Figure 18.

Figure 18 A typical photo SCR

Pulse transformers are often used to couple a gate trigger pulse circuit to high voltage thyristorized circuit
to obtain electrical isolation. Generally, these transformers are either 1:1 (two windings) or 1:1:1 (three
windings) types.
Important transformer design factors are -
a. Primary magnetizing inductance should be high enough so that magnetizing current is low, in
comparison with pulse current during the pulse time.
b. For unilateral pulse generators core saturation must be avoided.
c. Insulation between windings should be high for the applications including transients.
TA
Anode
Cathode
Gate
Output Control
Connections
Input Control
Signal AC or DC
LED LASCR
CASE
A
K
230VAC
Logic
Input
+5V
G
LOAD

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10. SCR

d. Inter-winding capacitance should be low because it may provide path for undesirable stray signals at
high frequencies.
Short Pulse Triggering
If triggering pulse is wide, the size and weight of the pulse transformer required is high. The circuit
shown in Figure 19 is used to convert wide pulse to short pulse. The differentiator converts the input
pulse to short pulse which is further amplified using transistorized circuit as shown in timing diagram.
Diode D1 allows only positive triggering pulse to the base of transistor and diode Dm acts as freewheeling
diode.


Figure 19 Pulse Transformer isolation: Short Pulse

Long Pulse Triggering
If triggering pulse is too short, it may not trigger the thyristor. The circuit shown in Figure 20is used to
convert short pulse to wide pulse. The integrator converts the input pulse to wide pulse which is further
amplified using transistorized circuit as shown in timing diagram. Diode Dm acts as freewheeling diode.

A
0 t
V1
+VCC
G
K
R
Q1
D1
C1
R1
Dm N1 N2
(a)
v1
t 0
V
VA
t 0
t
0
VGK
(b)

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10. SCR


Figure 20 Pulse Transformer Isolation: Long Pulse

Pulse Train Generator
In thyristorized ac circuits with inductive load, load current lags behind the voltage. In phase angle
control, it may happen that when triggering pulse is applied at low de angle, thyristor is reverse biased.
Therefore thyristor will remain off even though gate pulse is applied. To avoid this generally pulse train
is used to trigger the thyristor. Pulse train is generated using blocking oscillator as shown in Figure 21.
When control input voltage is high, pulse train will be generated and when it is low, pulse train will be
terminated.
A
+VCC
G
K
R
Q1
D1
C1
R1
Dm N1 N2
-
+
C
-
+
v1
(a)
v1
t 0
V
VA
t 0
Vm
t
0
Vm
VGK
(b)

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10. SCR


Figure 21Pulse Transformer isolation: Pulse Train Generator
Pulse Train Generator using timer and AND logic
Pulse train can be generated using timer and AND logic as shown in Figure 21. To avoid this generally
pulse train is used to trigger the thyristor. Pulse train is generated using blocking oscillator as shown in
Figure 22. When control input voltage is high, pulse train will be generated and when it is low, pulse train
will be terminated. V1 is the gating input and if it is high it will allow the pulsed to pass further.

Figure 22 Pulse Transformer isolation: Pulse Train Generator using timer and AND logic

+VCC
R
Q1
R1
Dm
v1
G
K
N1 N2
v2
Oscillator
AND
(a)
v1
t 0
V
t
0
VGK
(b)
+VCC
R
Q1
D1
C1
R1
Dm
-
+
v1
N3
G
K
N1 N2
-
+
(a)
v1
t 0
V
t
0
VGK
(b)

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Figure 23



Figure 24
7. Gate Protection Circuits
The gate signals are generally provided through pulse transformers for isolation purpose. The voltage
pulse may be of high voltage/ may be bipolar. Hence it may happen the power dissipation at the gate high.
Pulse may contain high frequency noise. In such case, there may be undesirable turn on of SCR. In order
to avoid such effects, it is necessary to protect the gate. Various gate protection schemes are used which
are discussed as follows.

LOAD

230V / 50Hz

HOT
NEUTRAL
TRIGGER
RG
R1
+
T1
-
XENON
FLASH
LAMP
OPTICAL
FIBER
LOAD

230V / 50Hz

HOT
NEUTRAL
+5V
RS
R1 R2
C
T1

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Table 1 Gate protection circuits
a) Gate cathode Resistance
 Increase dv/dt capability.
 Keep gate damped to assure VDRM
capability.
 Lowers turn off time (tq).
 Increases the latching and holding current.

b) Gate cathode Capacitance
 Increase dv/dt capability.
 Remove high frequency noise.
 Increases turn on and turn off time.
 Lowers gate signal rise time.
 Lowers di/dt capability.

c) Gate cathode Inductance
 Decrease DC gate sensitivity.
 Decreases turn off time (tq).

d) Series Resistor
 Limits the gate current
 Lowers dissipation in gate junction

e) Series Zener:
 Decreases threshold Sensitivity.
 Affects the signal rise time and di/dt rating.
 Isolates the gate.

f) Gate cathode LC Resonant circuit
 Frequency Selection.
 Unless the circuit is damped the positive
and negative gate current may inhibit
conduction or bring about sporadic anode
current.

g) Reverse Diode
 Supply reverse bias in off period.
 Protects gate and gate supply for reverse
transients.
 Lowest turn off time.

h) Capacitive coupling
 Isolates gate circuit DC component.
 In narrow gate pulses and low impedance
sources, the Igt is followed by reverse gate
signals which may inhibit conduction.

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8. Summary
SCR. is silicon controlled rectifier . It is widely used in ac to dc conversion. It has the capability
of controlling the output voltage. The SCR
1. SCR characteristics
2. Two transistor static and transient models
3. SCR turn-on methods
4. SCR turn-off methods
5. SCR protection and Triggering circuits
6. Gate protection circuits
7. Summary