Two Transistor Model of SCR.pdf

8/7/23, 11:19 AM Two Transistor Model of SCR
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Two Transistor Model of SCR or
Thyristor
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This article describes two transistor model of SCR or Thyristor. A
thyristor, also known as Silicon Controlled Rectifier (SCR) is a three-
terminal power electronic device used in various electronic applications
like rectifiers, SMPS, etc. as a switching device. It has three terminals –
Anode, Cathode, and Gate.
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An SCR or Thyristor is similar to that a diode except for the fact that it
has an additional control pin. Unlike a diode, an SCR needs positive
voltage across the gate terminal as well in addition to the anode and
cathode being connected to the positive and negative terminal of a
voltage source respectively.
Thus, the output from an SCR can be controlled by simply controlling
the gate voltage at an appropriate instant of time without having to
connect and disconnect the supply. This gives an added advantage to
the SCR over the diode in applications where voltage control is
required.
For a rectifier using diodes, the output will be there for the complete
positive half cycle and we can’t control it. But with SCR, we can decide
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when the gate voltage has to be applied and therefore we can give the
gate pulse at that instant to get the desired output. This increases the
efficiency of the entire system and allows smoother operations in the
case of motor drive applications.
Two transistors model of SCR
Unlike other power electronic devices, an SCR needs a positive pulse
across the gate terminal only once to activate. Once the gate current is
equal to or above the latching current, the SCR activates and the gate
voltage can be turned off. The SCR doesn’t need a constant voltage

across the gate terminal. This is because of its two-transistor mode of
working. Let us understand it in detail.
Two Transistor Model of an SCR
An SCR can be shown as two back-to-back connected bipolar junction
transistors as shown above in the picture. Both the transistors are PNP
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and NPN type respectively. Here T denotes the PNP type while the
NPN type transistor has been denoted by T . The base terminal of the
T transistor is connected to the collector terminal of the T transistor.
The collector terminal of the T transistor is connected to the base of
the T transistor. This is the back-to-back connected transistor model of
an SCR. The junction G in the image shown denotes the GATE terminal
of this SCR while A and K denotes the ANODE and CATHODE terminals
respectively.
It must be noted that for the NPN-type transistor to conduct, a positive
voltage is required at the base whereas a PNP-type transistor conducts
1
2
1 2
1
2

when there is a falling pulse or a negative voltage is applied at its base
terminal.
Now, in the above-given circuit, switch S is closed at an appropriate
instant time. This causes a positive pulse to appear at the base terminal
of the transistor T and it starts conducting. Assuming there is no
voltage drop between the collector and emitter of the transistor T , we
can say that it starts working like a short circuit. This causes the
negative terminal voltage of the battery to appear at the base of the
PNP transistor T .
As an NPN transistor needs a negative voltage at its base to start
conduction, the transistor T also starts conduction. Therefore both T
and T behave as a short-circuited path (assuming negligible voltage
drop) and thus a load current begins to flow from the supply V through
the load represented by resistance R. This is what constitutes the two-
transistor model of SCR or Thyristor.
Transistor latching in SCR
2
2
1
1 1
2

Latching of transistors in an SCR causes one transistor to drive the
other one and therefore there is no need for any external base voltage
supply in the case of the transistors or gate voltage supply in the case
of the SCR. Let’s understand the latching process.
In the above-given model, the switch S when connected, causes a
positive voltage to appear at the base terminal of the transistor T . As a
result, T starts conduction and the negative terminal voltage of the
gate supply appears at the base terminal of the transistor T . This
causes the transistor T to begin conduction as well.
2
2
1
1

Now as T is conducting, it causes the positive voltage from the source
‘V’ to appear at node G (assuming a negligible voltage drop across T ).
As the base of the NPN-type transistor T is in common with node G,
this positive voltage appears at the base of the respective transistor.
This causes the transistor T to continue conduction.
As T is conducting, it causes the negative terminal voltage of the
source to appear at its collector. The collector of the transistor T is
connected to the base of the PNP transistor T . Thus the transistor T
also keeps on conducting. In this way, both the transistors keep on
running each other which is known as the latch mode of working. That’s
why there is no role of the gate supply after turning on the latch
process within an SCR.
Once latched, both the transistors connected back-to-back
(representing an SCR) keeps on working and need no gate supply. This
makes SCR a unique power electronic device where you need a gate
supply only during the start and not for continued operation.
1
1
2
2
2
2
1 1

Here is an animation representing the two-transistor model of an SCR
and its working. Notice how the push button connected to the gate
terminal or the common base terminal is required only to start the
conduction process. Once that happens, there is no control of the push
button over the entire process.

Latching in an SCR using the two-transistor model animation
This is exactly what happens in a Silicon Controlled Rectifier (SCR). To
stop it from conducting, the current through the SCR has to be reduced

below the latching current so that the latching of the transistors could
be stopped.
Mathematical Analysis of the two-transistor
model
The thyristor when bisected from the middle, has two parts – an NPN-
type transistor and a PNP-type transistor connected back-to-back as
we discussed above. The picture given below is that of a thyristor
bisected from the middle.

Two-transistor model of SCR
Terminal A denotes Anode, K is the cathode and G is the gate while I ,
I , and I denote the anode current, cathode current, and gate current
respectively.
a
g k

Now, according to the transistor leakage current equation, the collector
current is given as
Therefore for the first transistor T1, we can say,
As for the first transistor, the emitter current equals to the anode
current I .
a

For the second transistor, T2
As the emitter current in the case of the second transistor is equal to
the cathode current of the second transistor I .
By applying KCL in the circuit, we can say,
Again, by applying KCL, we can say
Therefore, Ia can be given as
k

Hence,
Therefore, it is clear that with the increasing value of ( α + α ), it
approaches the value 1 (unity) and the anode current keeps on rising.
This rise in the value of ( α + α ) is the result of a cumulative increase
in the value α and α which begins with the supply of the gate current
I in the beginning.
This buildup of α followed by α again followed by α keeps ongoing
and the anode current rises. This buildup process is called the latching
of the SCR or Thyristor. That’s why the gate current is required only to
start the transistor’s latching process, which sets on as the anode
1 2
1 2
1 2
g
1 2 1

current is equal to the latching current. This can also be seen in the
characteristic curve of the SCR.
Characteristic curve of SCR

As you can see, the gate pulse (I ) is applied at a certain instant causing
the current to reach the latching current value. As the conduction
begins in the forward conducting region, I is no longer required and
hence it is equal to zero while the conduction goes on itself due to
latching.
It is also clear from the characteristic curve that the latching current
must be greater than the holding current for the forward conduction to
begin.
Read Next
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& Applications
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g
g

Silicon Controlled Rectifier (SCR) | Construction & Working
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1 thought on “Two Transistor Model of SCR or Thyristor”
Julius
April 6, 2023 at 10:11 am
The explanation I got for the working SRC was very extensive
and helpful for me. I love all these concepts explained here. I
wish I could reason and become like you people.
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