Power electronics digital notes b. Tech III year

two transistor model of SCR
. At the first stage when we apply a gate current I
g
, it acts 
as base current of T
2
transistor i.e. I
B2
= I
g
and emitter current i.e. I
k
= I
g
of the T,
2
transistor. Hence 
establishment of the emitter current gives rise α
2
as 
Presence of base current will generate collector current as 
This I
C2
is nothing but base current I
B1
of transistor T,
1
, which will cause the flow of collector current, 
I
C1
and I
B1
lead to increase I
C1
as 
And hence, α
1
increases. Now, new base current of T
2
is
,
This will lead to increase emitter current 

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14 | 
P a g e
and as a result α
2
also increases and this further increases 
.
As
,
α
1
again increases. This continuous positive feedback effect increases 
towards unity and 
anode current tends to flow at a very large value. The value current then can only be controlled by 
external 
resistance
of the circuit. 
Turn on methods of SCR 
The turning on Process of the SCR is known as Triggering. In other words, turning the SCR from 
Forward-Blocking 
state 
to 
Forward-Conduction 
state 
is 
known 
as 
Triggering. 
The 
various methods of SCR triggering are discussed here. 
The various SCR triggering methods are 

Forward Voltage Triggering 

Thermal or Temperature Triggering 

Radiation or Light triggering 

dv/dt Triggering 

Gate Triggering 
(a) Forward Voltage Triggering:- 

In this mode, an additional forward voltage is applied between anode and cathode. 

When the anode terminal is positive with respect to cathode (V
AK
), Junction J1 and J3 is forward 
biased and junction J2 is reverse biased. 

No current flow due to depletion region in J2 is reverse biased (except leakage current). 

As V
AK
is further increased, at a voltage V
BO
(Forward Break Over Voltage) the junction J2 
undergoes avalanche breakdown and so a current flows and the device tends to turn ON(even when 
gate is open)