Power electronics digital notes b. Tech III year

Two transistor analogy of SCR 
Basic 
operating principle of SCR
, can be easily understood by the 
two transistor model of SCR
or 
analogy of silicon controlled rectifier, as it is also a combination of P and N layers, shown in figure below 
 
 
Figure: 1.10. Two transistor structure of SCR 
This is a pnpn thyristor. If we bisect it through the dotted line then we will get two transistors i.e. one pnp 
transistor with J
1
and J
2
junctions and another is with J
2
and J
3
junctions as shown in figure below.
Figure: 1. 11. Two transistor structure of SCR 

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12 | 
P a g e
When the transistors are in off state, the relation between the collector current and emitter current is 
shown below
Figure: 1. 12. Two transistors connection of SCR 
Here, I
C
is collector current, I
E
is emitter current, I
CBO
is forward leakage current, α is common base 
forward current gain and relationship between I
C
and I
B
is 
Where, I
B
is base current and β 
is common emitter forward current gain. Let’s for transist
or T
1
this relation holds 
And that for transistor T
2 
Now, by the analysis of two 
transistors model we can get anode current, 
From equation (i) and (ii), we get, 

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13 | 
P a g e
If applied gate current is I
g
then cathode current will be the summation of anode current and gate current 
i.e. 
By substituting this value of I
k
in (iii) we get,
From this relation we can assure that with increasing the value of 
towards unity, 
corresponding anode current will increase. Now the question is how 
increasing. Here is the 
explanation using