Power electronics digital notes b. Tech III year

Figure: 1.8. V- I characteristics of SCR 

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Forward Blocking Mode
Now considering the anode is positive with respect to the cathode, with gate 
kept in open condition. The thyristor is now said to be forward biased as shown the figure below.
Figure: 1.9. Forward connection of SCR 
As we can see the junctions J
1
and J
3 
are now forward biased but junction J
2
goes into reverse biased 
condition. In this particular mode, a small current, called forward leakage current is allowed to flow 
initially as shown in the diagram for characteristics of thyristor. Now, if we keep on increasing the 
forward biased anode to cathode voltage. 
In this particular mode, the thyristor conducts currents from anode to cathode with a very small voltage 
drop across it. A thyristor is brought from forward blocking mode to forward conduction mode by turning 
it on by exceeding the forward break over voltage or by applying a gate pulse between gate and cathode. 
In this mode, thyristor is in on-state and behaves like a closed switch. Voltage drop across thyristor in the 
on state is of the order of 1 to 2 V depending beyond a certain point, then the reverse biased junction J
2
will have an avalanche breakdown at a voltage called forward break over voltage V
B0
of the thyristor. But, 
if we keep the forward voltage less than V
BO
, we can see from the characteristics of thyristor, that the 
device offers high impedance. Thus even here the thyristor operates as an open switch during the forward 
blocking mode. 
Forward Conduction Mode 
When the anode to cathode forward voltage is increased, with gate circuit open, the reverse junction J
2
will have an avalanche breakdown at forward break over voltage V
BO
leading to thyristor turn on. Once 
the thyristor is turned on we can see from the diagram for characteristics of thyristor, that the point M at 
once shifts toward N and then anywhere between N and K. Here NK represents the forward conduction 
mode of the thyristor. In this mode of operation, the thyristor conducts maximum current with minimum 

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voltage drop, this is known as the forward conduction forward conduction or the turn on mode of the 
thyristor.