Power electronics digital notes b. Tech III year

V-I Characteristics of a Thyristor 
Figure: 1.6. Circuit diagram for characteristics of SCR
From the circuit diagram above we can see the anode and cathode are connected to the supply voltage 
through the load. Another secondary supply E
s
is applied between the gate and the cathode terminal which 
supplies for the positive gate current when the switch S is closed. On giving the supply we get the 
required V-I characteristics of a thyristor show in the figure below for anode to cathode voltage V
a
and 
anode current I
a
as we can see from the circuit diagram. A detailed study of the characteristics reveal that 
the thyristor has three basic modes of operation, namely the reverse blocking mode, forward blocking 
(off-state) mode and forward conduction (on-state) mode. Which are discussed in great details below, to 
understand the overall characteristics of a thyristor. 
Reverse Blocking Mode of Thyristor 
Initially for the reverse blocking mode of the thyristor, the cathode is made positive with respect to anode 
by supplying voltage E and the gate to cathode supply voltage E
s
is detached initially by keeping switch S 
open. For understanding this mode we should look into the fourth quadrant where the thyristor is reverse 
biased. 

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Figure: 1.7. Reverse blocking mode of SCR
Here Junctions J
1
and J
3
are reverse biased whereas the junction J
2
is forward biased. The behavior of the 
thyristor here is similar to that of two diodes are connected in series with reverse voltage applied across 
them. As a result only a small leakage current 
of the order of a few μAmps flows. This is the reverse 
blocking mode or the off-state, of the thyristor. If the reverse voltage is now increased, then at a particular 
voltage, known as the critical breakdown voltage V
BR
, an avalanche occurs at J
1
and J
3
and the reverse 
current increases rapidly. A large current associated with V
BR
gives rise to more losses in the SCR, which 
results in heating. This may lead to thyristor damage as the junction temperature may exceed its 
permissible temperature rise. It should, therefore, be ensured that maximum working reverse voltage 
across a thyristor does not exceed V
BR
. When reverse voltage applied across a thyristor is less than V
BR
, 
the device offers very high impedance in the reverse direction. The SCR in the reverse blocking mode 
may therefore be treated as open circuit.