Thyristor turn off means it changed from ON to OFF state. Once thyristor is oON there is
no role of gate. As we know thyristor can be made turn OFF by reducing the anode
current below the latching current. Here we assume the latching current to be zero
ampere. If a forward voltage is applied across the SCR at the moment it reaches zero then
SCR will not be able to block this forward voltage. Because the charges trapped in the 4-
layer are still favourable for conduction and it may turn on the device. So to avoid such a
case, SCR is reverse biased for some time even if the anode current has reached to zero.
So now the turn off time can be different as the instant anode current becomes zero to the
instant when SCR regains its forward blocking capability.
ð‘¡ð‘ž=ð‘¡ð‘Ÿð‘Ÿ+ð‘¡ð‘žð‘Ÿ
Where, ð‘¡ð‘ž is the turn off time,ð‘¡ð‘Ÿð‘Ÿ is the reverse recovery time, ð‘¡ð‘žð‘Ÿ is the gate recovery time
At ð‘¡1 anode current is zero. Now anode current builds up in reverse direction with same
ð‘‘ð‘£/ð‘‘ð‘¡ slope. This is due to the presence of charge carriers in the four layers. The reverse
recovery current removes the excess carriers from ð½1 and ð½3 between the instants ð‘¡1 andð‘¡3.
At instant ð‘¡3 the end junction ð½1 and ð½3 is recovered. But ð½2 still has trapped charges which
decay due to recombination only so the reverse voltage has to be maintained for some
more time. The time taken for the recombination of charges between ð‘¡3 and ð‘¡4 is called
gate recovery time ð‘¡ð‘žð‘Ÿ. Junction ð½2 recovered and now a forward voltage can be applied
across SCR.
The turn off time is affected by:
1. Junction temperature
2. Magnitude of forward current ð‘‘ð‘–/ð‘‘ð‘¡
during commutation.
Turn off time decreases with the increase of magnitude of reverse applied voltage.