Correct Answer - Option 3 : less than the circuit turn-off time
The turn-off time of SCR:
- Once the thyristor is switched ON (the anode current is above latching current), the gate loses control over it. That means the gate circuit cannot turn off the device.
- For turning off the SCR anode current must fall below the holding current. After the anode current falls to zero, we cannot apply a forward voltage across the device due to the presence of the carrier. So, we must sweep out or recombine these charges to proper turn off of SCR.
- So, the turn-off time of SCR can be defined as the interval between anode current falls to zero and the device regains its forward blocking mode.
Circuit turn-off time:
It is defined as the time during which a reverse voltage is applied across the thyristor during its commutation process.
An SCR is turned off when its turn-off time is less than the circuit turn-off time.
Note:
If the circuit turn-off time is less than the device turn-off time then forward bias voltage gets applied across even before the thyristor could regain its forward blocking capabilities and gets turn on again or the device turn-off is unsuccessful.
