Definition reference for thyristor turn-off time (tq) From which instant is the turn-off time tq of a thyristor measured in natural or forced commutation analysis?

Introduction / Context:
The turn-off time tq is a key dynamic parameter of SCRs. It is the time required after current extinction before the device can again block a forward voltage without retriggering.

Given Data / Assumptions:

• Standard SCR definition used in datasheets.
• Natural or forced commutation brings anode current to zero.

Concept / Approach:
Internally, charge stored in the device must recombine or be swept out after current goes to zero. tq is measured from the instant the anode current falls to zero (not the voltage zero) until the SCR regains forward blocking capability.

Step-by-Step Solution:
Identify current zero crossing due to commutation.Start timing tq at i_A = 0.End timing when device can withstand specified forward voltage without turning on.

Verification / Alternative check:
Manufacturer switching waveforms annotate tq beginning at the current zero and ending at the recovery of blocking state.

Why Other Options Are Wrong:
Anode voltage zero: Not the reference used; voltage may not be zero at current zero in forced commutation.
Both zero: Unnecessary and often not simultaneous.
Gate pulse removal: SCR is a latching device; gate removal does not define tq.

Common Pitfalls:
Confusing reverse recovery of diodes with tq of SCRs; the definitions are analogous but not identical.

Final Answer:
when anode current becomes zero