Thyristor switching behavior in practice In an ordinary silicon controlled rectifier (SCR), how do the typical turn-on time and turn-off (recovery) time compare?

Introduction / Context:
This question tests understanding of dynamic switching characteristics of a silicon controlled rectifier (SCR). In power electronics, the relative magnitudes of turn-on time (the time to establish conduction after a gate trigger) and turn-off time (the time required to regain forward blocking capability after current goes to zero) determine allowable firing schemes, commutation methods, and maximum operating frequency.

Given Data / Assumptions:

• Ordinary line-commutated SCR (not a fast-switching GTO or IGCT).
• Conduction in the forward direction after proper gate triggering.
• Operation at line or low kHz frequencies typical of rectifiers and AC regulators.

Concept / Approach:
Turn-on involves carrier injection and spreading across the SCR structure. With adequate gate current, this process is rapid. Turn-off, however, requires removing stored charge from the device’s junctions and allowing minority carriers to recombine or be swept out. This recovery takes longer than turn-on, which is why ordinary SCRs are used mostly in line-commutated (natural commutation) systems where current naturally crosses zero each half-cycle.

Step-by-Step Solution:
Define t_on: time from gate pulse to full conduction.Define t_q (turn-off time): interval after current zero before the SCR can block forward voltage again.Physical reasoning: carrier removal and re-establishment of depletion regions take longer than carrier injection → t_q > t_on.Conclusion: turn-off time exceeds turn-on time.

Verification / Alternative check:
Typical data sheets show t_on in microseconds, whereas t_q is generally several tens of microseconds (or more) for standard devices, confirming the relationship.

Why Other Options Are Wrong:

• Equal times: not supported by device physics or data sheets.
• Turn-on longer than turn-off: contradicts typical values.
• “May be equal/less/more”: too vague and not representative of standard SCRs.
• Turn-off time equals zero: impossible; recovery is always needed.

Common Pitfalls:

• Confusing fast special devices (e.g., GTOs) with ordinary SCRs.
• Ignoring the impact of junction temperature and current on recovery time.

Final Answer:
turn off time is more than turn on time