Industrial electronics
Explainer
Turn-Off Time: The SCR's Recovery Pause
An SCR at zero current can still re-fire if voltage returns too soon.
Quick check · tap an answer to continue
Lesson notes
Key points
- After current reaches zero, stored charge carriers must recombine before the SCR can block forward voltage again.
- The device turn-off time, written t_q, is the minimum interval from zero current until forward voltage can safely reappear.
- The circuit must provide a reverse-bias interval, the circuit turn-off time t_c, longer than t_q.
- Inverter-grade SCRs have short turn-off times, so they suit high-frequency choppers and inverters better than converter-grade types.
Definitions
- Turn-off time (t_q)
- Minimum time after anode current zero before forward voltage can be reapplied without re-triggering.
Takeaway: Give the SCR reverse bias longer than t_q, or it re-fires.
Quick check: For safe turn-off, the circuit turn-off time must be:
- Equal to zero
- Less than t_q
- Greater than t_q
Stored charget_qt_c > t_qInverter grade
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