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This is a purely theoretical question. The values in the circuit below do not matter, and I understand the circuit is completely useless.

Consider if SCR1 and M1 are both turned on so that current is flowing through the resistor. If you turn off the MOSFET, will the thyristor turn off? If so, how long will it take, assuming it has a typical turn-off time of \$ \text{T}_q\$. Thanks a lot for the help!

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ The SCR will turn-off when the SCR current drops below the "hold current" specified in the datasheet. \$\endgroup\$
    – G36
    Jan 5, 2021 at 20:49
  • \$\begingroup\$ Thanks for the answer, @G36 . What is the importance, then, of the turn-off time? \$\endgroup\$ Jan 5, 2021 at 20:50

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If turning the mosfet off causesthe current through the scr to drop below its holding current, then yes, the scr will turn off. The mosfet could also be in parallel with the scr and when turning the mosfet on would cause the current through the scr to drop below the holding current. How long would it take? Maybe a couple of microseconds. Depends on the specs of the scr.[edit] according to the Motorola datasheet for the C106 scr, turn off time is around 40us.

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  • \$\begingroup\$ Interesting. Thank you very much! \$\endgroup\$ Jan 5, 2021 at 22:39
  • \$\begingroup\$ I have a question about your answer, actually, if you have a moment. Do you have any idea how the blocking voltages would be partitioned at the moment of turn-off? To begin, the MOSFET would have the entire DC voltage across it, but eventually the thyristor would also be able to block forward voltages again (after 40us). Would the voltage across the thyristor exponentially increase until it reaches, say, half the DC voltage (assuming both switches have equivalent off-state resistance)? \$\endgroup\$ Jan 5, 2021 at 23:18
  • \$\begingroup\$ SCRs tend to be leaky, so my guess is the MOSFET would have the higher voltage across it. The transition would depend of the capacitances of the two devices. Not something I'd be able to give a concrete answer to, I'm sorry. \$\endgroup\$
    – Kartman
    Jan 6, 2021 at 5:36
  • \$\begingroup\$ Thanks again. Consider, if you would, that there are snubber circuits in place such that the voltage is shared equally across the MOSFET and the thyristor when both switches are off. Do you have any idea how the voltages would be partitioned? After /$ T_q /$, I know they will be shared equally. But what about before the 40us elapses? \$\endgroup\$ Jan 6, 2021 at 17:05
  • \$\begingroup\$ You’d either have to measure or simulate using actual components. In reality, you’d just use a mosfet or IGBT. \$\endgroup\$
    – Kartman
    Jan 6, 2021 at 21:27

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