# Requesting clarity on the di/dt rating of thyristors

simulate this circuit – Schematic created using CircuitLab

I have a general question about thyristors. I know that the di/dt limitation of thyristors is due to high currents flowing through regions of the thyristor that have not yet fully reached a state of full on-state conduction/uniform current density. This can cause localized overheating.

However, I am wondering about this case: What if we preemptively turn-on the thyristor before any anode->cathode current flows. For example, consider that we turn-on the thyristor under zero voltage and zero load current. Then, after waiting a millisecond or so, we ramp up the current at a very high slope, such as at 20,000$$\\frac{\text{A}}{\mu\text{s}}\$$. In this case, will there be any such localized overheating/damage?

My gut says there shouldn't be, since other transistors can handle very high di/dt with no ill-effects. Please let me know your thoughts!

EDIT: For example, consider this circuit. First, we turn-on the SCR1 thyristor. Then we wait. Then, we turn on the MOSFET. Will this cause deleterious effects?

• You cannot turn on thyristor at zero/zero, they have a minimum trip current (for gate) and turn on voltage/current. ST Micro has a series of application notes on applying Thyristors and triacs . Feb 19, 2023 at 17:28
• I am assuming that we preemptively applying a gate->cathode voltage, that forward-biases the PN junction. Then, we reduce the thyristor to the same structure as a PN diode, right? Which can handle high di/dt? I updated the question to show a circuit of what I mean. Feb 19, 2023 at 17:30
• Turn-on at low sine voltage portion does of course limit the current. But then you're stuck at (near) 100% power factor. I'd recommend those STM appnotes for basics. Feb 19, 2023 at 17:36
• What about just the figure shown, with a DC source. You can assume that there is some inductance so the di/dt isn't infinite, but very high. I'm not concerned with turn-off for this example. Would the thyristor be damaged if a gate voltage is preemptively applied before the MOSFET is turned on? Feb 19, 2023 at 17:39
• How are you delivering "simmer" current to the thyristor? The gate alone will not reproduce the charge distribution that MT current has. If it's through another transistor... what's the thyristor doing at all, just use that? Feb 19, 2023 at 19:52