mazzilli zvs both igbt failure

Question

I try to build mazzili zvs with 2 IGBT toshiba MG75Q2YS40 75A 1200v. For the each fast diode, I use 2 pieces DSEC60-06 A 600v 30A in series to get 1200v.

for this, I use 2 power source:

• 12 volt for switch the gate.

• 0-80 volt (from variac + bridge diode + capacitor) for the main power

because the gate only use 12v power, I don't use zener. at first, no problem at all. it work normally.

Then I try to add the resonant capacitor by directly (without turn off the zvs) put second capacitor to the first capacitor in parallel, just by touch the second cap to the first cap (no solder,no screw).

when I do this, arc come out from the connection of the second capacitor and suddenly both IGBT failed together. at that time, I only use about 20 volt for the main power.I don't know exactly when the IGBT blown, when the second cap is connected or disconnected. because it's happened very fast.

I try to re-read the schema, but still don't find reason why both IGBT failed together, but the fast diodes is OK. maybe someone know why both IGBT suddenly failed ? thank you.

Answer 1

Adding the 2nd capacitor did disturb the oscillation and caused the Mazilli driver to latch up which killed an igbt that caused a huge voltage across the other igbt, that also killed the igbt at the other side because you have to put the new capacitor at the zero voltage crossing instant of time, which is impossible to do at such high frequencies, so at first kill the aux 12v power then add the 2nd capacitor, best decision would be to switch off both of the power sources.