SMPS failure, the Gates of the MOSFETs exploded?????? Answered
I've just build an SMPS using a TL494 chip using the power circuit from this circuit from uzzors2k, it's just a standard half bridge in my case using IRFP460's. The MOSFETs are driven through a small transformer that itself is driven with a H-bridge.
Yesterday I was trying it's first mains power test slowly ramping up the voltage with a variac when at about 180VAC i could hear some fizzling from the circuit and at 200V the GATE's of the MOSFET's exploded violently, at close inspection there was no shorts or flashovers and the gates were the only thing that was damaged at all.
One thing i noticed later was the output waveform of the power transformer, unlike the signal waveform it didnt exhibit the same 96% duty cycle but a 100% with a lot of ringing on the rising peaks of the square wave, i know this can probably be fixed by reducing the duty cycle but i cant see how it could've blew up the gates.
I've included some pictures so you can see what i mean by explode, as you can see on the left Fet the gate pin has completely blown off and the copper has melted slightly and the right image the same happened but i cracked the Fet open with a hammer to see if there were any clues inside.
I really dont have a clue why this happened and would appreciate some help, thanks in advance.