Electrical – Trouble shooting ZVS driver

flybackhigh voltagezvs

I've posted a few question on this forum about ZVF drivers and you all have been a big help and I'm hoping you can help me again. Forgive me if this is a bit long, I just want to be through.

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Above is the circuit and components I've used. I deviated from the usual online diagrams so I could run this at a much higher voltage than the usual 12-50v. I have a variac, full wave rectifier and large capacitor bank to hook all this stuff up to. When I work on this I connect my DDM to the cap bank to make sure I know what voltage I'm at. Also, at 100v input I don't draw more than 12amps from the variac. 

My first test run worked perfectly. In theory I should be able to turn this up to 150v, but I start at about 20v and stop at 100v because the new parts are kind of expensive and it really adds up when things break. The arcs were starting at about 2" and could be drawn 8-10" apart. So 100V input was good enough for me, tho I am very curious about 150v input. The MOSFET's broke for some unknown reason when I hooked it up to my 8 stage voltage multiplier. I figure the big static field messed them up.

I rebuild the circuit, hook it up and turn it on at 30v. I can hear the high pitched whine of the transformer. All seems good until I go to test it by drawing an arc from the flybacks secondary. The instant I draw an arc it shorts out. Another broken MOSFET. Good thing I have 2 more sets. I rebuild with all new components. Turn it on to 30V, hear the whining, go to test it with an arc, and instantly shorts out again. 

So I pull everything apart from both runs and test each individual component. In each run the only thing that broke was one MOSFET. I look at all the wires to make sure nothing is crossed and it all looks good. I even take my old circuit that uses IRFP260N transistors and test at 30V. Works just fine and I can draw arcs with no problems. Does any one know why my MOSFETS keep failing? I'm doing everything exactly the same as my first test run, I don't know why I'm getting different results. Im taking to working two from the failed runs to rebuild it again but im worried they will fail again and I dont know why. If they do I only have one set left and they would be doomed. These MOSFET's are about $20 a piece so I really want some input before I start tossing money away.

Also, can any one tell me the relationship between core gap and power draw?
Seems the higher voltage I go, the more current it draws and the larger the core gap needs to be. If I increase the core gap it also increases frequency. If I don't increase the core gap then it seems like maybe the core saturates and I get very angry chattering sounds when I turn the voltage up higher. If I press on I blow a MOSFET. 

Best Answer

Im a little late here but I know if you turn on the circuit with a power supply with a slow rise time then the MOSFETs wont start oscillating and they both stay on and short out. I always start my powersupply, then hook it up to the ZVS because it locked up every time I turned it on with the power supply switch.