I built a ZVS flyback driver a couple months ago, and it worked great and amazed people with the arcs it managed to create. I used 2 IRFP250 MOSFETs and the results were great. One day I was wanting even bigger arcs, and I combined two 12 volt batteries and a 12 volt car jump starter, for 36 volts and the arcs were giant. They worked for a while until the inductor started to get hot. The driver still worked (even with the hot inductor) until I turned it off and back on, when I had to replace the MOSFETs. The arcs stopped oscillating and it seemed like all of the current from the batteries was going to the inductor. The inductor got red hot and melted. I just recently replaced the MOSFETs and everything worked just like before. I started out at 12 volts, and no problem, but when I added 24 volts, the same thing happened as when I added 36 volts previously. The inductor got very, very hot, and when I switched it off and back on, the same thing happened. The MOSFETs are 30 amps, 200 volts! so I don't know why they would have failed with just two 12 volt batteries.
How can I prevent this from happening again and what possibly could have gone wrong? (I really don't think I was drawing 30 amps, but I guess if they failed, it did.)
I want to identify my problem because I don't think it is good to keep replacing my MOSFETs just for them to burn out again!