Anyway, I got the flyback transformer to spark today. I was running a 50/50 square wave into a 500V FET off a 12V supply. It sparked for two tenths of a second and then my digital isolator between the PIC side and the flyback driver side blew. I didn't know what all else blew, so I replaced everything from the isolator on up (isolator, driver FET, output FET). The kickbacks must be a lot more than with the ignition coils, with those I only ever saw 200V peaks when I switched the coils off and I didn't even need protection diodes on my transistors. This flyback did quite a number on my ckt.