over 90W of flyback (back EMF) energy? how to dissipate it? Answered
Continuing my work on the flyback driver, I am investigating methods and practices to protect my MOSFETs from overvoltage conditions. I have measured peaks of as much as 350V across a 250V MOSFET while drawing an arc, and it does not matter what I do it seems like I simply cannot limit the back EMF transients to <250V. I am really impressed by the rugged nature of these MOSFETs, although I have popped quite a few of them. Seeing that they are being bombarded with almost 400V transients, it is a wonder they survive as long as they do.
When I added a medium sized reverse biased zener diode in series with a 10 ohm 1 watt resistor, both went up in smoke, especially the resistor! The zener diode survived. I also tried a much larger screw mounted 30V zener diode w/o the resistor and it reached much over 100 degrees C after only a few seconds and I had very little output on the secondary. Judging by how quickly a 10 watt resistor heats up I assume that the back EMF very low impedance and has to be over 90W average!!! I'll need to try to use the integration feature on my scope to measure the average power dissipation accurately. In none of the configurations involving a resistor did I see the voltage clamped any lower than before. :(