- Joined Feb 22, 2015
First of all, I fully acknowledge that I did something stupid, so I'm not looking for "user error" as the root cause.
I was bringing up the circuit on the left. It's part of a buck-boost converter, but I've simplified the schematic for illustration. Q2/D2 (and Q1/D1) are actually one part, an FDPF085N10, an N-channel MOSFET with body diode. Vdd is 24 volts. When a button is pressed, a single 10-volt, 10 uS positive-going pulse is driven into the MOSFET by a driver (not shown).
I tested the MOSFET before using it, and it was good. Then I soldered it onto the PC board, turned on the power, and hit the button a few times, expecting the voltage to jump at least a little. Nothing happened. In fact, I found that my 24-volt supply had gone into shutdown.
Ok, my fault, I had inserted the MOSFET backwards, and in fact, gate and source were swapped, resulting in the circuit on the right. I removed the MOSFET and found gate, source, and drain fused together. I replaced the MOSFET with one oriented in the correct direction, and everything worked fine.
So the question: in technical terms, what was the mechanism of the failure?