I thought that would still require a fairly large heatsink?
Nope. A fully-turned-on MOSFET has very little voltage drop so hardly any Watts are dissipated.
But you don't know how many amps are running through them? Even at 3A these would be ~90C above ambient, unless I'm mistaken?
I think the four 19N20C mosfets are being used for input rectification (see link), I thought that would still require a fairly large heatsink?
http://www.thetaeng.com/FETBridge.htm
I think Dave swapped the input and output connectors.
Osram PT VIP 03 MID seems to be a HID ballast optimized for projection lamps. It even has a UART connection on the control board for controlling the lamp! The transformer next to the output is most likely the ignition transformer (insulated wire, multiple sections on the bobbin). The H-bridge probably generates a low frequency squarewave for the lamp. So it has almost no switching losses and only moderate conduction losses. The mosfet on the heatsink instead is probably used as buck converter for generating a constant current. Because it converts the rectified mains voltage into a lower voltage, it has quite high switching losses.
Yeah that sort of makes sense, I was going off the letter which said 60VAC in and 300VDC out (Are there lights that require 300VDC?).
I thought that transformer with the split bobbin was actually a common mode choke to filter the input AC.
If the side with the heatsink is the input, where is the rectifier? Unless the boards DC input AC output?
Anyway, still not closer to why the mosfets died, maybe the lamps just pull too much current?
![Confused :-//](https://www.eevblog.com/forum/Smileys/default/confused0024.gif.pagespeed.ce.5xOqKkq0Co.gif)