Alan B wrote: ↑
Nov 20, 2017 6:40 pm
Driving PWM into FETs requires more drive, and FETs in parallel may not share current very well. It helps to match them, but in any case you cannot assume the rating doubles with the second FET. There are guidelines for this, as I recall assume about 50% additional for the second FET. Again, counterfeit FETs will fare far worse as they are likely even less matched.
I definitely agree on the counterfeit parts.
However, well matched FETs (same part #/mfr, same run) share pretty well. MOSFETs have a positive temperature coefficient*; as they get hotter, their resistance goes up. Therefore the FET with the highest load tends to shed load as it heats up, and the cooler FET takes more of the load. This is a mechanism that allows paralelling MOSFETs with relatively good sharing, which is why you often see paralelled FETs in controllers.
This is in contrast to bipolars, which have a negative temperature coefficient - the hottest device takes more and more current as it heats up. This means you need ballast resistors to "force" a certain amount of sharing, but this (of course) increases losses.
(* - MOSFETs have requirements to maintain this positive coefficient, like sufficient gate drive.)