At 09:25 PM 9/1/06 -0400, David C Robie wrote:
FETS in parallel are an iffy thing. They gotta be matched perfectly, which is the only reason they work at all, rigorous laboratory matching before assembly. Slight manufacturing differences, slight differences in heat sinking, slight overload, or a spike from load disconnect will wipe the whole bunch of em out in microseconds. Most often noisily with shrapnel, faster than a fuse can blow. Seen a whole lot of those 'boom boom car' audio power amps where this has happened. You don't fix em, you chuck em out.
FETs actually parallel quite nicely IF you know what you are doing and set up layout correctly. Layout is critical. Modern EV controllers parallel FETS extensively. I've worked on one that used 28 in parallel (no lab matching needed). Nicely enough if a FET overheats from higher current it's resistance rises pushing current off to it's lower resistance brethren so they tend to share. You do have to make sure they all turn off and on simultaneously though (layout is critical) and the power paths have to be matched as well (I really should tell you layout is critical).
Yes and FETs often blow to protect the fuse, although I've seen them blow short and slag heat sinks too. Although a lot of times the reason they blow due to inductive spikes on turn off rather than current (did I mention layout was critical?)
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