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Re: (ET) 12V winch - PWM controller from scooter
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?)
From the Divided by a Common Language File (Edited to protect the guilty)
ME - "I'd like to get Price and delivery for connector Part # XXXXX"
Dist./Rep - "$X.XX Lead time 37 days"
ME - "Anything we can do about lead time? 37 days seems a bit high."
Dist./Rep - "that is the lead time given because our stock is live.... we
currently have stock."