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Old 01-26-2011, 08:56 AM   #4258 (permalink)
DJBecker
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Quote:
Originally Posted by MPaulHolmes View Post
For very high current applications, the B- bus bar's inductance starts to be important. So, during switching, some of the mosfets would turn on at different times, because of the potential difference from one end of B- to the other.
You avoid that situation with MOSFETs by having a gate driver voltage significantly above the threshold, and using per-gate resistors. The gate signal starts looking like a current source, and is less sensitive to minor variations at the source pin. If the MOSFETs are similar, they all switch about the same time because they are synchronized by the Miller knee.

I can see the problem you are talking about being much more of a issue with bipolar transistors, where even a minor difference in B-E voltage makes a big difference in collector current. You can't match/equalize base current with a series resistor because that kills your gain. The usual solution is a low-ohm resistor at the emitter. (I say "low ohm" rather than "small", because that fractional-ohm resistor usually generates a lot of heat.)
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