Sean Breheny wrote:
>
> Hi all,
>
> I have always heard people say "You can parallel MOSFETs without source
> resistors".
> Recently, I did a quick design for a 20 amp current regulator for some
> NiMH battery testing. Nothing very fancy, I essentially took two
> resistors in series (large, 300W resistors) and put three MOSFETs in
> parallel across one of the two resistors. The idea is that the seres
> combination is close to the right value to cause 20A drain,


Hi Sean, I have built a lot of high current dummy
loads etc very similar to what you are describing.

I really don't think FETs are the answer, typically
when you need linear region and high device dissipation
you are better off with some old fashioned power
transistors.

If I read you right you need up to 25v and 20A, this
is max 500w. My preferred method is to use big TO-3
darlingtons, with the large resistors as emitter resistors.
The resistors do the bulk of the dissipation, and also
do the current balancing. Setting base voltage gives
current control within a few percent. The big darlingtons
will give much better reliability in the linear region
with big dissipations, and easier current sharing. :o)
-Roman

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