> In my previous message, I wrote:
>
> > Bipolar transistors have negative temperature coefficient,
> > i.e. resistance goes down as they heat up. For two paralleled outputs,
> > the one with lower on-resistance will take more current, and will heat
> > up more, resistance becomes even lower, etc. The positive feedback
> > quickly causes current imbalance.
> >
> > MOSFETs are different, and have positive temperature coefficient, so
> > from this point of view, their outputs can be paralleled. However,
> > there can be other problems with paralleling MOSFETs.
>
> I just realized I switched the words "positive" and "negative" - sorry
> if it caused any confusion.

I think it was right the first time?

MOSFETs have a positive TC -> their resistance does goes up with temperature.

when paralleled, the FET getting hot increases its resistance, decreasing the
current it carries, tending to cool it down, ...

So even FETs without matched parameters (Vth, gm) will share reasonably well.


BJT's have a Vbe going down as temp increases and beta increasing.  Between
BJTs, the hotter transistor increases in beta, which decreases Vce, causing it
to carry more current.  The effect is not linear, and the power dissipation
goes up, causing it to get still hotter ...

The explaination above is generally true.  There are other factors which can
cause either type of transistor to behave in the opposite manner.


Regards


Jonathan King




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Jonathan King                        |   king@uicc.com
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