Theoretically, unlike the JFET or IGFET, MOSFET's, whether N or P channel
can be driven with either + or - Vg. That would imply bidirectional
application. Also theoretically, it should be safer to do so with
enhancement mode devices (that's the description I was looking for earlier).

I have designed also for Bipolar NPNs then slipped in N MOSFETs, since
another nice advantage is that you don't have to worry about gate leakage.
This has been a nice way to play with LED display brightness when needed,
due to the low Rds MOSFETs of today.

I always try to use N channel & P channels appropriately, but have seen all
N channel bridges and am itching to give this a try....Today it seems to be
a matter of how much complexity you want to make your gate
circuitry....which seems to be a matter of how hardened or protected it
needs to be. There are so many switching device types available that once
these decisions are made, it's fairly easy to read data sheets a make a
choice.

Chris

> > > Of course any MOSFET can be used as a high or low side
> switch, but some
> > > devices can be used bidirectionally, others not so well.
> Now let's see if I
> > > can remember which is which...
> It's trivial (use a P-channel MOSFET) when the voltage is Vdd, but for
> higher voltages (12V or 300V or 600V), you run into problems
> - for example,
> there are no common P-channel MOSFETs rated for >500VDC
> AFAIK. Also IGBTs,
> which come into their own at high voltages are only available
> in "N" type.
> To use N-channel MOSFETs (say) as a high side switch you need
> a drive voltage
> that is above the +ve rail.

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