William ChopsWestfield wrote:

>> initially, when the source of the high side fet is at 0V, applying
>> Vsupply+12 to the gate would cause Vsupply+12 of gate-source voltage on
>> the high-side fets.
> 
> Does that mean that N-channel mosfets are next to useless for supplies
> where Vsup + Vgsthresh > Vgsmax ?  Or does it mean that the high-side
> N-channel mosfet controllers are a lot more complicated than I thought ?

Probably the second (I don't know what you thought, so "probably" :)

This can be done in one of two ways (AFAIK). One is to generate an isolated
voltage that is referenced to the high-side MOSFET's source and is
guaranteed to be less than Vgsmax (or made so by using a zener); the
capacitor bootstrap controllers (e.g. HIP4081) and the gate transformer
circuits do this. Or you generate an auxiliary voltage that's guaranteed to
be more than (Vsup + Vgsrequired) and reduce it to less than Vgsmax with a
zener clamp to each MOSFET source. (The zener clamp to source is part of
any serious driver circuit I've seen, but of course in the latter case it
has to do more work.)

Many high-side driver applications work with supply voltages up in the 100V
or more range (industrial DC motors often have supply voltages of 90V or
180V). In these cases the auxiliary voltage method doesn't work well at all
:)

Gerhard

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