Gerhard is right. N-channel fets are certainly usable for applications
where Vsup + Vgsthresh > Vgsmax, but you need to generate a floating
supply which is referenced to the source and rises along with it, OR
you need some type of clamp.

Sean


On 4/29/07, Gerhard Fiedler <lists@connectionbrazil.com> wrote:
> 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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