Mike Keitz wrote:
>
> On Sat, 9 Jan 1999 15:15:37 EST Jon Petty <PHXSYS@AOL.COM> writes:
>
> >What's the difference between a NPN transistor and an NPN hexfet?
> >Under what conditions would you use a hexfet?
>
> There's no such thing as a "NPN hexfet".  The polarity is designated by
> the terms "N-channel" or "P-channel" devices.  "Hexfet" is one
> manufacturer's brand name.  More generically they're called "power
> MOSFETs" or just "power FETs".  Sometimes you'll see N-FET or P-FET to
> designate FETs of different polarities.
>
> Both types of devices have 3 terminals.  The Drain, Gate, and Source
> terminals of a FET correspond approximately to the Collector, Base, and
> Emitter terminals of a bipolar.
>
> The big difference between a bipolar and a FET is that the former is
> current-operated and the latter is voltage-operated.  Tunring a bipolar
> on requires about 0.7V but a continuous drive current to the base.  A FET
> requires several volts to turn on, but requires drive current only during
> the transistion time to charge up the ratehr large capacitance between
> the gate and source.  In most cases, much less drive power is required to
> switch a FET.  But for rapid switching, the driver needs to be able to
> supply large surges of current.
> <snipped>

  The thing that makes HexFETs nice IMHO is that they consist of "scads"
of small FETs in parallel, and they incorporate a built-in reverse
protection diode - they get their name from being made in a hexagonal
(honeycomb) pattern.  Internations Rectifier makes those.  I use them in
place of relays, and still add a protection diode quite often "for luck"
<G>  They're a good replacement for relays for most situations (read the
IR specs, get to know the rules, and you should have good luck with
them.)  If you drive one from a CMOS 4000 series gate, you want to put a
current limiting series resistor between 4000 output and the gate of the
FET so as to not blow the CMOS output stage up, and expect the gate to
take a little time to switch (1/RC right?) - for relay type sitations,
not a problem usually.

  Mark, mwillis@nwlink.com