> Other things being equal, N-fets are better (lower RDSon) than P-Fets,
> but they are a PITA to use on the high side, because their gates must be
> ~10V *above* the uper rail to switch on properly

Depending on the application (ie power being switched -> gate drive
needed) a voltage doubler made with a 555 will get you 24V for the
high side. It should be able to supply enough current to turn the FETs
off/on quickly and hard, and a 555 has fairly good current drive. Still
short of the (at least) 1A for big FETs though

Good hard and fast gate drive (both on and off) is the key

You could make a little buck SMPS with the PIC's PWM module (if
you're using a PIC that is), or any available/simple oscillator

You should also consider dead-time. This is built in to a good driver IC
and prevents both the high- and low-side FET on the same side being on
at the same time.

If they do happen to be on at the same time (even briefly, as one turns
on whilst the other is still turning off) at best it means excess=20
dissipation
and at worst destruction. But that largely depends on which is tougher,
the FETs or the power supply

The gates should have protection against EMF, hash etc from the motor.
A 15V zener paralleled with 270 ohm from gate to source is usually
sufficient. A fast Schottky diode across drain (cathode) - source (anode).
A reverse-series (back-to-back) 15V zeners (suitably rated) across the
motor terminals with 0.1uF ceramics from terminal to grounded case
will suppress a lot of hash at its source. If noise is really bad you may
have to add a couple of ultrafast diodes too.

FETs are very good at switching power but they can be quite frail
electrically.

Joe=20



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