> Does anybody have any good information about high voltage H-bridges?
(about
> 200V @ 4A)
>
> I was thinking about IGBT but I'm a little fuzzy about how to drive the
high
> side of the bridge.

OK - now I read the original question :-).
SUMMARY TO DATE:

At the speed you have since suggested (20 Hz to 200 Hz) FETS are probably an
easier solution. They are available, and cheap - a TO220 package will easily
handle the dissipation.

Bob Axtell has suggested using P Channel FETS for the high side drivers,
driven by optoisolators. P channel FETs cost more for the same rating and
are getting rare at 200 volts BUT they do remove the need for a high side
driver voltage above the positive rail.

People have suggested high side supply generators using either a discrete or
integrated solution. This is a common way to generate a high side drive
voltage. Only one supply is needed for two high side drives PROVIDED that
the supply is isolated by drive resistors (and possibly diodes) as
requisite. Such supplies use the cycling of the main switches as the source
of the AC drive voltage to fill and then discharge the 'pump" capacitors. IF
your main switch runs very slowly or stops the high side drives can run out
of drive. At the speed range you  have mentioned you should be easily able
to design the driver to have enough drive at the lowest frequency used.

It was noted that if you are using PWM then you can turn the high side
FET/IGBT on and PWM only the lower bridge element. This allows slow speed
drive for the high side (more suitable for optos) and higher speed PWM for
the low side.

ALSO

You can make simple power supplies which produce the high side drive voltage
independent of the main switching and this will then work as slowly as you
wish (even "DC"). A 555 has often been used for this. Custom ICs are
available.

The high side drive voltage can be referenced to the high side positive rail
(rather than the high side emitters) as long as the saturation voltage is
not extremely variable and as long as there is suitable protection on the
gates to stop them being damaged when first driven. eg imagine a high side
drive +10v above the high side rail. Imagine a 200v high side voltage. If
using eg optos, at first turn on  you have about +210v Vge. As long as there
is a gate drive resistor and a ge protection zener the device will soon
reduce Vge to below zener voltage as it turns on. Having the high side drive
supply float relative to the emitter removes this need BUT can make
replenishing the drive supply more challenging (depending how you do it).

For "playing" a 9 volt battery makes a superb floating supply :-).


regards

        Russell McMahon

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