With non- complementary bridges you have a problem. Therefore it is nice
to either buy insulated VFETS (modified TO220 case - several types), or
modify the scheme (if necessary) to use a single power supply for
everything in a complementary bridge and then you have two big heatsinks,
one for transistors with grounded case and one for those with case at Vcc,
the latter being well insulated on heat resistant insulators. The
transistors themselves need no insulation in this case, assuming that you
know where ground currents go.

As to sizes etc, it depends on what you do with the drivers. Know that a
simple fan will reduce the heatsink requirements two to three times.

I find a thermistor watchdog that latches a relay using a thyristor
invaluable during smoke let-out phases. It is set to trip the thyristor at
100 degrees C (roughly), uses a 358 opamp, and is powered by the circuit
it is to keep the smoke in. I leave it in the circuit until I am sure that
the smoke does not try to escape.

One last item: I have learned the hard way that with power electronics you
*have* to make eyelets on each component and each and every wire (even low
power) connected 'in air'. I am always pleasently surprised that the
circuit can reach the melting temperature of solder and not fall apart or
drop high voltage/high current wires onto something. Form tight closed
eyelets and put them over the pins (f.ex. of TO3 cases), and make sure
that they are not under mechanical tension so they cannot spring off and
cause solder to fly about if it comes to it.

Peter

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