Why did my drive blow up?

My first attempt at building a high-current H-bridge failed miserably.
I was trying to drive a pair of windshield wiper motors on a robot with
a new H-bridge that I built using discrete MOSFETs (HRF3205) with
HIP4081A drivers. I got the design from a Nuts & Volts article. One of
the MOSFETs literally exploded. I don't think it was a build problem
since my friend built a drive from the same design and tested it
completely with a small motor. Two of his MOSFETs exploded when
connected to the big motors.

The only thing I can think that would have caused this is flyback
voltages that the MOSFET zener diodes did not suppress. I thought that
the zener diodes built into the MOSFETs would protect the circuit from
flyback voltages (so did the author of the Nuts & Volts article). Maybe
these built-in diodes are only effective for small loads.

Am I right about the cause for explosion? If I need to add diodes for
flyback suppression, how should I size them? I'm building this drive to
handle 20A.

Thanks,

BRW

.

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