Lateral MOSFET torture...

[analog_guy]

It should not be surprising that one can destroy a lateral FET with an overcurrent condition. Even if the FETs have external protection zeners, they will not clamp at a sufficiently low voltage to prevent overcurrents from potentially destroying the FETs. Typically the zeners are selected to prevent gate breakdown, and this voltage limit is around 12-14V, more than enough to drive the FETs past their SOA.

The real question is how fast any overcurrent protection circuit needs to respond to prevent damage to the FET. Over short intervals an audio amp power supply can provide tens to hundreds of joules of energy, so we cannot count on the power supply providing limiting. One way of looking at the situation is to consider the thermal time constant of the device vs. that of the protection circuit. In the case of a fuse the thermal time constant for a bondwire will be less than that of a fuse, since the bondwire is physically much smaller. It is possible to mitigate this problem by paralleling many output devices, but this adds capacitive loading and cost.

IMO, the best approach is to employ an active overcurrent circuit (monitoring the current delivered from both the positive and negative rails) and to set the current trip point somewhat less than the current SOA of however many devices are paralleled. The amp I am designing uses slaved overcurrent monitor circuits that remove gate drive to the output devices if an overcurrent condition (>80 amps) is detected on either the pos or neg rails. Once the overcurrent circuit is tripped, the output device gate drive remains off until the amplifier is power cycled. The time from overcurrent detect to gate turn off is about 10 usec. Since the overcurrent condition cannot occur for loads less than 1 ohm, the amp should never trip unless its output is shorted or is driving an unrealistically low impedance.

[djk]

The Hafler DH500 had three pair of 8A Hitachi in parallel, 10A fuse.

The DC off-set was enough to blow this fuse into a short on the output jacks, ±93V rails.

The only time you could blow the outputs was with sustained clipping into 2R. The external 10V zeners would blow, and then the gates would follow.

I had one with a blown bias transistor, biased itself into class A. I had up-graded the stock 35CFM fan to 110CFM, so it would run for about five minutes before it would overheat. Did this many times before I figured out what the problem was. That was 1982, the amp is still in service in 2007 in a friend's PA.