This is a followup to:
Simple ESD protection for MOSFETS
I built the circuit described in the above post using FQP30N06L MOSFETs and after several hours of testing two of the MOSFETS died at nearly the same time. I am not certain why. The big worry is that when they failed, they became dead shorts which, if I hadn't gotten to the kill switch in time would have damaged the solenoids.
Anyway, I am considering going to TIP122s because they have a similar pinout (BCE vs GDS); replacing the 100ohm gate resistor with a 330ohm base resistor; and dropping the TVS diode.
The main reason I'm considering making these changes is familiarity and robustness. I've never seen a Darlington fail without an obvious cause (mostly accidental over-current), and I don't have much experience with power MOSFETs.
Are there any big disadvantages that I might not be considering? And is it reasonable to suggest that Darlingtons are more robust, or at least more forgiving?
@Andy The TVS diode that I used is the P6KE6.8CA-E3/54
Best Answer
The parts you used are robust when used properly. I suspect you did not give them enough gate voltage to turn them on fully and that caused their failure. There may also have been transients on the 50V supply that caused the parts to avalanche. They are not guaranteed to be reliable with only 5V drive- you need a 'logic level' MOSFET, and I would not use a 60V MOSFET on even a regulated 50V supply with inductive loads floating about.
When properly rated and properly driven the MOSFETs will run extremely cool and should last indefinitely. Large power MOSFETs have a large gate charge so casual ESD events actually are not normally an issue. You just have to make sure they're either FULLY ON or FULLY OFF or you'll get excessive internal heating and perhaps violate the safe operating area (SOA) specifications. And make sure they don't see excessive voltage when off from drain to source. That means enough gate voltage supplied fast enough turn turn on and the opposite to turn off. So a pull-down resistor (in parallel with maybe a 12V zener for gate voltage protection) will prevent some problems, but you must use a MOSFET specified for 5V drive if that's what you are using.
You might consider something more like an Infineon IPP50N10S3L-16, which is rated at 100V (5x more margin than the thin 10V you have with a 60V MOSFET) and has 21m\$\Omega\$ max Rds(on) with 4.5V drive. There are other choices- you can do a parametric search.
Unlike BJTs, there extra voltage rating costs Rds(on) vs. silicon area and also makes logic level more difficult, so the voltage rating should be enough to be reliable against transients etc. but not too much, if that makes sense to you.