Sigurthr wrote ...

If they're not getting hot but they're dying it's almost always an overvoltage issue. Just because you only placed 18V on the drain doesn't mean that's all that is going to be there. When operating an inductive load with a single ended configuration you get reflected flyback spikes whenever the switch opens up. I've personally observed a 12V supply cause spikes well over 400V.

My advice: use an external fast antiparallel diode rated for very high voltage, and consider placing a flywheel one across the primary. Other than that you can attempt Class-E configuration.

hen918 wrote ...

Try looking at the gate voltage on a scope I had a very similar problem when parasitic inductance was causing my gate voltage to spike several volts above the maximum rating.
Working absolutely fine, stone cold, and then tic, my power supply tripped. Sure enough, two dead MOSFETs. (they were in an H-bridge)

Wolfram wrote ...

Antiparallel diodes don't do anything against inductive switching spikes, which appear in the forward direction. TVS diodes for clamping or RC snubbers to absorb the spikes are possible solutions. The most important part is to minimize the energy available in the spikes in the first place.

Antiparallel fast diodes generally make sense when switching a series resonant load below the resonant frequency, which is a separate problem. The point is to avoid forced recovery of the internal MOSFET diodes which are usually quite slow.