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I forgot to mention, I found a way to decrease the oscillations.. Obvious really but it has a downside.
I add gate resistance! :P
I threw a 100ohm pot in to see the effect of changing gate resistance on supply ripple.
Turning the mosfet on more slowly allows time for the inductance to be overcome.
I can reduce the amplitude of ripple from ~30V to ~5V.
The frequency of oscillation is nearly unchanged.
At higher currents I imagine this would fry the mosfet as it has to survive high currents with a large Rds during the longer transistion time. I remember playing with heat dissipation formulae for switching power supplies, I might have to revisit them.
Still, with 30 times the current draw, the starter should cause 30 times the ripple, or 900V (still 150V with slow turn-on). Seems excessive, but maybe *that* is what is killing my mosfet...
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