Is there any equation to calculate the maximum gate voltage that can be applied to the gate of a power MOSFET without oxide breakdown, other than the generic equation: E = V/tox
That's it, only you don't know Ecrit (unless the mfr likes
to share detailed qual data from test-to-fail) and you
don't know V in the off state (being dropped by the neck
depletion region, more or less) and tox isn't always
shared either.
The mfr tells you what they'll stand up for, beyond that
it's on you. And they design them such that Ron is all
in before you hit Vgs(max) so there's no real reason to
go over the top.