Thanks for bringing this up. I know what you mean.
But if i put in values of
Stray C (interwinding) =10pF
dV = 400V
dt = 20ns
.
The 3 A came from the 300 kV/us figure you gave earlier. You gave no other numbers earlier, so I worked with that. You never mentioned anything about the actual numbers you were expecting, but those only yield 20 kV/ us. Why are you even bringing up CMTI if your real worry is something else?
By the way, many engineers seem to be able to use high-side driver ICs successfully, though they do have their gotchas. IR211x has been around for decades, and there are many more choices today. Careful attention to layout prevents a lot of negative voltage issues.
FWIW, it's been a few years since I worked above 200 V, but I had good luck with SiLabs (now Skyworks) Si827x series drivers at the time, switching GaN FETs at ~350 V in about 3-4 ns.
John