Electronic – High side switch won’t turn off

As Ignacio is trying to say, you need to pull the gate of the FET up to the 350V supply rail to turn it off. This makes the VGS ~= 0V. Which means the FET is off.

To turn it on, you need to reduce the voltage on this gate, perhaps the full 350V is not healthy for it. Perhaps drop the voltage at the gate to +330V to turn it on. This will make the VGS = -20V which is plenty. But always check datasheets.

This is a very high voltage switching circuit and your BJT will need to sustain the 350V as well, like I said. I suggest an opto-coupler to isolate the signal from the 350V supply.

Ok, CC mode is almols like an inductor. You have to add a serious diode with anode on the GND and cathode on the output. The current then will flow through it without damaging the transistor.

Upd: Oh, i known what is your problem. Remove the capacitor. And make the resistor between the gate and driver like 100r or even 0. You must close the transistor as quick as you can, otherwise the current times voltage is too high- power that heats it up exists for too long. Keep the diode, of course. In case your transistor's gate voltage is limited, make sue you get the maximum out of it.

Upd: It's still gets burned, but now there is no power or overvoltage on the drain. There is still overvoltage on the gate. Change the resistors so they will be still much smaller than the initial 100k, and at the same time will not drive the gate beyond it's absolute maximum rating. Change R1 to 10k and R2 to 2k.