Apologies for the very poorly written previous question.
We have a circuit that consistently burns out our MOSFETs when we attempt to use them at full power, 250V, but they seem totally fine at 200V.
The circuit is used to power a solenoid to make a small robot kick a ball. At the top, there is a capacitor charged up to 250V, next the solenoid with a snubber diode (unidirectional zeener with a 330v breakdown voltage). The MOSFET is off until we want a kick at which point we turn the MOSFET on and allow the cap to drain through the solenoid.
The mosfet we are using is the STD18N55M5, this one, it is rated to 16A continuously and 64A pulsed. The Drain source breakdown is 550V. I don't know exactly why there is a resistor or a diode alongside the MOSFET but I assume they are similarly for spike protection.
I am new to the project so I don't know a lot of the details but I have been tasked with trying to discover why our MOSFET keeps burning out above 200V. My initial instinct is that the 16A is just too low for 250V but I have no evidence to back this up. Another possibility is that the zeener is somehow breaking down at 250 rather than 330 and shorting the MOSFET between 250 and ground but that seems unlikely.
I don't really understand how one calculates the current through an inductor and MOSFET since there are no resistors in the path which seems like it would create infinite current which obviously isn't the case. Any insight or suggestions for things to try would be greatly appreciated. Thank you!
EDIT: the cap is 1500uF, 250V. The flyback zeener is this one digikey.com/product-search/en?vendor=0&keywords=F4115CT-ND and the mosfet is the d package with no heat sink