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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

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ I think there are about a thousand other questions dealing with that you need a freewheeling diode \$\endgroup\$ – PlasmaHH Nov 4 '15 at 21:47
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    \$\begingroup\$ You mention a capacitor, but I don't see one in your schematic. You also mention a MOSFET, but only show a switch. \$\endgroup\$ – Olin Lathrop Nov 4 '15 at 22:21
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    \$\begingroup\$ There are too many confusions and missing knowledge to be able to answer this question in a reasonable space in a way you can understand. The lack of attention to details exhibited here confirms that this question just isn't a good fit for this Q+A site. \$\endgroup\$ – Olin Lathrop Nov 4 '15 at 22:24
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    \$\begingroup\$ Fix up the question dude. \$\endgroup\$ – Andy aka Nov 4 '15 at 22:26
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    \$\begingroup\$ Did you clamp the gate well below its V(GS,max). Due to fast rising drain voltage and C(GD) the gate voltage may be pulled way above its maximum voltage. \$\endgroup\$ – jippie Nov 5 '15 at 5:51
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Making a few assumptions from what you wrote and the partial circuit diagram, the un-clamped inductive kickback from the solenoid the instant the transistor is switched off, is likely exceeding the max Vds rating of your transistor and destroying it.

But like the folks above mention, in electronics, the details are important and you have provided very little information regarding the actual values of the components involved.

I hope that helps a little.

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  • \$\begingroup\$ After some testing, this turned out to be the problem, thanks! \$\endgroup\$ – Indigo Nov 9 '15 at 22:01
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If your wires are long, then D1 only clamps the inductive spike from the solenoid, and not the spike from the wires between the FET and that solenoid.

Be sure that D1 connects between the capacitor, and the directly to the drain of the FET. Similarly, D2 should be very close to the FET.

Note that the ground connection is equally important -- be sure the '-' end of the capacitor is close to the FET's source.

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