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I'm using this depletion-mode MOSFET in a high-voltage power-management circuit. My 500V DC supply isn't high grade, and now that I've blown two of these MOSFETs (gate shorts to drain) I'm wondering if I can put any components on either side to protect it? I was getting weird results with the first one before realizing it was blown. I might have blown the second one (not sure at which point in my testing it failed) simply by turning the high-voltage supply on with it wired like this:

schematic

simulate this circuit – Schematic created using CircuitLab

For protection purposes: If it simplifies things I never want to see more than 200mA through the circuit. If no better ideas I guess I could put a fast-blow fuse in front of V+? But I'm not sure if that's the only thing that can blow this MOSFET. E.g., I'm beginning to wonder if having no load on the drain can cause problems.

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  • \$\begingroup\$ What is this schematics? I don't see any reason why this shoudn't blow off. \$\endgroup\$ – Marko Buršič May 10 '16 at 18:11
  • \$\begingroup\$ @MarkoBuršič - Explain why? Lack of load on drain? I couldn't find anything in the specs constraining drain voltage other than to be less than 1000V over source. And the power resistor on source should limit current to 5A even if the MOSFET isn't adding any resistance (which, according to its spec sheet, it should). NB: The MOSFET is heat-sinked. \$\endgroup\$ – feetwet May 10 '16 at 18:20
  • \$\begingroup\$ Please explain what you want to acheive, maybe I am wrong, but this circuit can just blow your MOSFET and nothing else. Perhaps you will get some information to modify this, if you specify the purpose. \$\endgroup\$ – Marko Buršič May 10 '16 at 18:24
  • \$\begingroup\$ Goal is to provide constant current to a varying-resistance load on the drain side. Which it does quite well at low voltage. It also does it at high voltage until something blows the MOSFET. The circuit has to be able to survive an effectively zero-resistance load (i.e., a short) on the drain to V+. If that would be expected to blow the MOSFET I would be interested to know why, but even more interested to know how I can protect and recover the circuit from that condition. \$\endgroup\$ – feetwet May 10 '16 at 18:38
  • \$\begingroup\$ Few notes: 0.2A*1000 Ohm=200V, A Vgs of -200V is just too much. Second, Vds =500-200=300V, P_dissipating=300*0,2=60W, which is also too much. Not sure about the schematics, if this circuit can really work as constant current source, don't know where you got it. \$\endgroup\$ – Marko Buršič May 10 '16 at 18:53
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The common way to protect a MOST gate is to use a Zener diode between gate and source. Your MOST has a max Vgs of 20V so add a ~15V zenned diode in reverse bias and you'll be fine. There are also devices called transils, which are specialized for over-voltage protection but do basically the same.

enter image description here

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  • \$\begingroup\$ So if the MOST failed with a short between gate and drain then it must have been caused by excessive voltage difference between gate and source? \$\endgroup\$ – feetwet May 10 '16 at 18:15
  • \$\begingroup\$ If you get a G-D short it will kill the MOST and the Zener diodes with it. Maybe when connecting the transistor the gate caught some static or got charge through capacitive coupling and the voltage blew the gate oxide. Hard to tell what are you trying to do with this circuit, but do try the protection diodes first. They may point you to the source of the problem \$\endgroup\$ – user68591 May 10 '16 at 18:32
  • \$\begingroup\$ I have only observed that the malfunctioning MOSTs have a G-D short. I.e., I take it as evidence that the MOST is blown. But I don't know why or how it happened. I just added clarification of the circuit's purpose in question comment. \$\endgroup\$ – feetwet May 10 '16 at 18:43
  • \$\begingroup\$ So I want a Zener or TVS to shunt current when Vgs exceeds ~15V. I.e., "clamping" voltage should be under 15V? And, presumably, all other voltage specs for such a diode will be under 15V so any such will do, and in either orientation? Even though this circuit could present a voltage difference in the hundreds of volts? Or is there some other specification I need to watch? \$\endgroup\$ – feetwet May 10 '16 at 22:08
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This datasheet provides detailed recommendations on employing TVS diodes to protect both the more sensitive gate and the entire MOSFET against transients, by connecting the diodes like this:

MOSFET Protection circuit

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  • \$\begingroup\$ @uhoh – I just updated the link, but the essence of the recommendations are summarized in the circuit diagram. \$\endgroup\$ – feetwet Mar 18 '18 at 3:22

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