Electronic – How to protect the MOSFET

circuit-protectionmosfet

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.

Best Answer

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.

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