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what happens if I exceed the maximum rating of the drain-to-source voltage of an N-MOSFET

I am using NTNS3C94NZ so 12 V in this case

datasheet: https://www.onsemi.com/pub/Collateral/NTNS3C94NZ-D.PDF

does it act as a zener? as a short? is it destructive? what voltage/current can it handle?

I did the test with 20 V, it seems that the over-voltage on the drain leaks to the gate, enables the transistor, that start conducting and pulls the drain low.

so externally, it did act as a ~14 V Zener Drain-to-Source with 20 mA current (dependent on my voltage supply strength). It seems to be still functional after that as the current was limited.

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    \$\begingroup\$ "Stresses exceeding those listed in the Maximum Ratings table may damage the device. If any of these limits are exceeded, device functionality should not be assumed, damage may occur and reliability may be affected.". Simply don't exceed the maximum ratings, else unpredictable potentially damaging things happen. \$\endgroup\$
    – Tom Carpenter
    Commented Nov 6, 2019 at 12:33
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    \$\begingroup\$ Like you said, it starts to act as a zener and if power dissipation isn’t limited, fail shortly after. \$\endgroup\$
    – winny
    Commented Nov 6, 2019 at 12:36
  • \$\begingroup\$ @TomCarpenter I know, but the design flaw is there, I will have to test it \$\endgroup\$
    – Rémi Baudoux
    Commented Nov 6, 2019 at 15:12
  • \$\begingroup\$ App note on avalanche pulse ratings: infineon.com/dgdl/… \$\endgroup\$
    – sstobbe
    Commented Nov 6, 2019 at 20:02

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Most MOSFETs have a protection circuit between the Gate and Source, consisting of back-to-back Zener diodes and series resistance. In this case it is shown in the datasheet:-

enter image description here

The protection circuit is usually included to prevent damage from static discharge during handling, not to clamp excessive voltage during normal operation.

Your FET has no specification for current through the protection Zeners, so you must assume that any significant amount could kill it. However the most common failure mode is a short, so if your FET is still working properly and within its specs (particularly Gate-Source leakage current) then it is probably OK.

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    \$\begingroup\$ high voltage was on D. I have protection diodes on both G and S in the microcontroller. And the current flowed from D to S. \$\endgroup\$
    – Rémi Baudoux
    Commented Nov 18, 2019 at 23:01

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