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I have a system that has two fan banks driven by MOSFETs. The fans are mounted on two sides of the enclosure on ~2 ft cables. After one second from system turn-on the microcontroller turns these FETs on. When the fans are already connected before system turn-on this is no problem.

What has been seen is that if the fans are disconnected during operation the system will reset. Also, if the system is running with the FETs powered and the fans are connected the system can reset. This is not consistently reproducible.

There are positive and negative spikes on the gate from -3V to +9V. I'm decently confident this is what is killing the micro and causing a reset. What I don't understand is how there can be a significant energy burst when the inductive load is removed from the system. Just to be clear the state of the FET is NOT being changed by the micro - there is no turn-off/turn-on control - only installation or removal of fans.

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ Normally people use a series resistor between MCU and gate circa 100 ohm. \$\endgroup\$
    – Andy aka
    Commented Mar 2, 2020 at 17:16
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    \$\begingroup\$ I'm saying "gate-drain capacitance (a.k.a. reverse transfer capacitance) and dv/dt" and "better to place a resistor (e.g. 47R) between mcu's pin and FET's gate." then leaving. \$\endgroup\$
    – Rohat Kılıç
    Commented Mar 2, 2020 at 17:17

2 Answers 2

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As the fans are "disconnected", that is , wires physically removed, sparks are generated.

Those sparks arcs will explore all possible paths for the charge to return home.

Grounds are upset

VDD is upset

And the FET drain-gate capacitance conducts that spike into the MCU output.

Thus you have an ESD event, yet you expect continuous operation.

Review your gnd_ing.

And place a resistor between FET gate and the MCU.

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  • \$\begingroup\$ This makes sense, I didn't think there would be energy stored in the FET itself. Series resistor and TVS have been added. \$\endgroup\$
    – pbandjazz
    Commented Mar 5, 2020 at 14:44
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The snubbing action of your side is lost if you disconnect the fan and leave the snubber.

Try adding a snubber diode into each fan. That way the snubbing action will take place within the fan assembly and not kick may stuff into your circuit.

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