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I've made a switching circuit for 12V devices using a P-channel MOSFET. For most standard applications, it works as expected, but the MOSFET gets easily damaged if the user connects a BLDC motor. I am trying to figure out why it is happening as I have a flyback diode (D1) which should protect from this situation.

Circuit diagram

What's happening here? Is the diode placed wrong? Did I use the wrong diode for this purpose? What else can I do to protect this circuit?

  • I would like to support 12V user devices up to 10A
  • I need to use P-channel MOS due to some reasons

Flyback diode (1N4001) datasheet

P-MOS (IRF9540N) datasheet

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  • \$\begingroup\$ Is the external ground connected to the ground of your board? \$\endgroup\$
    – Ferrybig
    Commented Nov 5 at 8:09
  • \$\begingroup\$ @Ferrybig - Yes it is \$\endgroup\$
    – michalo
    Commented Nov 5 at 18:15
  • \$\begingroup\$ Aside from the accepted answer and valid comments, I'd also like to see a capacitor from +12 to GND near the MOSFET. \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented Nov 6 at 17:08

6 Answers 6

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At 10 amps, if you don't have a decent enough heatsink, the MOSFET is likely to burn: -

enter image description here

It's unlikely that the problem is due to a faulty flyback diode based on what information you have provided.

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    \$\begingroup\$ Yes, it would appear you need a heatsink or a MOSFET with a low enough on-resistance so a heatsink is not required (dissipation below a watt). \$\endgroup\$
    – Carl Rutschow
    Commented Nov 4 at 15:12
  • \$\begingroup\$ Thank you - I will definitelly switch to MOSFET with lower Rds(on) \$\endgroup\$
    – michalo
    Commented Nov 5 at 18:17
  • \$\begingroup\$ ...and what is at least equally important is that the flyback diode can handle the current. If the MOSFET switches 10A also the diode must handle 10A. In addition an enormous amount of energy (magnetic and mechanical) may get dumped in the diode if the MOSFET turns off. \$\endgroup\$
    – Curd
    Commented Nov 5 at 19:59
  • \$\begingroup\$ The flyback diode need not handle 10 amps continuously but, to uncover what rating it should have requires more knowledge about the load leakage inductance and duty cycles involved @Curd \$\endgroup\$
    – Andy aka
    Commented Nov 5 at 21:15
  • \$\begingroup\$ @Andy aka the diode may need to handle 10 A nearly continuously if the motor acts as generator after the MOSFET turns off. Then not only the magnetic energy stored in the inductance but also the energy stored in the mechanical system connected to the motor gets dumped into the diode (and wiring resistance). See my answer. \$\endgroup\$
    – Curd
    Commented Nov 6 at 7:57
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Is it the diode placed wrong?

enter image description here

The diode does protect the Drain from pulses below GND. No external diode is present to protect against pulses above 12V.

Did I used wrong diode for this purpose?

Please add information about switching speeds and, if possible, oscilloscope measurements.

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I don't think the problem is surge voltage. If you have a BLDC, remember that it is not just a motor, but an integrated inverter which drives an AC motor. That inverter usually has a DC bus with some capacitors, and driving capacitors with MOSFETs means big inrush current, which probably is what's killing yours.

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As others mentioned, heat is a problem. The mosfet may get thermally damaged before it even has a chance to warm up.

The gate is driven off through a 4k7 resistor. That same gate has on the order 1nF capacitive coupling to the channel. Transients on the channel couple to the gate and can easily drive it past the V(GS) limits.

A 1N4007 is too slow a diode. Use UF4001..7 if you can instead.

There is nothing protecting the drain from going beyond supply rails either.

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Been many years but I would try using a Varistor for the surge voltage issue.

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If the MOSFET is switching an inductive load of 10 A also the flyback diode must be able to handle that current.
A 1N4007 diode does not (or only for a short time; only 1 A continuously).

Depending on the amount of energy stored in the load (including the mechanical system connected; e.g. rotating mass) the diode may get damaged and then also the MOSFET.

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