Based on the helpful advice from the community, I realised the first schematic I made would not work as a protection for reverse polarity well due to a couple of flaws including the diode having to be reversed and the circuit not working without a load + the risk of creating an oscillator.

So I updated the schematic, very similar to the one proposed by Tim in the question linked above, BUT, (as of how I think it's logical), with the NMOS's pins flipped. (source to GND and Drain to output). This is it: enter image description here

Let me know if I got the working of this right based on the following description:

System ratings: Vin=55V, Vz=35V, For use in a buck converter. NMOS is chosen over PMOS because they have a much lower Rds(on)

OPTION 1 - CORRECT POLARITY enter image description here When the polarity is right, the mosfet is pulled to Vin through Rm23, so it starts conducting a bit and Vgs will for a short moment be <=55V (Red Arrows). Once it starts conducting and GND hits the zener (Black arrows), the voltage across its leads becomes Vin-Vz=55-35=20V, which are applied to the gate (purple arrows). Vgs=Vg-Vs (GND=0V)=20V-0=20V, perfect!


enter image description here

When the polarity is WRONG, the mosfet gate is pulled to GND, and it doesn't conduct anything. Happy ending.

Did I get it? Is my understanding of this circuit right? Tim had the drain and source reversed, why did he do that? The MOSFET will only turn on if the Vgs is positive


2 Answers 2


Let me know if I got the working of this right...

The zener diode is no longer protecting gate and source from excessive over-voltage in your modified circuit. That's a showstopper in most cases. Now you have reversed drain and source, the internal body diode will conduct on the reverse voltage you are trying to prevent.

...based on the following description

Irrespective of the description, the circuit is wrong and won't work hence, a description of a wrong circuit is irrelevant.


Nope. This won't work as reverse polarity protection. Look at the body diode of that mosfet. It will conduct if the polarity is reversed.

Try to swap source and drain.

  • \$\begingroup\$ If we reverse the MOSFET's source and drain, then will it work? And if yes, how does it? In our example of Vin=55V and Vz=35V, how can we calculate the voltage at the mosfet's gate? It must be positive \$\endgroup\$
    – Mito
    Commented Jul 30, 2023 at 15:02
  • \$\begingroup\$ @Mito I think it should. \$\endgroup\$ Commented Jul 30, 2023 at 15:46

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