Reverse polarity protection with a P-MOSFET

Reverse voltage protection with P-FET
Reverse voltage protection with P-FET

Reverse polarity protection can easily be done with a single diode. The problem with that however is the voltage drop (usually 600mV) that with  large currents can mean a substantial powerdissipation. Using a Skottky diode for its smaller voltage drop is risky as the reverse current through a Skottky is not negligable.
A better way is to use a P-MOSFET as shown in the circuit above. This method is explained very well  in an Afroman youtube video.
Though the direction of the P-Fet may seem a bit counterintuitive, but realise that when the Vgs is in conductive range, the FET will  basically operate as a Switch or in fact a  resistor with very low resistive value, allowing current in both directions.
However you need to pick a FET with low VGS(threshold) so that the device is still offering a tiny volt drop at low battery voltages AND you’ll need a FET with low RDS(on) so that at  your peak current  the FET is dropping less than say 100mV. Other than the FQP4706 the DMG3415U may be a good pick. With 1.8V gate drive it has 0.071Ω resistance.

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