High- vs Low-side MOSFET reverse-polarity protection

[jessenator]

Curious if there are disadvantages in our types of circuits to having MOSFET reverse-polarity protection be on the "low side" of the circuit; i.e. on the ground side. This is assuming we're working 100% with negative-ground circuits.

Most of the resources I've seen are for old fx retrofitting (thanks to Keen's musique site and other old forum posts) are high-side designs using a P-channel MOSFET. It seems that using N-channel MOSFETs on the high-side pose a potential problem and more complexity with its signal routing, making it not the best choice in this instance.

Designing Reverse-Polarity Protection with MOSFETs

Overview of MOSFET reverse-polarity protection: NMOS vs PMOS topologies, operation, device selection, efficiency vs diode drop, and key PCB design considerations.

www.aivon.com

This article linked above, on the other hand, shows examples of low-side protection using an N-channel MOSFET.

I suppose one disadvantage would be all GND would have to pass through it, and it alone in one's power filtering scheme, which would either a) eliminate the option of a singular GND plane and require star wiring, or b) require an ancillary supply breakout board with star wiring, allowing the main effect board to have a GND plane.

Any other considerations?

 

[GAStan]

I suppose one disadvantage would be all GND would have to pass through it, and it alone in one's power filtering scheme, which would either a) eliminate the option of a singular GND plane and require star wiring, or b) require an ancillary supply breakout board with star wiring, allowing the main effect board to have a GND plane.

I think you have Ground and Negative Common mixed together.

On all of the PPCB boards I have seen there is only one wire connecting Power In Negative to the board, this could be used as your protection point.

 

[jessenator]

I think you have Ground and Negative Common mixed together.

On all of the PPCB boards I have seen there is only one wire connecting Power In Negative to the board, this could be used as your protection point.

Apologies. I used the word in describing Keen's write up, but I'm not looking to retrofit/modify existing boards.


This would be for a separate power module PCB.

 

[jessenator]

So I breadboarded up the low-side setup (I sadly don't have a p-channel MOSFET to try high-side), and this is what I'm reading:



I wonder if I need to experiment with R1 to balance the rating of the MOSFET so I can get closer to 0V.

edit: 470k nets me ~150mV, which is about the floor. 100k shows ~160k.

 

[jessenator]

Okay, went back and additional resistor is required to get it down, close to 0V.



I've got 2mV with a 100k network:



I'd like to think that 2mV is a safe value for most semiconductors, anyway.

 

[vigilante398]

The biggest advantage to low-side polarity protection is that you can use an N-channel MOSFET, which tend to have a much lower RDS(on) than a comparable P-channel MOSFET. For pedal applications I would say the difference is basically academic, it won't affect the overall operation of the circuit either way. It's not difficult to find a P-channel MOSFET with a low enough RDS(on) to be a negligible voltage drop, and at the same time it's not difficult to carefully plan a board for a switched power return with an N-channel MOSFET.