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NMOS Reverse Polarity Protection

cl10Greg

Mar 20, 2014
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I found an article that covers reverse polarity protection with a NMOS. A generally good practice is to have a pull down resistor to ensure that the transistor is turned off. This doesn't seem like it would work for reverse polarity protection though. If you had a pull down and had it hook up in reverse, it would turn it on and defeat the purpose. Anyone have any insight in this or am I misunderstanding a concept?
 

BobK

Jan 5, 2010
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Okay, I have looked for a circuit and found this:

http://www.ti.com/lit/an/slva139/slva139.pdf

The MOSFET is being put in backwards from it's usual usage, i.e. with an N-MOSFET the drain is connected to the battery - and the source to the load. Normally, you would not do this because the body diode would conduct and you could never turn it off. But in this clever usage, when the battery is backward neither the body diode nor the MOSFET will conduct, and with the battery connected the gate will be at the battery + voltage and the source near the battery - voltage, which makes it conduct, shorting the body diode and having a much smaller voltage drop than a diode would.

No pulldown resistor is needed, because when the battery is right, the gate is always connected to b+ and when the battery is backwards it is connected to battery -. It can never float.

Bob
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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I found an article that covers reverse polarity protection with a NMOS.

NMOS is a logic family. What Bob describes above has nothing to do with NMOS. I'm pretty sure that's what you meant though.

Often a P Channel mosfet is used to allow a common ground.
 

Harald Kapp

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Okay, I have looked for a circuit and found this:

http://www.ti.com/lit/an/slva139/slva139.pdf

This circuit will work fine as long as the max. VGS is observed.
It will not work for high voltage MOSFETs where VBAT=VDS > VGSmax because the gate will break down.
It will also not work for low VBAT if VBAT < VTH (threshold voltage) because in that case only the body diode will turn on. A Schottky diode is a better choice in this case.
 

(*steve*)

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Where I live we often refer to N-channel MOSFETS as NMOS and P-channel MOSFETS as PMOS :rolleyes:

You learn something every day. This is the first time I have heard of this abbreviation being used with this meaning.
 
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