MOSFET for reverse voltage polarity protection instead of a diode

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skylen
 
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MOSFET for reverse voltage polarity protection instead of a diode

Post by skylen »

Has anyone here used a MOSFET for reverse polarity protection? How has that worked out for you? What did your circuit look like?

Background

Instead of a diode, with a forward voltage drop of at least 0.3 V for what I have seen even in Schottky diodes, have you ever used a MOSFET for reverse-voltage protection? One point to remember is that MOSFET must be connected in such a way that the body diode is forward biased in the direction of normal current flow.

References for using a MOSFET for low voltage-drop reverse polarity protection

- Low-Voltage, Reverse-Battery Protection Circuit Using the TPS61200 Boost Converter (see section 2.1, “Reverse-Protection Circuit” on p. 2; this shows a configuration using an n-channel MOSFET and on using a p-channel MOSFET).
- Overvoltage Protection in Automotive Systems (section “Reverse-Voltage Protection” and Figure 2, use of p-channel MOSFET for reverse-voltage protection).
- Pololu Baby Orangutan B Schematic (see p-channel MOSFET Q1 on input to U1).
- Forum discussion “MOSFET for Reverse Polarity Protection”
- “FET Supplies Low-Voltage Reverse-Polarity Protection”, Electronic Design article.

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ImaginaryAxis
 
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Re: MOSFET for reverse voltage polarity protection instead of a diode

Post by ImaginaryAxis »

I have never used one to date since I usually use a diode for one off type of projects. One of main advantages of using the body diode of a FET is the low on resistance; nMOS have lower rds(on) compared to a pMOS of the same die size. This helps save battery life especially for low power applications; i.e., 3.3V and 2.7V.

skylen
 
Posts: 56
Joined: Fri Sep 04, 2009 5:54 pm

Re: MOSFET for reverse voltage polarity protection instead of a diode

Post by skylen »

You make a good point about the supply voltage being a significant factor in the benefit of using a MOSFET instead of a diode. For low voltage (< 5 V or so), 300 mV is a significant proportion of the supply voltage and so a lot of energy is wasted in a diode. For higher voltage applications it is not such a large factor. For 24 V and higher, a Schottky diode would probably be completely sufficient.

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