Reverse Polarity Protection

Owned by Jason Skells
Last updated: 2021-12-18
3 min read

Introduction

Reverse polarity protection (RPP) is a circuit used to protect your board and its components against reverse polarity (ie. plugging in the power and ground backwards, or in reverse). A RPP circuit works by allowing current to flow only in the correct direction. There are a couple ways to do this using:

  • Diode

  • MOSFET (NMOS or PMOS)

How each of these are implemented

Diode

  • This is the most simple type of RPP circuit and we can implement it by simply placing a diode in series with our power supply voltage or ground.

  • We prefer to use Schottky diodes over other types of diodes due to the low forward voltage drop. This means less heat is generated and thus we can get a higher efficiency from our circuit.

2 Possible Implementations of RPP Using a Schottky Diode

Pros

  • Cheaper than other solutions

  • Easy to implement

Cons

  • Lower efficiency when compared to the other implementations using transistors

  • With high-current applications, the diodes can overheat - we can put multiple diodes in parallel to compensate for this though

 

MOSFET (NMOS or PMOS)

  • This implementation is done by placing a PMOS on the high-side, with the gate connected to the ground, or by placing an NMOS on the low-side, with the gate connected to the battery.

  • NMOS FETS are typically cheaper and have a lower Rdson than PMOS FETs

  • Using an NMOS FET can leave us with a floating ground (ie. the automotive system’s ground in the figure below is not 0V) since we will have a voltage drop across the NMOS’s Rdson. This can affect sensitive circuits and reduce the accuracy of sensors, especially if we are comparing a voltage to ground.

  • A PMOS FET typically has a higher Rdson than a similar sized NMOS, however, since the PMOS is in line with the high-side, we will not lift up the ground reference.

2 Implementations, the First Using a PMOS and the Second Using an NMOS

  • Looking at the PMOS implementation, we can see that if flip V_BAT, we will apply a high voltage to the gate. Do to the nature of a PMOS FET, this will essentially switch the FET OFF, stopping any current from flowing through the drain-source channel.

  • Looking at the NMOS implementation, we can see that if we flip V_BAT, we will apply a high voltage to the drain, and a low voltage to the gate. The NMOS FET will be in a OFF state and will not allow current to flow into the systems ground pin via the drain-source channel.

Pros

  • Much higher efficiency compared to the diode implementation.

Cons

  • Slightly more costly (the Schottky diodes are surprisingly expensive at ~$1.3 for a 40V 45A diode and some MOSFETS are around that price too)

  • FETs are larger than diodes and thus will increase the board size

 

 

References

https://www.ti.com/lit/an/slva835a/slva835a.pdf?ts=1639787369441

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