The motivation for this circuit is pretty simple. Sometimes, someone will plug a battery in backwards, or attach power cables backwards. The last thing you want is for your carefully designed circuit to explode in sadness, tears, and magic smoke, so a reverse battery protection circuit can be pretty useful.

A really easy way to do reverse battery protection is to insert a series diode into your circuit, between the power supply and your circuit. During normal operation, the diode allows current flow, and the circuit operates normally. In the reverse battery connection case, the diode is reverse-biased, no current flows, and the circuit is protected.

This, however, means that in the normal operating case, the diode is constantly dissipating power as there is current flowing through it and a voltage drop across it (P = IV), so we’re wasting a lot of energy. Additionally, due to the voltage drop across the diode, we may not have the full operating voltage supplied by the battery or power supply.

A different method involves adding a series fuse and an anti-parallel diode. In the normal, the fuse carries current, and the anti-parallel diode is reverse-biased and effectively does nothing. In the reverse-battery case, the diode is forward biased, and in theory, carries the lion’s share of the current. Essentially we present the power supply/battery with a short (in the form of the diode) in series with a fuse.

In theory, the fuse will blow before the circuit blows, but in practice, the diode typically carries so much current that the traces on the PCB leading to it tend to blow before the fuse does, ruining the board and possibly frying the circuit.

The circuit in the figure is one way of handling the problem, which I feel is superior to the other discussed methods. Looking at the schematic, it’s very similar to the first described case of the series diode. The Virtual GND node is used as the circuit’s actual ground. The MOSFET has a much lower voltage drop across it than the diode for the same current, decreasing the power dissipation.

One important difference is that power MOSFETs have an anti-parallel body diode, which results from the structure of the MOSFET. In standard usage, the diode is reverse-biased. However, if we used the MOSFET in that configuration in the reverse-battery protection circuit, the diode would conduct if the battery was reversed. Thus, we have to flip the MOSFET upside down.

In this case, the source and drain are swapped, so the diode is reverse-biased in the reverse battery case. The zener and the resistors connected to the gate of the MOSFET are just to turn it on. The zener clamps V_GS to some above threshold value (and below the maximum gate voltage). R₁ and R₂ are large-valued resistors on the order of 10k or above.

In the forward-biased case, R₂ pulls the gate up to the zener clamp voltage, and the transistor turns on, allowing current to flow normally, with low dissipation in the MOSFET. The key is to choose a large power transistor with a low on resistance. We can safely ignore the long switching time of such MOSFETs as in this case, the MOSFET doesn’t switch at all, except at power on and off.

In the reverse-bias case, the body diode is reverse-biased, and R₂ pulls the gate well below the source, shutting off the transistor, preventing current flow, and protecting the circuit. Even though it has a higher component count, this design is more power-efficient than the series diode, and is more effective at reverse-battery protection than the anti-parallel diode with series fuse. The power diodes required for the other designs often take up the same amount of area if not more than the MOSFET.