Many years ago, an old engineer gave me a valuable piece of advice, suggesting that a common mistake we make in trying to make our designs foolproof is to underestimate the ingenuity of fools. One of the simplest things we can do to thwart those ingenious fools is to prevent the power to our circuit being connected the wrong way. I’d recommend you consider this for all your designs even if there is only the most remote possibility that a reverse polarity could occur.
Let’s say we have a circuit powered by 2 AA cells that draws up to 100mA and operates over the 3.0V to 2.0V range, which is about what we can expect over the battery’s useful life. Our circuit can’t tolerate a reverse connection of the batteries so we can be assured that sooner or later someone will do exactly that. How can we protect the circuit?
The simplest approach is to add a diode in series with your power supply. As shown at left in Figure 1 below. This works well but there will be a forward voltage drop across the diode of 0.7V – 1.0V. This drop represents 25% or more of our available voltage when the batteries are fully charged, and up to 50% if the batteries are at the end of their life. This is clearly pretty inefficient. There must be a better way.
We could always replace the standard diode with a Schottky diode which has a typical forward drop in the range 0.2V – 0.5V. This is much better, but still reduces our efficiency by up to 25% as the cells reach the end of their lives.
Another very neat approach is to use a P-Channel MOSFET as shown at right in Figure 1. When power is applied with the correct polarity, the MOSFET’s body diode will conduct, bringing the source terminal to one diode drop below the supply voltage. The MOSFET will then turn on, shorting the diode, since the gate is now at a lower voltage than the source.
If a reverse voltage is applied, the body diode will be reverse biased, and the MOSFET will be biased off since its gate voltage is higher than its source voltage, and no damaging reverse currents will flow.
In the normal mode of operation, the voltage drop across the MOSFET will be defined by the on-resistance which is typically just a few tens or hundreds of milliohms. In the example above, a RQ5A030AP MOSFET would drop less than 10mV at 100mA. Nice.
You should check the MOSFET drain-source and gate-source voltage ratings to make sure your chosen MOSFET is operated within its safe range. In this case the MOSFET can tolerate a drain-source voltage of -12V and a gate-source voltage of -8V, so we are good. We also need to be sure our MOSFET gate-source threshold is low enough that it will turn on with the lowest likely input voltage. Again, the datasheet suggests we will be fine as the VGS threshold is -1V, and at 2.0V we are well within the saturation region.