In Power Protection Circuits, I outlined some methods of circuit protection. In this article, I'll take a closer look at the practical aspects of using a P-Channel MOSFET circuit for reverse polarity protection.
Both the series diode and crowbar circuit have been seen to have some rather large limitations. This circuit isn't any more complex and yields substantially improved performance.
I used an IRF-9630 P-Channel MOSFET that I had on hand for this test. This is a 6.5 amp, 200 volt unit with an RDS resistance of 0.8Ω. A MOSFET with a lower RDS would be a better choice but this unit will illustrate the principle.
The MOSFET is connected in an unconventional manner here. The source terminal would usually be the input and be connected to the supply voltage; the drain would be the output and connected to the load. In this arrangement, the MOSFET uses its internal body diode to conduct when the power supply is correctly connected. If the power supply is reversed, the MOSFET is turned off and no current flows to the flow. I'll leave the detailed explanation to Wikipedia. (See the comments below for an excellent explantion on Hack-A-Da)
A slight variation of this circuit is shown in the comments below for use when the supply voltage exceeds 12 volts.
What does this mean in practise? I connected the IRF-9630 as shown about and plotted output voltage vs load current as shown by the blue line below.
The first thing to notice is that there is no immediate insertion loss just for putting the MOSFET in the circuit - the 5 volt supply is still 5 volts at no load. This is already an improvement over the 0.4 volt drop just for adding a series diode! As the load current increases, there's a very linear drop in output voltage. The red line in the graph shows the I-R voltage drop for 0.8Ω, so the response is close to what we would expect for the drain-source resistance of 0.8Ω.
The key to better performance of this circuit is a MOSFET with a lower RDS. MOSFETs with an RDS in the few milliohm range are not uncommon, so performance can be considerably improved.
The current required to turn off the MOSFET if the power supply is connected backwards is very small. It does not have the limitation of the crowbar circuit that extra current must be available to shut it off.
The advantages of this circuit are a small voltage drop (even better than shown here with a better MOSFET) and protection with a reversed power supply without the need to make the power supply larger than needed. The voltage drop is small enough (again, with a properly selected MOSFET) that a 5 volt regulated wall wart will provide acceptable voltage to the circuit. A MOSFET may be slightly more expensive than a single diode but the cost to implement this circuit will be lower.
This method provides reverse polarity protection with few complications.