Power Protection Circuits - A Closer Look at the P-Channel MOSFET Circuit

P-Channel MOSFETIn 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.

MOSFET Test Plot

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.

Conclusions

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.


Posted: 2 years 10 months ago by Jon Chandler #9866
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This is such a simple and effective way to protect against reversed power supply polarity that I'm surprised it's not more widely used.

I will probably incorporate it into my timer project if it comes to pass, as it will have a terminal strip for a regulated 5 volt supply. Since the connections would be made by others, this is cheap insurance against errors. I need to think about adding an LED next to the connector that illuminates on reversed polarity. "If the red LED adjacent to the power terminal strip is illuminated, READ THE INSTRUCTIONS"
Posted: 2 years 10 months ago by jmessina #9869
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Here's a really good explanation of how it works.

http://hackaday.com/2011/12/06/reverse-voltage-protection-with-a-p-fet/

Pay close attention to the last 30-45 seconds.
Posted: 2 years 10 months ago by Jon Chandler #9872
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Very good explanation Jerry! Thanks for posting the link.
Posted: 2 years 9 months ago by Jon Chandler #9878
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The Hack-A-Day link Jerry posted by Afrotechmods has one important addition when using this method for voltages greater than around 12 volts.

For some MOSFETs, the gate-source voltage is limited to 10-12 volts even if the device is rated to switch higher voltages. Afrotechmods recommends adding a Zener diode of about 10 - 12 volts and a resistor to clamp the gate-source voltage to the safe range. See the diagram below.



This is only a consideration if the supply voltage is greater than ~ 12 volts. I would definitely add the Zener and resistor for automotive applications where the nominal 12 volt supply can swing wildly.
Posted: 2 years 8 months ago by Jon Chandler #10200
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A nice addition to this circuit is the addition of a bipolar LED and resistor across the input lines, set up to show green on correct polarity and red on reversed polarity. At a tiny cost increase over a single LED you have a power on/reverse polarity indicator.

Also see the comment above about adding a zener diode and resistor to keep the gate/source voltage within range when using this circuit above 12VDC.

I'm in the process of designing some DIN-rail mount modules for an industrial application that will be powered from 24v connected to a terminal block. You can be sure this circuit was the first thing I added to the design!

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