# Reverse polarity protection - PMOS vs Schottkey diode

I have a circuit running from 2 AA cells in series, meaning I get 3.0V when fresh (or 2.4V if NIMH cells) and it goes down to 2.0V, that's when my circuit cannot function anymore (no DC/DC onboard). It's a very low current circuit (<30mA 99% of the time). The circuit is very very cost-aware as it's going to mass production. Every cent counts.

I want to create the an efficient reverse polarity protection. Right now I am using A schottkey diode and getting ~0.24V voltage drop across it. I guess I am wasting some good battery juice this way when my circuit shuts down even before the battery was really exhausted.

What will be the advantages & disadvatages of switching to PMOS style protection?

I am confused on what are the parameters & numbers I need to verify in the PMOS datasheet so it can provide polarity protection for this low voltage/low current input?

• Can't you get (mechanical) reverse polarity prevention from your battery holders? – Tut Sep 18 '13 at 12:18
• @Tut - I've used a reverse MOSFET protection circuit in a manufacturing situation after battery holder based protection proved less reliable than was desirable. – Russell McMahon Mar 26 '15 at 15:22

I'm assuming the type of circuit you are thinking of is this: -

For use on your battery powered circuit I see little to say against it. A couple of things though; you need to pick a FET with low $V_{GS(threshold)}$ so that the device is still offering a tiny volt drop at low battery voltages AND you'll need a FET with low $R_{DS(on)}$ so that at 30mA (or whatever your peak current is) the FET is dropping less than (say) 100mV.

Maybe the following FET is a decent example: -

With 1.8V gate drive it has 0.071$\Omega$ resistance. At 100mA drain current this device will "lose" 7.1mV.

• thanks so much! so I've chosen this PMOS: Diodes DMP2305U. Based on cost consideration. Do you think that placing a 47uF capacitor just after the Source connection (as a main decoupling cap for my circuit) could cause a in-rush current that will destroy/hinder the PMOS functionality? – mindbomb Sep 22 '13 at 13:18
• @mindbomb OK dude, the 47uF shouldn't be a problem even though the max current rating is 10A because the transistor will turn on slower because the source is being held low by the 47uF. I's be tempted to do a couple of tests on it though. What is the current limit of the power supply - you ought to consider this too (as a precaution). – Andy aka Sep 22 '13 at 15:23

If cost is really such a high priority as you say, then why is reverse polarity protection needed at all? When it's more important that the circuit be cheap than robust, you make it cheap at the cost of robustness. If someone installs the batteries backwards, oh well, that's their problem.

Of course the battery polarity needs to be clearly labled, but that can often be done at no extra cost. Unless you are doing your own battery holder as part of the custom case or something, the cheapest battery holder is probably clips that mount directly on the PCB. You can draw obvious battery outlines to show polarity on the silkscreen below where the battery gets installed.

• Also, since AA cells have a "bump" on the positive poll, it's pretty easy to mold the battery holder to prevent contact if the battery is inserted reversed. – DoxyLover Sep 18 '13 at 16:56
• @DoxyLover I recall seeing something like that, a battery clip that prevents reversed batteries, and it was patented, no less - by Microsoft, I think. Can't find the link now, unfortunately. – Anindo Ghosh Dec 2 '13 at 13:39
• @AnindoGhosh Maybe you refer to what is reviewed in this EEVblog video. – Lorenzo Donati Aug 16 '14 at 13:58