I need to make a reverse polarity and over-voltage protection for a fairly powerful device (45W @ 12V input) and after some research I noticed a weird peculiarity. A typical reverse polarity protection circuit is shown below; it uses a p-channel MOSFET in a high-side configuration as a switch.

N-Channel MOSFETs tend to have lower Rdson, they are much more common and usually cheaper, so, as I guess, using a N-Channel MOSFET is preferable. The same schematic also exists with a N-Channel MOSFET, but to put the N-Channel MOSFET in a high-side switch configuration a charge pump or some other type of dc-dc inversion is required (second schematic).

But why not just put the N-channel MOSFET on the low side like on the third circuit? For some reason I've never seen in any device I've put my hands on a protection circuit with a low-side protection. So, my question is:

Why is it preferable to put the protection switch in the high-side configuration rather than the low-side?


*This peculiarity carries onto the over-voltage protection as well. The PMP10737 TI reference board, for example, uses a P-Channel MOSFET for the overvoltage protection; however, to prove my point that N-Channel MOSFET are better for this purpose, the same board uses a N-Channel MOSFET with a complex IC to drive it in the reverse polarity protection! *

schematic

simulate this circuit – Schematic created using CircuitLab

N-Channel MOSFET with a charge pump N-Channel MOSFET low-side configuration

  • 1
    It may have something to do with your circuit ground no longer being the same as your input ground? I'm not sure. – Hearth Nov 11 at 23:19
  • Incidentally, your drain and source should be the other way around in the third schematic. – Hearth Nov 11 at 23:21
  • @Felthry good point, but that is a concern only if some part of a device is protected (like some discrete car board for example); however, I've seen high-side disconnecting literally everywhere on the power input of the whole device. Even fuses are placed usually on the positive, not negative rail for some reason. – sx107 Nov 11 at 23:24
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    For high-power systems, especially mains connected ones, it's better to disconnect the high side than the low side, because a break in the low side means that your entire circuit is now live. – Hearth Nov 11 at 23:25
  • @Felthry >> even mains connected ones << these are AC devices, insert the power plug the other way around. – sx107 Nov 11 at 23:26
up vote 2 down vote accepted

It depends on your application.

The main issue with low-side protection is that you are disconnecting your ground reference. Many different systems work on the assumption that the 0V/Ground/Earth is shared between the devices. There can be many obvious and hidden ground connections.

If by way of example you have a circuit that is connected to ground by some other means - such as a USB device connected through shield to a PC which is in turn connected to earth and from earth back to your supply negative terminal. In this scenario, your low-side reverse polarity protection is effectively bypassed through this other current path.

If on the other hand you are using a battery connected only to your device, then there is no harm in doing low-side protection as there are no hidden ground paths that can bypass it.

Switching the high side on the other-hand is usually not an issue, as you would typically connect all the grounds together and have an individual power supply - it's unlikely there will be a hidden path from the power supply positive terminal through another device (*).


(*) not impossible - some systems, e.g. some cars, have positive earth, meaning the positive terminal of the supply is effectively the common terminal (car chassis).

  • It's worth noting those cars are quite old cars, but not really that relevant I suppose. – Hearth Nov 11 at 23:25
  • @Felthry indeed, but worth mentioning by way of a counter example. – Tom Carpenter Nov 11 at 23:26
  • Is ground reference disconnection the only concern? – sx107 Nov 11 at 23:27
  • @sx107 pretty much AFAIK. For battery apps there is really no issue with doing low-side protection. In fact some simple Li-Po charger circuits use low-side switching to control charging. – Tom Carpenter Nov 11 at 23:29
  • @sx107, sneaky ground connections can really burn you if you switch on the low side. I have had problems with this even when I was sure I had considered every ground path. In addition to USB, there are also audio connections, maybe an input jack for a power adaptor, etc. For low voltage circuits where the current is not too high (a few amps), there are plenty of choices of low Rds PMOS. It is not a matter of whether NMOS has a lower Rds, but whether PMOS Rds is lowe enough for the application. – mkeith Nov 12 at 1:57

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