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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

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    \$\begingroup\$ It may have something to do with your circuit ground no longer being the same as your input ground? I'm not sure. \$\endgroup\$ – Hearth Nov 11 '18 at 23:19
  • \$\begingroup\$ Incidentally, your drain and source should be the other way around in the third schematic. \$\endgroup\$ – Hearth Nov 11 '18 at 23:21
  • \$\begingroup\$ @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. \$\endgroup\$ – sx107 Nov 11 '18 at 23:24
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    \$\begingroup\$ 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. \$\endgroup\$ – Hearth Nov 11 '18 at 23:25
  • \$\begingroup\$ @Felthry >> even mains connected ones << these are AC devices, insert the power plug the other way around. \$\endgroup\$ – sx107 Nov 11 '18 at 23:26
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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).

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  • \$\begingroup\$ It's worth noting those cars are quite old cars, but not really that relevant I suppose. \$\endgroup\$ – Hearth Nov 11 '18 at 23:25
  • \$\begingroup\$ @Felthry indeed, but worth mentioning by way of a counter example. \$\endgroup\$ – Tom Carpenter Nov 11 '18 at 23:26
  • \$\begingroup\$ Is ground reference disconnection the only concern? \$\endgroup\$ – sx107 Nov 11 '18 at 23:27
  • \$\begingroup\$ @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. \$\endgroup\$ – Tom Carpenter Nov 11 '18 at 23:29
  • \$\begingroup\$ @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. \$\endgroup\$ – mkeith Nov 12 '18 at 1:57
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Resistance in either line can cause problems for your circuit, but as we typically design circuits with 0V being the reference used for the whole system, ground lift is a bigger problem than power drop. All of the currents in your system - including switching currents for power supplies, high speed signalling currents etc - go through that resistance and produce a voltage difference. That voltage noise is then coupled into everything that depends on that voltage reference. It'll usually be sensitive analog circuits where that bites you.

I wanted to talk about one huge advantage of putting your reverse polarity protection in the low side though. Consider the situation where your device is connected to a bunch of other devices, all referenced to mains earth. For example, you're designing a USB device which will be plugged into a PC, but needs an external 12V supply for its power requirements. The PC's power supply negative, chassis, etc are tied to the ground of the mains AC power supply, and therefore so is your device, through the ground wire and shield in the USB cable. Now imagine that your external power supply is similarly referenced to ground. You have a reverse protection diode (doesn't matter what kind) in the high side of your device's input, which would protect it just fine if you plugged in your power cable backwards but your device was otherwise isolated. But in our case, the negative terminal is mains earth, and you've just connected 12V to that terminal, without anything protecting it. Current will flow in through the negative terminal of your device, out the USB port, through the PC and back to the power supply through mains earth. You might just blow a fuse, if you're lucky.

If the diode/mosfet protection was in the low side input, this wouldn't happen. You'd be shorting the high side input to ground (which is probably fine, assuming you're not also getting power from the USB rail, or a battery or something, in which case you'll need to protect that too), but the 12V connected to your low side input terminal doesn't do anything.

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