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I want to add a reverse-polarity diode to my design.

It has an input of 24 V (230 Vac to 24 V power supply) which is fed to a 24 to 12 V converter. I plan to draw up to 4 A from the 24 V power supply. The PCB works in a generally room temperature environment and has an SBC, mini pcie port and an RS-485 comm line.

I was planning on adding a diode with a serial fuse too, in parallel to the 24 V in order to prevent reverse-polarity connection damage.

I also want to add a surge protection to the same power terminal.

  • Can I just use a TVS (bidirectional or unidirectional) instead of the diode?
  • Do I need to have both?
  • I know that a TVS adds a substantial amount of input capacitance. Should I be worried about it?
  • Do I need to have other things in mind, when designing the circuit?
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@metsik: Below is an NMOS circuit example for reverse polarity protection. You can see that this is simply a mirror of the PMOS circuit posted by Lundin.

I recommend using a 10V Zener so as to protect the FET gate. This is enough to fully saturate the FET, without getting close to the max Vgs (usually 20V).

Do you need hot-swap protection on this circuit? I would submit you only need it if you are switching this circuit to a live 24DC supply (hot swapping). Most off-the-shelf supplies can power up into a capacitive load (in a controlled manner). Also, a larger concern than the TVS will be the input capacitance you need to stabilize your flyback.

enter image description here

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  • \$\begingroup\$ Thank you. I wont need to hot- swap. \$\endgroup\$ – metsik May 2 at 5:27
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Without any statistical evidence for it, I think the most common solution for rugged industrial/automotive solutions is a P-MOSFET (with built-in diode) with zener + resistor for polarity protection. Then TVS after that for the sole purpose of getting rid of spikes. Example (component values may be tweaked):

schematic

simulate this circuit – Schematic created using CircuitLab

Optionally with a fuse before the MOSFET. N channel versions exist too if you are very picky with Rds(on).

For surges, a big bulk cap is usually good enough for most applications. For the more extreme scenarios with huge surges, there's specialized "surge stopper" power management IC you can use: https://www.analog.com/en/products/monitor-control-protection/surge-stopper-overvoltage-overcurrent-protection.html.

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  • \$\begingroup\$ For surges, a big bulk cap is usually good enough - can you expand on this? Having a big bulk cap can actually make inrush worse. \$\endgroup\$ – Reinderien Apr 28 at 13:47
  • \$\begingroup\$ @Reinderien Oh yeah it gives a huge inrush at power on. But then it typically doesn't matter because you haven't started yet. These caps help from surges when you are already up and running. It's a pain to combine them with fuses though, very delicate balance in that case. \$\endgroup\$ – Lundin Apr 28 at 14:01
  • \$\begingroup\$ You can also mirror this circuit and use an N-channel mosfet on the return leg if you can tolerate separate grounds. \$\endgroup\$ – Troutdog Apr 28 at 14:11
  • \$\begingroup\$ @Troutdog I plan on designing a flyback after that protection circuit, so I will have seperate ground either way. how does a protection circuit with an N-MOS look like? does that mean adding a 3rd ground? \$\endgroup\$ – metsik Apr 29 at 7:06
  • \$\begingroup\$ @Lundin, the velue of the 24v zenner seems to result in a 0v voltage across R1. woldnt it put the PMOS in an ohmic-cutoff state? also, as I see it, the added TVS will result in high inrush current that I wish to avoid. is there another way to aproach it without introducing voltage spikes and high inrush current to my 24- 12v converter? \$\endgroup\$ – metsik Apr 29 at 7:19

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