Is using a TVS diode better than using a parallel reverse-polarity protection silicon diode?

Question

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?

Answer 1

@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.

Answer 2

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):

^{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.