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I was looking at this open-source power supply circuit for an automotive CANBus device. I am wondering it would include both a Schottky Diode (D1) and a MOSFET (Q1) on the power supply line. My understanding is that both of these are to provide reverse-voltage protection.

Is redundancy of two reverse-voltage devices just that, to have a failsafe? Or do these two devices have different characteristics that make it desirable to have both?

12V Power Protection Circuit for Automotive CANBus Device

Edit: Here is a link to the schematic: https://github.com/macchina/m2-hardware/blob/master/M2/Interface%20Board/Interface%20Board%20Schematic.pdf

It is from the open-source Macchina M2 CAN Bus network analyzer.

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  • \$\begingroup\$ @MassimoOrtolano good to know, I added the link \$\endgroup\$
    – Jonathan Hale
    Apr 1, 2021 at 16:07

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The MOSFET/PNP circuit looks to me more like load-dump over-voltage protection, rather than reverse-polarity protection.
If the input voltage rises much above the D3 zener voltage then Q2 begins to conduct, pulling the PMOS gate up towards its source and switching it off, which cuts off the output.

schematic

simulate this circuit – Schematic created using CircuitLab

enter image description here

An automotive electrical environment can be really nasty, and 'load-dump' (if you're not familiar with the term) refers to a situation where you may have a heavy load on the electrical system drawing a large current from the alternator, and then this load is suddenly switched off.
The alternator takes a finite time to react, so for some period it's trying to push the same amount of power into the vehicle's electrical system as it was before - but since there's no longer that load drawing a lot of current, the voltage rises - and this voltage rise can be quite significant.

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  • \$\begingroup\$ That simulation is very helpful, thank you. I added reverse voltage to the sweep and confirmed that this circuit does not do reverse voltage protection. \$\endgroup\$
    – Jonathan Hale
    Apr 1, 2021 at 16:43
  • \$\begingroup\$ If there are significant load dump voltage rises, would it make sense to move those 3 filter capacitors to the other side of the MOSFET? It seems like a load dump would blow them out (they're 25V rated). \$\endgroup\$
    – Jonathan Hale
    Apr 1, 2021 at 21:00
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    \$\begingroup\$ Well... in theory that SMBJ14A TVS should clamp to about 23-24V, but I'd certainly feel safer with those 25V rated caps on the other side of the protection circuit. They do provide some noise filtering though (along with the ferrite bead) which might prevent the protection circuit from kicking in for very brief spikes at the input. Maybe consider bumping their voltage rating up to 35V or more. \$\endgroup\$
    – brhans
    Apr 1, 2021 at 21:23
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The MOSfet is not configured for reverse polarity protection.

It is arranged so that it will turn OFF on over voltage. So it is over voltage protection.

At voltage below 16V the MOSFET will be turned on by bias through R4.

When the input voltage goes over ~17V the transistor Q2 turns on by current through the zener diode D3. Q3 takes away the bias voltage for the MOSFET and turns it off to protect the following circuit.

The MOSFET does not do reverse polarity protection because of its internal body diode.

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  • \$\begingroup\$ OH OK, I thought the Zener was just there to protect the gate of Q2. But now that you explain it, I see how that would be used to switch off the MOSFET, isolating during an overvoltage condition. \$\endgroup\$
    – Jonathan Hale
    Apr 1, 2021 at 16:13

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