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I'm using this MCU board that is PoE-powered and it gets out 12V from pin 20. I'm also using an LM2596 in order to get those 12V and then power out 5V to power all board peripherals.

Also, I'm using the same LM2596 to manage the DC input (8-36V).

The issue is that I need that the MCU board will output 12V or will get the 12V from the DC side, because I need also the 3.3V that comes from MCU board.

At this time, I need a sort of switch mechanism that implements this logic:

  • When DC Input is on, board will be powered from DC input (12V through pin 20 and the MCU 3.3 step-down converter will power up the 3.3V branches)
  • When PoE is on, board will be powered from PoE and the 12V gets through pin 20
  • When both PoE and DC input are there, it will be powered from the DC input.
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  • \$\begingroup\$ powered from the input that has most high voltage - why? Do you expect brownouts? \$\endgroup\$
    – Reinderien
    Dec 11 '20 at 14:19
  • \$\begingroup\$ I mean that if there are two powers, one at 11,89V and the other one at 12,01 it will pick the second one. \$\endgroup\$
    – VirtApp
    Dec 11 '20 at 14:30
  • \$\begingroup\$ I understand. But why? What if one is 12 and one is 13? At what threshold do you start to worry about damage to the rest of the circuit? \$\endgroup\$
    – Reinderien
    Dec 11 '20 at 14:31
  • \$\begingroup\$ You're right, the external DC input will accept only 12V not more \$\endgroup\$
    – VirtApp
    Dec 11 '20 at 14:33
  • \$\begingroup\$ Also, i've made an edit, my bad...When Both PoE and DC input is there, it will be powered from the DC input \$\endgroup\$
    – VirtApp
    Dec 11 '20 at 14:37
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The simplest incarnation of a power selector is two low-forward-voltage-drop Schottky diodes:

schematic

simulate this circuit – Schematic created using CircuitLab

This will not do anything "intelligent" - it will roughly do what you asked originally, which is to pass the highest of the two input supply voltages to the power rail of the board. If you need anything more complicated you're probably going to need either integrated or discrete high-side switches, but there's not enough detail in your requirements to say whether that's needed. You're going to need to figure out:

  • How much current can the POE and PSU inputs provide at most?
  • How much current will the 12V input draw at most?
  • Does the board have (any, or enough) over-voltage and/or over-current and/or TVS protection for your application?
  • Does the board have enough filtration for your application?
  • Do you need fusing?
  • What, if any, specific behaviour do you want to happen on brownouts and switches in either direction?
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