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I am currently designing a PCB for a school team and the PCB has three modes of power. V_USB(5V), V_EXT(5V regulated) and V_Poe(5V). How do I go about creating a power selector that has the hierarchy (EXT-> PoE -> USB). I have looked long and hard for this and discovered some posts that help. For example, so far I think I can use of using this circuit:

Circuit

with a P-MOSFET connected to the second supply(gate connected to first supply) that is normally off. So it would go: (PoE compared with USB, with Mosfet on USB) compared with (EXT with whatever wins the first round(MOSFET here)

However, I scoured digikey for a P-MOSFET that is in depletion mode and I get 0 results.

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  • \$\begingroup\$ You could just use three diodes star-pointed together and whichever supply produces the larget voltage will supply all the current. Beware that the volt drop in a diode can be between 0.3V and 1V depending on current taken. \$\endgroup\$
    – Andy aka
    Oct 24, 2015 at 22:04
  • \$\begingroup\$ I don't think that would take care of the hierarchy problem. Also, can you tell me why you circuit works the way it would? \$\endgroup\$
    – Manan S
    Oct 24, 2015 at 22:12
  • \$\begingroup\$ It operates as the one in your circuit except there are three diodes instead of two and three independant voltage sources feeding the three diodes. How does your 2 diode circuit provide the thing called "hierarchy"? \$\endgroup\$
    – Andy aka
    Oct 24, 2015 at 22:17
  • \$\begingroup\$ hey sorry, the circuit I meant to convey in my original message is found here: imgur.com/dxX2KKm. \$\endgroup\$
    – Manan S
    Oct 25, 2015 at 2:03
  • \$\begingroup\$ That won't work for two reasons. First the parasitic diodes will always cause the mosfets to conduct and second, the mosfets are wired as source followers and the regular fet conduction will be poor. \$\endgroup\$
    – Andy aka
    Oct 25, 2015 at 8:54

1 Answer 1

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You could use an old-school relay switching network like this...

schematic

simulate this circuit – Schematic created using CircuitLab

This is just an outline of the circuit. If you implement it using real relays, you would probably want to put a fly-back diode in series with each relay coil , although some relays have these built in.

You could also implement the same idea using solid state components like transistors, resistors (wired as an inverter) and normal MOFSETs in place of the relays.

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  • \$\begingroup\$ How would the implementation look like with the components you mentioned? I don't have much experience with them, but the I'd like to learn \$\endgroup\$
    – Manan S
    Oct 24, 2015 at 23:30
  • \$\begingroup\$ It is simpler if you can switch the low side (the ground connection) rather then the high side (the V+ connection). Would that work for your application or do you require that all the grounds be tied together and the power line be switched? \$\endgroup\$
    – bigjosh
    Oct 24, 2015 at 23:43

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