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In a circuit I have three possible power sources: (a) USB with 5V (500mA max), (b) 7.4V+ via a buck converter (3A max) and (c) a direct pin to the 5V rail in the cicuit, 3A max.

The problem is now when source (c) is connected I'd like to disconnect the buck converter (as I fear there might be some problems when it is powered with 5V in reverse direction and open-loop at its input) with a diode or MOSFET, but at 3A the voltage dropt at the diode is too high (the microcontroller needs 4.5V at least). Using different sources or a different microcontroller is not possible.

So how could I do this then? Is it that of a problem when there is about 2.5V at the buck converter in reverse mode, which happens when I use a MOSFET as diode (gate to ground, source to 5V rail and drain to the converter), or can I "disconnect" it complete without voltage drop when there is a power source connected to pin (b) and nothing to pin (c)? I am not really experienced with MOSFETS sadly :/ It should be possible to tune the converter to more than 5V at least, I think up to 5.5V max.

Also, do I need some kind of protection for the USB-connection? Like also disconnecting it with a MOSFET when there is another source connected? Or can the internal diode of those USB-hubs handle my situation?

Thanks for your help.

UPDATE

Oh I totally forgot to mention that I will be using a LT2596 buck converter with the following setup, but with 5V+ setup (or without the two resistors at the top when I choose the 5V fixed version):

LM2596

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    \$\begingroup\$ What circuit is the buck regulator - it may be perfectly fine but without a circuit it's guess work. \$\endgroup\$ – Andy aka May 24 '14 at 17:02
  • \$\begingroup\$ 0.5V / 3A = 167mohm \$\endgroup\$ – Ignacio Vazquez-Abrams May 24 '14 at 22:02
  • \$\begingroup\$ I am sorry but how does this calculation help me? \$\endgroup\$ – gummiwipfel May 25 '14 at 17:35
  • \$\begingroup\$ It's the maximum R_ON for the MOSFET. \$\endgroup\$ – Ignacio Vazquez-Abrams May 26 '14 at 20:44
  • \$\begingroup\$ A popular solution is to use a point-of-load switching regulator feeding the target device, with the multiple voltage sources hooked to the regulator's input via diodes. You may be able to find a buck regulator with less than 0.5 Volts minimum headroom (so that even the 5 Volt USB supply delivers the desired minimum of 4.5 Volts to the load), failing which a buck-boost auto switching regulator for less that $3 does the trick. \$\endgroup\$ – Anindo Ghosh Jun 27 '14 at 8:39
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Rather than approaching the issue from the point of view of the assumed solution, here is an alternative, and extensively used solution:

Use a point-of-load switching regulator to supply the desired 5 Volt supply to the target device (your microcontroller board), sourcing the supply for the regulator from whatever upstream voltage source is available.

Any number of voltage sources can be hooked to the regulator using diodes, one per source line.

By using an auto-switching buck-boost regulator, great flexibility is obtained in upstream voltage options, while maintaining the target 5 Volt output from the regulator. This particular example supports adjustable output from 1.25 to 35 volts, with input ranging from 5 to 30 Volts, with a current rating of 4 Amperes.

Here is a simplified schematic for the specific application requirements stated in the question:

schematic

simulate this circuit – Schematic created using CircuitLab

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You should be fine just using a SP3T (single pull triple throw) rocker switch. Just make sure the current rating is adequate and that it is break before make (breaks connection before making connection with next position).

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