A PCB circuit must have capability to be powered from a bench power supply, or USB supply. The bench power supply can be replaced with an external power supply of fixed voltage as well.
The external supply range is 5V to 12V. The USB supply is 5V. What is required is that, both can be connected at same time without malfunction. Only one of them can be attached for normal system operation.
The way to do this, as my knowledge, is to use diode OR operation with the two DC sources. I am not exactly sure why this works in theory or practice. I should choose shottkey diodes since they can have lower Vf. Also, the reverse breakdown of these should be larger than 12V. The current rating of both should be what the circuit power demands if one of them is active i.e if the circuit demands 1A, then both of these must be rated at 1A.
Is this approach correct? Also, does it matter if the USB power is coming from a "floating" device e.g laptop running off batter and not connected to the mains.