I’m a beginner and acutely aware of my knowledge gaps! I've done my reading before bringing this question, but I didn't find (or understand) what a solution might be. Your help is much appreciated 
Context: I have built a file server using a headless Raspberry Pi 3B and two 3.5 HDDs. It is powered by a 100w AC/DC PSU which outputs DC 12v and DC 5v rails.
I would like to add a separate DC power input so that the server can be run from a 12v battery/regulator unit when AC power is not available. I intend to use a 12vDC-5vDC buck converter to create a 5v rail from the 12v DC source.
The DC power input is NOT intended for seamless redundancy for the AC/DC PSU. The server will be deliberately powered up in either ‘ac mode’ or ‘dc mode’ using an On-Off-On switch. Automatic switch-over between supplies is not needed.
Problem: I am under the impression that I should prevent current from ‘flowing back’ into the output terminals AC/DC PSU when the server is being powered in ‘DC mode’. One way of achieving this is to place suitable diodes between AC/DC PSU’s DC output terminals and the rest of the circuit. However, I believe that this will result in a voltage drop when the server is powered in ‘AC mode’. Under-voltage of 0.3v on the 5v rail may cause instability in the Raspberry Pi.
Question 1: is it best practice to prevent current ‘flowing back’ into the output terminals of the AC/DC PSU if the PSU is not in use? What problems might this cause?
Question 2: Is it possible to prevent current from ‘flowing back’ into the AC/DC PSU by means other than diodes? I have been considering using relays, but I know little about them. I've also heard talk of MOSFETS, but again, they're beyond my competence at this time.
Thanks
Context: I have built a file server using a headless Raspberry Pi 3B and two 3.5 HDDs. It is powered by a 100w AC/DC PSU which outputs DC 12v and DC 5v rails.
I would like to add a separate DC power input so that the server can be run from a 12v battery/regulator unit when AC power is not available. I intend to use a 12vDC-5vDC buck converter to create a 5v rail from the 12v DC source.
The DC power input is NOT intended for seamless redundancy for the AC/DC PSU. The server will be deliberately powered up in either ‘ac mode’ or ‘dc mode’ using an On-Off-On switch. Automatic switch-over between supplies is not needed.
Problem: I am under the impression that I should prevent current from ‘flowing back’ into the output terminals AC/DC PSU when the server is being powered in ‘DC mode’. One way of achieving this is to place suitable diodes between AC/DC PSU’s DC output terminals and the rest of the circuit. However, I believe that this will result in a voltage drop when the server is powered in ‘AC mode’. Under-voltage of 0.3v on the 5v rail may cause instability in the Raspberry Pi.
Question 1: is it best practice to prevent current ‘flowing back’ into the output terminals of the AC/DC PSU if the PSU is not in use? What problems might this cause?
Question 2: Is it possible to prevent current from ‘flowing back’ into the AC/DC PSU by means other than diodes? I have been considering using relays, but I know little about them. I've also heard talk of MOSFETS, but again, they're beyond my competence at this time.
Thanks
