# Dual Power supply switching

I have two PCBs: PCB1 and PCB2. Both PCBs have a 5 V power supply connectors. Both PCBs are also connected with each other via extension connector. When 5 V connector of PCB1 is supplying power, then it will run both PCBs and when 5 V connector of PCB2 is supplying power, then it will also run both PCBs. The issue here is if both power connectors are connected then I want that only 5 V connector of PCB1 should supply power, 5 V connector of PCB2 should not conduct. I am stuck at this issue and unable to figure out any solution yet. I am attaching a block diagram to help understand better.

if both power connectors are connected then i want that only 5v connector of PCB1 should supply power, 5v connector of PCB2 should not conduct.

This can be done with a modified power ORing:-

simulate this circuit – Schematic created using CircuitLab

V1 and V2 are the individual supply inputs for PCB1 and PCB2, respectively. Blue and grey lines are the common supply rails through the interconnection (NOTE: GNDs are shorted together through common rail).

• When V1 is present and V2 isn't, common rail will see V1 so PCB2 will be supplied from V1.
• When V2 is present and V1 isn't, common rail will see V2 (minus a diode drop) so PCB1 will be supplied from V2.
• When both V1 and V2 are present D1 will block current flow so the common rail will see V1 only, therefore PCB2 will be supplied from V1.

Considerations for D1:

• Current ratings should be high enough than the total consumption (average/steady state and peaks).
• Forward drop should be low enough to keep losses minimum.
• Reverse breakdown voltage should be higher than any of the supply voltage's maximum value (For your case I'd put 5V + 10% at least).