Backup Power Switch

I have two DC power sources 5.0vdc each.
A - primary
B - secondary

B is always connected.
Trying to figure out a circuit that would use source A unless its not available.
There are a lot of of OR-ing controllers that allow you to do this, but they depend on small voltage difference between the sources and basically select the source with higher voltage.
Both of my sources are at 5.0V but "A" must have priority.

Thank you,

Boris.

Let's assume your load is happy with a supply of 5V +/- 0.1V.
If you could raise A to 5.7 and B to 5.5 then you could simply connect each via a respective diode to the load (which would receive either 5.1 from A or 4.9 from B). In the presence of A the B diode would have only 0.4V across it so would pass a negligible current.

How much current does it need to switch?

Is it a hot swap? Can you use a relay?

Current - 1A
Yes, hot swap (so no interuption in the power supply)

Boris.

Put a diode in series with the the B power supply and keep them both hooked up.