Low cost power supply redundancy

I'm looking at coupling the output of two computer PSUs, driving a fairly stable high power project (around 300-400W peak) most likely using all output rails (12V, 5V, 3.3V) all well within the rated limits of a single PSU. Ignoring issues balancing load between rails on a single PSU, I want to use a second supply as a backup to take the load if the first one fails.

I have two questions for the electrical engineers out there:

1) can I simply use two high current power diodes as simple backflow prevention to allow failover to occur reasonably transparently, by using a common ground and tapping off +12V between the diodes below?

+Vin (1)   ~ o----->|----o----|<-----o ~   +Vin (2)
+Vout


I'm thinking of using one or more bridge rectifiers to do this, purely from a cost perspective - they're cheap and ready to use (rated ~50A, maybe more). I'd be applying +V from each PSU to the each of the two AC inputs and taking the output off the + terminal. Is there any problem inherently with this design?

2) I'm having trouble understanding the datasheets, I'm trying to work out (a) what the forward voltage drop will be if I feed in 12V, 5V or 3.3V, hence what the heart dissipation requirements will be, and (b) determine if the voltage drop or heat dissipation requirements are going to be too high for my requirements, in which case I'll work out a relay-based solution instead. Take this one for example:

http://www.farnell.com/datasheets/2341003.pdf

It lists "Forward voltage per leg" as 1.2V, is this the voltage drop? At what temperature? Or is it the minimum forward voltage?

Should I be looking at Schottky diodes instead, as a 1.2V drop is significant on a 3.3V line? Are there any other options I could consider?

• shottkies are better for voltage drop than silicon. Best of all are FETs, but you will need to drive them on, so more complicated, and more expensive than simple diodes. Mar 8, 2018 at 10:06
• Yeah, I was trying to keep this side simple, but FETs are another option, you're right. Mar 8, 2018 at 10:35