# What is ORing and why is it named ORing?

I read some documents like the PoE application, it used ORing to protect the reverse input current. I would like to know what ORing is. I googled it, but I cannot find the answer. It seems like a diode or something the datasheet uses diode to protect, but I am not sure what this is.

• Can you provide a reference?
– Frog
Jul 14, 2021 at 9:47
• In the power supply context, ORing uses a logic term to describe an analogue circuit function, described well by @TomCarpenter answer below. Jul 14, 2021 at 9:52
• Or-ing, not O-Ring, in case that's how you were reading it. Jul 14, 2021 at 9:55
• @AndrewMorton I have seen O-Ring mentioned in a presentation in front of the entire engineering department. Jul 14, 2021 at 10:39

OR is a logical function - the output is high when either input is high. ORing is simply a way of saying perform the logical OR of a set of signals.

In the context of power supplies, when ORing is used, the output (the device) is powered by whichever supply typically has the highest voltage.

A simple power OR gate can be made by diodes in series with each supply, which prevent power from the higher voltage supply backflowing into the lower voltage supply.

• EEC- No, there are other techniques besides diodes that can be used. FETs are used quite often as they offer lower ON resistance and so lower voltage drop than a diode. Jul 14, 2021 at 11:17
• @EEC this might be a good read. Product specific, but it gives some insight. Keyword: "Ideal Diode" and "Load switch" Jul 14, 2021 at 11:29
• I parsed it a O-Ring and wondered what it was talking about. Jul 14, 2021 at 20:01
• @Joshua - I couldn't decide between or-ing, o-ring and 0-ring. These all sound just like they could be a specific technical term. :D Jul 15, 2021 at 6:58
• @Joshua O-ring, usually used as a seal to couple two parts. Very common part.
– Mast
Jul 15, 2021 at 9:07