Single rail /> multi rail

I have seen it stated that single rail power supplies are better/more stable than their multi rail counterparts. I used to believe that this was the case too, but I have recently learned that this is actually an all too common misconception!

I would explain in detail, however there is already a very good explanation on which I think that everyone should read before developing their own opinion with regards to rail distribution:

In short:

Rail distibution has less to do with the deliverance of power to devices in your system, and is in fact more to do with over current protection (OCP) circuitry. The difference between a single and multi rail power supply could be as simple as the configuration of a single control chip.


At low wattages there is little advantage to having many rails, at high wattages however, (1000w+) multi rail configurations become almost a necessity, as an 80+ amp OCP threshold is unlikely to prevent things from catching fire in the event of an electrical short ;)



Great link; I've read it many times. I, personally, use single 12V rail PSUs on all of my systems. At wattages above 1000, most people are using extremely overpowered PSUs, and not taking use of more than 800W at the max. They aren't running the risk of blowing a cap on that 12V rail, if it is a single rail. Multi-rails at lower wattages can drastically reduce efficiency, taking an 80+ Gold system to 80+ efficiency. Nobody really needs more than 1000W, so multiple, weaker rails are pointless.


I think you should read it again ;)

In a power supply with many 12v rails, there is still only one 12v regulator. Groups of wires are simply monitored for over current protection separately. So there is actually no measureable efficiency deficit either!


Capacitors can discharge very quickly, and in switched mode psu's (pc power supplies for example), high quality low ESR caps are used. So drawing excessive quantities of current from a well-designed power supply doesn't generally cause a cap to blow (barring cheapies of course :P ). Also as I mentioned before, there is only one regulator, and thus only one capacitor serving all of the rails designated to carry a given voltage.

Here is a diagram that should help to clarify things:

The only difference between the two is the positioning of current limiting hardware.

And before you ask, no the current limiting hardware does not have a measureable effect on efficiency ;) In fact, if any of these current limiters reaches it's 'limit' then the power supply deems it an overload and shuts off immediately as if there were a short.

"But what if you load one limiter with lots of hardware, won't the supply shut down erroneously?"

Yes, but only if the power supply is very poorly designed. In practice designers will spread the load of each device across many rails. A pcie power cable for example has 3 physical 12v wires in it, so the load from one pcie cable can easily be spread across 3 rails.

I have not yet found a single substantial piece of evidence to suggest that a single rail is any more stable/efficient than multiple ones, or vice versa. In fact the few pieces of evidence that I have come across seem to indicate that a well-designed multi rail psu is actually a little safer.

Anyone else have any thoughts/opinions?

Having multiple strong rails is fantastic, but a lot of multi-rail PSUs are running 3 or 4 weak, shoddy rails. If I were to ever need more than 500W, I would ptobably step up to a nice multi-rail system, but for the majority of users, it isn't necessary.