Looking at the Wiki on computer PSUs, the rise and fall of multi-rail PSUs was a result of regulatory requirements, and was brought in as part of the ATX 12V v2.2 spec.

3.2.4. Power Limit / Hazardous Energy Levels
Under normal or overload conditions, no output shall continuously provide more than 240 VA under any conditions of load including output short circuit, per the requirement of UL 1950 / CSA 950 / EN 60950 / IEC 950.

The requirement was later deleted from the ATX 12V v2.3 spec.
Single vs. multiple +12 V rail


{I'd always thought it was a cost thing- lots of lower current rails would be cheaper to build than one honking great rail for a high power PSU.}

---

Grant
Darwin NT

Single rail is certainly easier to design and manufacture, the component count is lower, and not just capacitors but power devices, resistors, would components etc..
Internal cabling is very easy to manage in both single- and multi- rail designs, and external, is up to the user. I don't know of a single-rail computer PSU that doesn't have a whole variety of cables and or sockets sprouting from it. Indeed being presented with a totally de-labeled PSU without taking the case off it is all but impossible to tell what type it is.
Properly designed a multi-rail should be more efficient across a wider range of loads, but that isn't always so, but most consumer grade PSUs are a long way down on the best industrial design devices, but then with industrial devices they are designed against a known load whereas consumer devices have to cope with a much wider range of loads.

---

Bob Smith
Member of Seti PIPPS (Pluto is a Planet Protest Society)
Somewhere in the (un)known Universe?

Where dual rail PSU's really fall down is in systems with GPU's that pull all their power from the motherboard which usually leads to the motherboard rail becoming unstable and overloaded while there is almost no load on the other rail.

With multiple rails you must work a balance out between those rails while good single rail PSU's don't require that at all and why I prefer to use good single rail PSU's wherever possible over multi-rail jobs.

Cheers.

The problem I see with multiple rails is, do you know how to balance the loads...

Therein lies the design compromise and the usage and utilization problem...

Single rail design is the obvious way to go. Better lower supply ripple and lower noise. Far more responsive to load. And no concerns for the user about needing to load balance on separate less powerful supply rails.

Additionally: Cheaper to make due to fewer supply ripple capacitors needed. The single rail controller should be no more expensive than for multiple rails, and you can get a cost reduction for spreading the load over more phased power transistors... Less supply ripple for the supply capacitors also so a further cost reduction there also.

No brain needed: Go single rail.


Shame that Marketing and profits artificially bump up the price...

Happy cool crunchin',
Martin

---

See new freedom: Mageia Linux
Take a look for yourself: Linux Format
The Future is what We all make IT (GPLv3)

Many thanks for the replies. Not quite sure I'm any the wiser though. It would seem to me that splitting the load 2 or 3 ways would balance out the power load for stability rather than shoving it all down 1 rail, but I appear to be wrong. Anyhow the low end 750W max PSU's that I would ever need come with multiple rails anyway.

I'm simply not in the market for 3 x GTX 1080 in a single machine like some people can apparently afford. Distributed computing is simply a hobby to me, not a way of life, and is funded accordingly on a pension. My low end machine runs a 750Ti quite happily on a 550W PSU, I only upgraded that because it was an office Dell machine with a 350W PSU meant for 9-5 buisness use, not 24/7 crunching. My top ma/chine is designed as a server anyway, and has 2 x GTX960 cards in it and it came with an 850W PSU as standard, more than adequate.

But the replies are appreciated, thank you.

The problem I see with multiple rails is, do you know how to balance the loads, or would you, as most do, run the psu output cables as neatly as possible. An example, of neatly, being to take one output and run it near the front of the case and power all the hdd's, the cd/dvd drives and the front panel fans. Not realising that the start/stall current of a motor is, as a rule of thumb, about 3.5 times the nominal running current.

Many thanks for the replies. Not quite sure I'm any the wiser though. It would seem to me that splitting the load 2 or 3 ways would balance out the power load for stability rather than shoving it all down 1 rail, but I appear to be wrong. Anyhow the low end 750W max PSU's that I would ever need come with multiple rails anyway.

I'm simply not in the market for 3 x GTX 1080 in a single machine like some people can apparently afford. Distributed computing is simply a hobby to me, not a way of life, and is funded accordingly on a pension. My low end machine runs a 750Ti quite happily on a 550W PSU, I only upgraded that because it was an office Dell machine with a 350W PSU meant for 9-5 buisness use, not 24/7 crunching. My top ma/chine is designed as a server anyway, and has 2 x GTX960 cards in it and it came with an 850W PSU as standard, more than adequate.

But the replies are appreciated, thank you.

And to keep it simple.
A single rail PSU should be more stable, but may be less efficient at part load. But like many things it depends on the exact design.....

---

Bob Smith
Member of Seti PIPPS (Pluto is a Planet Protest Society)
Somewhere in the (un)known Universe?

Is there a definitive answer to which is better and why, single rail or multiple rail PSU's. The dearer up-market ones seen to have single rail designs.

https://www.youtube.com/watch?v=rC-ImSeYn2A

---

_________________________________________________________________________
Addicted to SETI crunching!
Founder of GPU Users Group

Is there a definitive answer to which is better and why, single rail or multiple rail PSU's. The dearer up-market ones seen to have single rail designs.