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Trend towards single-rail PSUs?

D

drizzt81

[H]F Junkie
Joined
Jan 21, 2004
Messages
12,361
Hello,

I am by far not expert on PSUs and have not kept up with them much. Not to long ago, I remember that multi-rail PSUs were all the rage, since the customers were told that loading one rail more won't affect the output of the other rail, i.e. your GPU pulling massive power won't affect the CPU stability. Over the past few days I have seen two threads linking to 1kW PSUs with a single rail. To me this makes it seem that currently the "trend" of PSU manufacturers is to go back to a single-rail design.
Is there any specific reason for this? I can imagine that split-rail designs may have been too complicated for some users, especially in light of GPUs being the "king of power consumption" these days and some GPU rails being designed with insufficient current capabilities. Are people looking to buy single-rail PSUs to "KISS" and not have to worry about what connector is attached to which rail, goes into what component?
 
I think it has a lot to do with KISS. When psu manufacturers are uncertain as to how much power new gpu's are going to use, it makes more sense to put out single rail units, in case the new cards needed more power than what was being delivered on any one rail. I prefer them myself as it just makes things easier.

I believe Intel had specified a maximum amount of amperage allowed on each rail (I think 22a?), so psu's have been coming out with 5+ rails, which I think is rather ridiculous. Again, much simpler to have it all on one rail.
 
Yeah the ATX spec limited the 12v to 240VA. People were already violating that some time ago though as I have an almost 2 year old SuperFlower with 35a on one 12v among many many many other examples.

The big reason for the shift back to single 12v is things like SLI that draw all their 12v power from 12v1 on an ATX12v2.0 PSU which is shared with everything in the system except the processor. When that happens you can overload the rail. The split 12v idea migrated from the serverside of things and it really is a good idea there.....but there you don't have 2 XXXX19000000xxxx xtxxx Super Special Edition SLI GPU's on the same rail as your RAID array....you simply have your motors for you RAID pulling from the 12v.
 
There's a possibility that there could be an issue with safety on some single-rail PSUs.

One of the reasons for multiple rail PSUs was simple resistance. The typical wires that come from a PSU are between 16 gauge and 20 gauge. Putting a huge amount of power through a wire, say, at 18 gauge, isn't very smart due to the resistance. Also, the connectors add resistance. So let's say you have a PSU with one +12V rail at 60A.

Theoretically, this means you could draw all 60A through one connector. Unfortunately, this means that connector/wire will be heating up immensely due to the resistance. This can be inherently unsafe, cause failure, and even fry itself at some point. So what the ATX spec did was say you can't draw any more than 18A on a single +12V rail.

Which was supposed to mean that the Over-Current Protection on each +12V rail was limited to a certain percentage more than 18A, for safety reasons. This also assisted in load balancing.

Unfortunately, right around this same time, GPUs started sucking down immense power, and so did processors. The P4 processors were very power hungry, as were the GeForce 6800 series and the Radeon X800 series. So now, it's actually possible to draw more than 18A (216W) on the +12V rail, which means we need to have a bit of load-sharing on the rails.

What's going on now is that people are re-specing their PSUs to have a large, single rail. This makes it more obvious to the user how much power can be drawn before OCP kicks in. Multi-rail PSUs can sometimes be confusing that way.

For example, the Corsair HX620 has three rails rated at 18A each. So you'd assume OCP kicks in at 18A, correct? But it also shares power across all three rails, which means that if one rail needs 24A and another needs 11A, it'll adjust itself automatically. So why rate it at 18A? Well, for one thing, when we designed the spec, 18A was the max you could rate each rail at and still meet the ATX 2.2 spec. That's changed slightly. The HX620 has a 50A combined +12V OCP point. So if anybody ever figures out how to draw 50A or more from the +12V rails on the HX620 in real-world usage, it should trigger the OCP and shut itself off.

Anyway, there are pluses and minuses to both designs. I think the best design is a hybrid of the two, quite honestly. I think safety is the most important thing, we're dealing with huge amounts of power here, if something goes wrong or if a PSU is poorly designed, it can be a huge safety issue. After safety, I think stability and power are the most important.

There are lots of companies that sell good PSUs out there, but only a handful that sell great PSUs. I think we would like to be known as a company that sells a great PSU. As such, we're always looking at the next-gen and trying to figure out what the best course of action is. We're testing stuff daily, comparing various designs and contemplating on new specs and cool possibilities. I read this forum and others to find out what you guys want, and then I work with our engineers to design that.

I helped design the HX series to meet your guys' needs, and I think that is going very well. I hope any future products we release meet those needs even better, and I'll work hard at making that happen.
 
Those are good points, but how likely is it that somebody is going to be drawing such a large amount of power through only one connector? Maybe if they attached a Y cable to one molex on the PSU and then from there branched off to a bunch of harddrives and gpus or something, but at the very least you've still got some of that 12v going to the motherboard.
 
I generally cant see a problem with having such high power on a single rail if the wire gauge is fattened and connectors with a large enough contact surface area are used.
Unfortunately I dont believe the ATX spec covers this method of reducing contact and wire resistance, hopefully an ammendment will be made to allow it so ATX compliance can still be achieved.

A downside of using a single rail is reduced noise isolation.
A dual rail PSU (conforming to the ATX specs) uses one rail for the CPU and the rest of the system on the other.
As long as the rails are independant of each other, this should give a higher stability power supply to the CPU.
With more rails, isolation can be further improved but with a reduction in max power from a single rail.

tbh though, it shouldnt matter if multi rail PSU's are used as long as more than one power connector can be connected to high power devices.
The 8800 series of cards use 2 of the more robust 6 pin power connectors.
Adapters are supplied to allow each 6 pin connector to use 2x4 pin molex supplies, theoretically allowing up to 4 power rails to be used!
 
No offense redbeard, I normally don't buy Corsair products, but the Corsair PSUs are TREMENDOUS. Love them. I sell stuff from time to time and I reco ur PSU's. I like the 520 for myself actually.

Thanks
 
Chernobyl1 said:
tbh though, it shouldnt matter if multi rail PSU's are used as long as more than one power connector can be connected to high power devices.
The 8800 series of cards use 2 of the more robust 6 pin power connectors.
Adapters are supplied to allow each 6 pin connector to use 2x4 pin molex supplies, theoretically allowing up to 4 power rails to be used!
Click to expand...

The reason more than one 6-pin connector was used is because the spec for the PEG connector only allows 75 watts to be used through one. And since the 8800GTX does about 165W at full load, it gets 75W through one connector, 75W through the motherboard's PCI-E slot, and the other 15W from the 2nd PEG connector.

At least that is my understanding. The reason the 75W per connector spec exists, though, is for safety reasons. You might be able to do double that on a connector, but really a 2x3 connector is 3 ground and 3 +12V wires. At 75W, those three +12V wires are pushing a little more than 2A each. If they started drawing 5-6A each, they would be running fairly warm, especially with the connector.

I don't disagree that as long as the load is spread out over multiple connectors, it should be fine, but from a PSU manufacturer's standpoint, you can't guarantee that somebody won't do something stupid and just buy your PSU as a 2nd PSU to run their watercooling/video cards/hard drive RAID array/optical drive duplication farm/etc, and run it all off a single molex.
 
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