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sluggo

about "multi-rail" supplies

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Many folks wonder about "single-rail", "dual-rail", "triple-rail" supplies and which is better or more appropriate for what they plan to do with their machine. I just wanted to clear up what seems to be a common misconception, which is that these rails are independent of one another. In fact, with the exception of one family of supplies (that I know of), there is only one +12 rail in every ATX supply on the market. All the "rails" labeled on the power supply case are actually the same current source and labeled as separate rails only to comply with a UL requirement for consumer-class supply cabling.

 

Every ATX supply (with the one exception) has a single winding on the secondary of the main transformer that generates the +12 voltage. The output of this transformer is filtered and fed to all the +12 circuits in the PC. There are no other isolation or protection devices that would constitute a separate "rail", and all of the +12 lines in the PC go back to a single point in the PSU.

 

The one exception that I know of to this is the new Enermax Galaxy line, which starts at 850W for around $300. It has two separate secondary transformers for +12 and has separate output specs for each.

 

To sum up, the only number that really counts w/r/t +12V is the total +12 current available from the supply. All the "multiple-rail" designs you see are just a single +12 supply with one point of regulation and protection. I hate to see people get all wrapped around the axle on this one issue, which really shouldn't be an issue at all.

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Understand what you said, still confused.

 

You say all +12v "rails" are supplied from the same current source.... how come with one of Mushkin's power supplies, it has 4x +12v rails, each at 20A. How come it cant put out a straight 80A? Its "Rail Fusion" limits it to a mere 48A.

 

What Im saying is how can they rate it at 20A per rail, but when you combine them, you only get 48A max?

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What Im saying is how can they rate it at 20A per rail, but when you combine them, you only get 48A max?

Good question. The short answer is that the supply is capable of sourcing 48 Amps of +12, and the "4x20" specification is simply marketing fluff. Much as it pains me (as a former power supply designer) to see "marketing" applied to something so basic to the operation of a PC, I suppose it was inevitable.

 

The long answer is that the "split-rail" language that you see being used had it's birth in a well-intentioned but widely misinterpreted section of the ATX 2.2 spec. In that spec, a recommended guideline was proposed that would limit the energization (total energy available) of any particular line to 240VA in order to comply with a long-standing UL spec for user-accessible areas of listed equipment. The 2.2 spec also recommended, but did not require, an independently regulated +12 supply for CPU power. To this date, no PC-grade ATX supply has fully implemented either of these recommended specs. What they have done is simply to bundle cables in such a way as to create the "plumbing" for independent 12V rails, but the electronics are unchanged - it's simply a single 12V supply capable of prodigious output.

 

The latest word from Intel is that in the next released version of the ATX spec (we're up to 2.9 on the preliminaries already), the recommendation for 240VA limiting is gone altogether. There will not even be the appearance of a requirement for multiple independent rails.

 

The thinking behind the 240VA limit from UL is a little fuzzy. It's a spec that's been in place for a long, long time, and whether it's to reduce the likelihood of a self-sustaining fire or to protect the user from electrocution has been a matter of much discussion and one for which UL seems to offer strangely little guidance. My interpretation had always been that it was there to protect users of 240V AC lines from getting blasted into next Tuesday if they did something stupid. In no way does a 12V line offer any threat of electrocution, so applying a 240VA limit to user-accessible areas of a PC makes not a lot of sense to me. I think Intel and the ATX spec-builders may have finally gotten UL to come around to this way of thinking. We'll see.

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So the 4x +12v rails each at 20A is a gimmick?

 

This is making it harder and harder on me to not grab a stellar PC Power and Cooling 1kW power supply next time I upgrade some parts... something about 72A continuous, 76A peak on the +12v.

 

Who did you design power supplies for? This is something that interested me, being in college for computer/electrical engineering currently.

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Note that at least one of the PC P&C power supplies at 1000 watts is labeled as a single rail design. While they may not be perfect, they do tend to label their products much more realistically than many other brands.

 

For cheap 'quad-rail' products, note that technically any 'rail' they have is rated at delivering up to 20 amps (or whatever their rating is) but they simply cannot supply that much power to all 4 rails simultaneously. So basically they can get away with saying 4x20 amp rail design since each one can supply that juice independently, but not together.

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So the 4x +12v rails each at 20A is a gimmick?

 

This is making it harder and harder on me to not grab a stellar PC Power and Cooling 1kW power supply next time I upgrade some parts... something about 72A continuous, 76A peak on the +12v.

 

Who did you design power supplies for? This is something that interested me, being in college for computer/electrical engineering currently.

For consumer ATX supplies, the "4x +12 rails" simply reflects a marketing trend that's caught on. Your supply's +12 output is 48A with at least 20A available from individual cable bundles (and probably more), but in no case can you draw more than 48A from the supply on +12, so 80 Amps (as technodanvan said) is simply not going to happen.

 

The Mushkin supply is still good for 576 Watts on +12, which is a lot. Personally, though, if I was spending that sort of money I'd at least expect it to have active power factor correction, which it does not. The Corsair 620HX that I use has 600W capability on +12 (listed as "3x18"), has active PFC, is rated for full output up to 50 degrees C, is dead quiet and comes with a 5 year warranty. It's the same price as the Mushkin.

 

PCP&C makes a quality product, no question. A 1kW supply would be way more than I need, and I suspect is more than most non-quad, non-SLI, non-RAID users would need. How much you need is up to you, but I'd look closely at your power budget before pulling the trigger on that purchase. You can choose from several very, very good supplies and spend no more than $150.

 

I used to design mainframe power systems for one of the large computer companies. Power is still a good field, but the vast bulk of the basic consumer electronics product design work is now done in Taiwan and China with controllers made by companies like Maxim, TI, and PI. Right now I'd be looking more closely at automotive and solar applications. Those look like they'll be fun for the next 15 years or so.

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What Ive got right now is just fine for what I do... no complaints with my Antec. Probably wont get a PC P&C or anything of that caliber until I upgrade from my current rig, and with being in college, I wont be upgrade for a long time... coupla years, hopefully sooner though. ;)

 

PSU design seemed like fun... in my intro to engineering class, for my group design project, we made a 120v AC -> 5v DC @ 3A power supply. Seeing how much went into something as simple as that made it really interesting to look at the guts of the PC P&C supplies.

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To sum up, the only number that really counts w/r/t +12V is the total +12 current available from the supply. All the "multiple-rail" designs you see are just a single +12 supply with one point of regulation and protection. I hate to see people get all wrapped around the axle on this one issue, which really shouldn't be an issue at all.
Not exactly true.

 

Although common voltage/current source, independant current limiting, voltage regulation and filtering are all possible and are offered to one degree or another on many "multi-rail" PSU's.

 

Dave

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