Single-rail versus multi-rail power supplies

Single-rail versus multi-rail power supplies

Lines are being drawn in the power supply war. On one side, we have vendors such as Tagan pushing “multi-rail” designs on its big 1KW+ PSUs. On the other side we have PC Power and Cooling which is pushing a single-rail design 1 Kilo Watt model.

If you think of each of these rails as its own power plant, a multi- or split-rail design breaks up the power among multiple power plants. So, on Tagan’s TurboJet TG1100-U95, you get four power plants, each capable of producing 20 amps of 12 volt power or 240 watts per rail for a total of 960 watts. PC Power and Cooling single-rail Turbo-Cool 1KW-SR features 72 amps of 12 volt or 864 watts continuous and can peak at 960 amps all one single rail.

Which design philosophy is better? I won’t draw any conclusions here but I can give you each side’s spin on the issue.

First up is Tagan, which argues that it’s about death and following the rules. A multi-rail design, the company says, is much safer and some international safety organizations don’t allow you to output more than 20 amps per rail. More than that, the company tells me, it could possibly kill you.

Tagan also says a multi-rail design complies with the EPS12V spec which everyone agrees to follow. If hardware vendors follow the EPS12V specifications for power requirements, they shouldn’t violate an EPS12V PSU.

As the sole PSU vendor pushing a single rail design for big PSU’s, PC Power and Cooling’s argument is quite intriguing. Even though the company once also pushed a multi/split rail design, the company has since decided that the single rail is the future.

The problem with multi-rails, the company says, is that power tends to get stuck on the individual rails. If the PSU, for example, allocates 36 amps of power from rail 1 and 2 to the CPU but the processors only consume 22 amps – the rest cannot be reallocated to the GPU or hard drive array. With a single-rail design, if the CPUs only use 22 amps of juice, the rest can be sent to the GPUs or whatever else needs the 12 volt power because it all comes from a single bucket of power.

As for spec’s, PC Power said the spec’s were written in the days when CPU power consumption was a runaway freight train. With Intel and AMD pushing low power chips, locking up X amount of amps for CPUs that will get used is the wrong way to do it. As far as safety goes, PC Power says it has certifications from UL and other international test labs that say it’s kosher.

The practical upshot, if both have done their homework on their PSU designs, is both will work. I think the single-rail certainly sounds more efficient with its send power where it’s needed outlook. But multi-rail designs should and have worked as well up to this point.

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Reloadron

Well time to jump in. I tend to lend myself to no more than two rails of 12 Volt power. I only say two in keeping with the PSU Design Guide.

Looking at main motherboard PSU connector. The earlier 20 pin versions allowed for a single 12 Volt line on pin 10, and the newer 24 pin versions allow for only two 12 volt lines on pins 10 and 11. Cheaper PSUs use AWG 18 wire with a rating of about 9 Amps and better units use AWG 16 wire having an 11 Amp rating. Bottom line the maximum current on a 20 pin connector for 12 Volts is between 9 and 11 Amps and on a 24 pin connector 18 and 22 Amps. The latter exceeding the typical 18 Amps of most PSU 12 Volt rails.

The only other 12 Volt power hungry devices in the machine are the GPU and CPU each generally having additional 12 Volt connectors. We are seeing the number of individual wires for each of these connectors growing recently. OK, I can see a seperate 12 Volt line for the processor simply because the specification calls it out, however, I can see no need for additional individual 12 Volt rails. For what reason?

There seems to me to be just too much hype surrounding all these multiple 12 Volt rails not to mention the hype in general as to PSUs and how much power is really needed. In many PSUs I have taken apart all roads seem to lead to Rome meaning all the 12 Volt rails lead to one or two diode bridges and often a single transformer. The rails offer at best overcurrent protection. No magic and nothing special.

One rail, maybe two. That is my story and I am sticking to it!

Ron

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