Old Fart OverClocker
Join Date: Aug 2004
Location: Las Vegas, NV.
SLI & Crossfire Power Supplies
SLI & CF PSU's
Originally Posted by webstien
I had to make this
As you will see all PSU's recommended for highend systems are Single +12V Rail PSU's.
This has now become necessary for several reasons:
PSU manufactures are now assigning the rails on their multi-rail (3 or more +12V rails) PSU's in whatever fashion pleases them.
Motherboard manufacturers are now requiring additional power connectors, beyond the standard ATX & P4/EPS connectors.
Sometimes the interaction of these unique rail assignments & additional power connectors results in mobo/PSU incompatibility.
All of these problems are of course caused by the ever higher power demands of ever more powerful GPU's.
Most Dual +12V Rail PSU's are not effected as one rail "should always" be used exclusively to power the CPU via the P4/EPS connector, however Dual Rail PSU's that follow the specs can not power highend SLI systems.
Therefore all PSU's recommended for highend systems now need to be Single +12V Rail PSU's.
Please note that the Seasonic built Corsairs & Antecs (HE & Trio) are, regardless of advertising,
Single +12V Rail PSU's, in that there is no per-rail current limiting, meaning that any +12V rail can pull whatever +12V amperage is available. This is also true of the Seasonic M-12's.
The Corsairs are built to higher standards than the Antecs.
From PC P&C's Power Supply Myths Exposed!: http://www.pcpower.com/technology/myths/
8. ARE MULTIPLE 12-VOLT RAILS BETTER THAN A SINGLE 12-VOLT RAIL?
With all the hype about multiple 12-volt rails (ads claim that two rails is better than one, five is better than four, etc.), you’d think it was a better design. Unfortunately, it’s not!
Here are the facts: A large, single 12-volt rail (without a 240VA limit) can transfer 100% of the 12-volt output from the PSU to the computer, while a multi-rail 12-volt design has distribution losses of up to 30% of the power supply’s rating. Those losses occur because power literally gets “trapped” on under-utilized rails. For example, if the 12-volt rail that powers the CPU is rated for 17 amps and the CPU only uses 7A, the remaining 10A is unusable, since it is isolated from the rest of the system.
Since the maximum current from any one 12-volt rail of a multiple-rail PSU is limited to 20 amps (240VA / 12 volts = 20 amps), PCs with high-performance components that draw over 20 amps from the same rail are subject to over-current shutdowns. With power requirements for multiple processors and graphics cards continuing to grow, the multiple-rail design, with its 240VA limit per rail, is basically obsolete.
PC Power and Cooling is once again leading the industry. All of our power supplies now feature a large, single 12-volt rail. The design is favored by major processor and graphics companies, complies with EPS12V specs (the 240VA limit is not a requirement) and is approved by all major safety agencies such as UL and TUV.
From Silverstone: http://www.silverstonetek.com/products-da750.htm
"With the knowledge that single +12V rail power is the next generation power supply, SilverStone Technology introduces Decathlon series, offering enthusiasts more choices with single rail power selections to handle their system."
FSP Group (Fortron Source) BoosterX 3, 300W SLI Graphic Power Supply
Why Recommend Only Single +12V Rail PSU's for 600W or more?
An example of multi-rail PSU vs. mobo incompatibility:
DFI high performance mobos tend to be non-standard & very picky about PSU's & RAM.
The DFI Expert will not even boot with the now discontinued PC P&C Turbo-Cool 850W (3x +12V rails) or
most (non-server edition) PC P&C Turbo-Cool 1KW (4X +12V rail) PSU's, when the extra power connectors required/recommended by DFI are used. The use of these non-spec connectors crossed 2 of the +12V rails, the PSU saw this as a short & immediately shut down. PC P&C now offers ONLY single +12V rail PSU's!
Many mobo manufactures are putting non-spec power connectors on their mobos to feed these power-hog GPU's & it seems history is repeating.
The OCZ PowerStream 600W often can not power SLI rigs that the PowerStream 520W (single +12V rail) can easily power, this is due to the dual +12V rail design & the design's "per rail" current limiting. This was proven at DFI Street & agreed to by the OCZ engineers.
There are many similar examples, but none as thourghly researched as these & with the full agreement from the manufacturers.
By ATX/AMD/Intel specs the current per rail can not exceed +12V@20A, so no dual rail PSU that complies with the official specs can power a highend SLI/CF system.
There are no real specs for PSU's with more than 2X +12V rails & each manufacturer does as they please, so here are some examples.
Example for a 3X +12V rail PSU:
+12V1 powers the CPU via the P4/EPS connector
+12V2 powers the mobo, drives, fans & everything except the GPU's via their power connectors
+12V3 powers the GPU's via their power connectors
+12V1 & +12V2 are fine & can easily do their jobs within the +12V@20A per rail limits
+12V3 may not be able to do the job depending upon the GPU's.
Example for a 4X +12V rail PSU:
+12V1 powers the CPU via the P4/EPS connector
+12V2 powers the mobo
+12V3 powers all drives, fans, etc.
+12V4 powers the GPU's via their power connectors
+12V1, 12V2 & +12V3 are fine & can easily do their jobs within the +12V@20A per rail limits
+12V4 may not be able to do the job depending upon the GPU's.
R600's apparently will be able to draw up to 270W each, 540W per set, now 75W per card, 150W per set can come from the PCIe mobo connectors, so that leaves 195W per card, 390W per set that must be powered through the GPU power connectors, this is all +12V, 390W / 12V = 32.5A, as only 20A is available we have a problem.
Use other GPU's with other wattages, but the problems remain.
The same problem can occur with large HDD arrays, as each HDD pulls almost +12V@3A during spin up,
if you have 8X or more HDD's, without staggered spin up, then you could have a PSU OCP problem at boot.
Now since we have no idea, short of disassembling the PSU, to determine what rail powers what &
it changes not only from brand to brand, but it even changes on different models & without public notice on different revisions of the same model, therefore the ONLY universally safe & sound practice is to exclusively recommend single +12V rail PSU's for 600W or higher PSU's.
I HATE typing this much, but everyone willing to read this deserves a through explanation.
Dedicated Multi-GPU Graphic Power Supply Unit
From $66: http://froogle.google.com/froogle?so...line&scoring=p
If you have a good PSU, but need more power for SLI,
just put this +12V (only) GPU PSU in an empty 5.25" bay!
Jason's (EOC) Reviews: http://www.extremeoverclocking.com/articles.php#13
jonnyGURU's Reviews: http://www.jonnyguru.com/reviews.php
Legit Reviews: http://www.legitreviews.com/articles/power_supply/
Nvidia SLI Certified: http://www.slizone.com/object/slizon..._powersupplies
For more information: http://forums.extremeoverclocking.co...2&page=1&pp=20
Last edited by davidhammock200; 03-18-2007 at 11:30 PM.