Corsair sheds light on Haswell PSU support, lists compatible units

If you were reading TR last week, you probably know about Haswell's new low-power state, which causes compatibility problems with some PSUs. When the story broke, Corsair said it believed all of its PSUs were compatible. However, it was still checking to make sure.

Corsair has now publisehd a full Haswell compatibility list. According to that list, all of the company's AXi, AX, HX, TX-M, TX, and GS units support Haswell, as do the CX750M and CX750. Lower-wattage CX-M and CX units are "likely compatible," as are all VS-series models, but Corsair says it's still in the process of validating those.

Corsair has also shed some additional light on the reasons behind Haswell's PSU compatibility woes. We already covered the broad strokes last week, including the fact that Haswell's reduced sleep-mode power consumption trips up the protection circuitry of some PSUs. Corsair's post explains exactly why that is:

According to Intel's presentation at IDF, the new Haswell processors enter a sleep state called C7 that can drop processor power usage as low as 0.05A. Even if the sleeping CPU is the only load on the +12V rail, most power supplies can handle a load this low. The potential problem comes up when there is still a substantial load on the power supply's non-primary rails (the +3.3V and +5V). If the load on these non-primary rails are above a certain threshold (which varies by PSU), the +12V can go out of spec (voltages greater than +12.6V). If the +12V is out of spec when the motherboard comes out of the sleep state, the PSU's protection may prevent the PSU from running and will cause the power supply to "latch off". This will require the user to cycle the power on their power supply using the power switch on the back of the unit.

While we are still working with Intel on the details of the testing methodology they use to check PSUs for Haswell compatibility, it is already known that a power supply that uses DC to DC for the non-primary rails (the +3.3V and +5V) will not have an issue with the new low power sleep states. This is because a DC to DC buck converter is used to convert +12V to +3.3V and +5V. This means that no matter what load the CPU puts on the power supply, there will always be a load on the +12V because the +12V is required to provide power to +3.3V and +5V.

The firm adds that all units marked as 100% Haswell-ready in the compatibility list use DC-to-DC conversion. Lower-end CX-M and CX models, as well as the VS series, apparently use a different conversion method. Corsair likely needs to use a different validation scheme to determine Haswell-compatibility for those units—hence their status as "currently validating."

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