High efficiency PSUs causing boot problems w/some mobos?

[mcaren]

I recently purchased the following two components as part of a new computer build:

FSP Group (Fortron Source) AX400-PN, RoHS, 12cm FAN, version 2.2, 2 SATA, 20+ 4 pin, PCI Express, 400W Power Supply

Intel BOXDG965WHMKR LGA 775 Intel G965 Express ATX Intel Motherboard

I could not get the motherboard to post -- I finally contacted Intel who allowed me to return the motherboard for a replacement.

I now have the replacement and am having problems with the new motherboard. I briefly get power to the fans, and then they power down, followed by another brief power-on -- then nothing. I've tested the motherboard to death and had begun to wonder if this new board was dead, but I found a posting on another forum that has me wondering...

It turns out that high efficiency power supplies -- such as those that meet 80 Plus standards but also many others that don't -- typically need a higher minimum load on the 12V line(s) than older or less efficient power supplies.

It also turns out that some recent vintage motherboards have a bit of a delay before the 12VDC power is provided to the processor. This might be to stagger the 12V power load so that the surge to the PSU is reduced. As many PSUs have a current limiter to keep the maximum power on any DC output to <240VA (to meet UL/EN safety standards), this may be useful with high power systems; If the current limit is triggered at boot, the PSU (and the system) would power down immediately.

In a minimalist system where on-board graphics is used, this 12V turn on delay in some motherboards causes the initial total current on the 12V line to drop below 1A. If the delay is long enough, then the PSU will not start.

Too low a 12V load on startup is the most common cause of a high efficiency PSU not starting with the kind of low power minimalist system that many SPCR readers build.

I had this PSU tested (assuming that if it provided electricity that it would not be the source of the problem) - it tested fine. I have contacted FSP to find out if this is an issue -- will post when I hear from them. Some of the messages I've read about this explain that adding video cards (or other electronic devices that add to the PSU load with no lag) allows the motherboard to boot. I don't have a video card to test -- will probably try an old PSU in the morning to see if that works.

Have any of you heard of this? mcaren

 

[doolittle]

Well the AX series are really FSP's budget line (no active PFC and only 70%+ efficient) so that may not be the case.

But there is nothing stopping you from getting some 10 ohm/20w power resistors to put across each rail (everyone has a few of those laying around right? ). Each should give 1.2A/14.4w on your +12v. Maybe try the PCIE connector first (if it has one) to simulate a GPU load.

You can also try to jump-start the psu moboless and see if it fires up at all. Beep codes may be useful if you remove critical components like the cpu and/or memory.