Efficient PSUs that don't start

Silent Mike's Blog

The following discussion has been added as a section to the Power Supply Fundamentals & Recommendations article. It addresses the issue of modern, high efficiency power supplies that sometimes don't work with some modern motherboards.



Higher efficiency PSUs generally tend to need higher minimum power on the 12V line in order to simply run. Typically, we're talking about 1A or greater. Older, less efficient PSUs have much lower minumum current needs, under 0.5A and often ZERO.

In some recent motherboards, there are various time delays implemented in order to ensure that the PSU (and motherboard) is not subject to a huge current surge when everything turns on all at once. Many Asus boards have been identified as doing this, al though you won't get Asus to talk about it — I tried — they will say it's proprietary information they don't want to share with competitors. They are not the only board makers doing this.

The practice began during the peak of the Prescott era when startup surge became quite serious, but before the 80% efficient power supplies became common. Board makers extended the practice to AMD boards as well.

So this means, for example, that there could be anywhere between tens and hundreds of millseconds between different portions of the board and components being powered up. Just how much delay there is and how much power the CPU/VGA draw affects whether one of these high efficiency PSUs will actually start. Sometimes, adding HDDs will help, sometimes not — they may not pull current soon enough after the power button is pressed to change the current demand the PSU "senses". If the current sensor detects too low a load, the power supply usually does not start.

I don't have concrete information about the time delays involved. However, the Asus boards that would not start with some high efficiency Seasonic PSUs also would not start with some high efficiency PSUs from Antec, Fortron-Source, and Enhance.

A sure-fire way to tell whether too-low 12V start current is the problem is to hook up a known working, older, generic 300W PSU to the afflicted system. If max power was the problem, it would have a hard time starting, or not start at all. But invariably, with these too-low 12V start current situations, such PSUs (even several years old ones that long precede 24-pin ATX outputs, etc.) will start the system fine.

The reality is that most of the better brands like Seasonic and the others mentioned above are going for high efficiency because it is one of the big differentiators between PSUs today, and also very high power output. There are few PSUs that put less than 80% of the total power rating available on the 12V rail. For a 400W PSU, this typically means 320W is available on the 12V lines. You simply don't get a surge that big at startup with most computers, enthusiast or not, so even older PSUs should start fine on most systems

The upside of all this is that most PSU makers are aware of the issues here, and they are implementing solutions. The simplest one is to add just enough internal resistance on the 12V rail to ensure that there is enough current draw to start the PSU even with no 12V draw from the outside. This naturally drops the hard-earned efficiency down a notch, but it is in fact, what some PSU makers have done.

I know that Seasonic has quietly implemented an active circuit that automatically inserts enough of a load so that the 12V line always sees the minimum load, at least, but then this extra resistance is removed when the load gets higher, so that turn-on is never a problem, and high efficiency is maintained at normal and high power operation.

Which Seasonic models? I believe all the current sleeved output cable S12s, the S12-80+ models and the soon to come M12s.

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