Problems between recent ATX12V v2.xx compliant power supplies and many motherboards have been raised often in the SPCR forums. The most common symptom is an explicable failure to boot. This problem seems to crop up with surprising frequency in the SPCR forums.
In the past few weeks, we've tried to replicate the problem in our lab with a variety of different motherboards and power supplies. We also spent time talking with both PSU and motherboard engineers from many companies. We believe we now understand the problem: It relates to high efficiency PSUs and minimalist low power systems.
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.
Usually an older or less advanced, less efficient PSU will not exhibit this problem with the same system; their minimum 12V power requirement is lower.
Adding more load on the 12V line is the obvious way to overcome the problem, but the common strategy of adding another hard drive does not always work. The typical HDD may take a little too long to ramp up to full current demand on startup, due to its electro-mechanical nature.
A more sure-fire way of ensuring startup is to add a video card. This is not always desirable, as total heat and power consumption will be increased; low power/thermals are usually key reasons for such systems in the first place. Other add-ons such as a sound card, modem card, or TV tuner card may help breach the minimum 12V load. Another fan or two could also work.
A technical solution is to add an artificial load of 6~8W on the 12V line. This could be anything from a DC light bulb to a passive >10W resistor. The problem again is that total power will be increased. However, if you can figure out how to add this load, you can probably also figure out how to add a switch to take the extra load out after bootup. (NOTE: Be aware that sleep mode could well put the system back down to less than minimal load, however. This will cause problems.)
Some PSU makers are aware of this problem, and some of them are working on a dynamic loading solution that will ensure proper operation even with extremely low load -- without affecting high efficiency.
ADDENDUM, Aug 24/06
As mentioned above, 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 fairly 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 -- 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 in the peak of the Prescott era when startup surge became quite serious, and the 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".
I don't have concrete information about the delays involved.... however, some of 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 bog-standard 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 this 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. Trust me, you simply don't get a surge that big at startup with 99.99% of computers, enthusiast or not.
The upside of all this is that most PSU makers -- well, Seasonic, at least -- 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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Last edited by MikeC on Thu Aug 24, 2006 2:25 pm, edited 2 times in total.