Power Supply Fundamentals

Power | Reference|Recommended
Viewing page 5 of 5 pages. Previous 1 2 3 4 5


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.


Increasingly, switched mode power supplies (SMPS) are designed with an active power factor correction (PFC) input stage. This is mainly to meet new regulations aimed at restricting the harmonic content of the load current drawn from power lines. Both users and power companies benefit from PFC, as does the environment.

Power Factor Correction (PFC) can be defined as the reduction of the harmonic content, and/or the aligning of the phase angle of incoming current so that it is in phase with the line voltage. Mathematically, Power Factor (PF) is equal to Real Power (Watts) divided by Apparent Power (Volt*Ampere). The basic concept is to make the input look like a pure resistor. Resistors have a power factor of 1 (unity). This allows the power distribution system to operate at maximum efficiency, which reduces energy consumption.

Non-PFC power supplies use a capacitive filter at the AC input. This results in rectification of the AC line, causes high peak currents at the crests of the AC voltage. These peak currents lead to excessive voltage drops in the wiring and imbalance problems in the three-phase power delivery system. The full energy potential of the AC line is not utilized. Nonlinear peak currents also distort output voltage and create harmonics. There is an international standard for controlling harmonics (IEC100-3-2) and PFC is mandatory for home appliances consuming 70W or more power in EU nations as of January, 2001.

PFC circuits are classified into two types: active and passive.

Passive PFC uses passive elements such as a ferrite core inductor on the input source to create a countering reactance. While easily applied to the existing power circuitry without much modification, the power factor is low (60 - 80%), the AC input must be chosen (115VAC / 230VAC), and the harmonics produced from the difference between the capacitance and the inductance are hard to control. Significant electromagnetic noise can result.

Active PFC uses switching regulator technology with active elements such as IC, FET and diodes, to create a PFC circuit This circuit has a theoretical power factor of over 95%, reduces total harmonics noticeably, and automatically adjusts for AC input voltage. However, it requires a complex EMI filter and an input source circuit, and is more costly to build.

The benefits of high PF for the user comes from the reduced AC current drawn by high PF PSUs, not in any savings from electricity bills, except in the case of commercial utility users who do pay for V(oltage) x A(mperes). There are two broad consequences:

Less stress on the AC electrical wiring: The lower current drawn by a high PF power supply means that there is less stress on the electrical wiring of the building. This can be a big plus in the case of older building with lower capacity AC wiring. It is certainly easy to see the benefits in a enterprise setting where dozens or hundreds of PCs are drawing power. If the total current load from the IT department could be reduced by 30% or more, this would be very signficant in direct electricity savings, reduced airconditioning cost, and possible avoidance of building AC re-wiring.

Lower UPS costs: Lower current draw also means that smaller capacity Uninterruptible Power Supply (UPS) units can be used. As UPS units are priced in direct proportion to their current capacity (VA), a PF of 0.98 versus one of 0.6 can traslate into a 40% reduction in purchase cost. Again, in an enterprise setting with hundreds or thousands of PCs, the savings can be very significant.

PFC Myths

There are myths about power factor correction that continue to be propagated by well-meaning people. Let's tackle the two most common ones:

Does higher PF reduce my electricity bill? No, if you are a home user. If you are an enterprise running hundreds of PCs and pay not only for power but also VA, then yes. For more details, see PFC discussion above.

Does PFC make a power supply more efficient? Not in the normal way that power supply efficiency is defined, which is the power loss (to heat) as a percentage of total AC input in AC-to-DC conversion. However, in the sense that Apparent AC power (VA) is lowered, PFC does reduce energy consumption.

Power factor correction is applied by an input circuit which uses a small amount of input power. With two PSUs that are identical, equipping one with PFC will cause a typical efficiency drop of 2~4%. Many PSUs that have Active PFC also have high efficiency, as APFC is usually found on higher quality PSUs, but the two are not intrinsically related.


We've often noted in the past couple of years that the retail power supply scene seems to be attracting new brands constantly. We've also noted that most of these new brands are selling products designed and built by other companies, Original Equipment Manufacturers (OEM) and Original Design Manufacturers (ODM) who are happy to customize an existing model for a large volume customer. Some SPCR readers have expressed interest in finding out who is making what. We've known for a long time how to find out, but never thought it important enough to pass this information. While knowing who manufactured a product can be helpful in assessing quality, we feel our reviews provide plenty of information to make that assessment.

Recently, thanks to Hardware Secrets, another PC hardware website, the details of how to identify a PSU's manufacturer have been spelled out for all to see. Here's a quick summary:

Underwriters Laboratories maintains a certifications directory that is accessible from their website, http://database.ul.com/cgi-bin/XYV/template/LISEXT/1FRAME/index.htm. Almost every computer power supply is certified for safety by UL, and the label on most certified power supplies is marked with the UL logo that contains the UL File Number for the manufacturer.

The UL file number on this SilverStone power supply is E166947. It's obviously made by Enhance Electronics... but most labels will not identify the manufacturer so plainly.

Simply enter the number in the "UL File Number" data entry box on the UL Certifications Directory web page linked above, then hit the search button on that page. The results are not infallible, however. Sometimes, the brand owner will have its own UL File Number, in which case, its name will come up in the search. Often, the UL File Number will be that of the manufacturer, in which case, that manufacturer's name will come up in the search.


There are many informative discussions in the SPCR PSU forum. Several of these threads have been turned into stickies that always appear at the top of the forum so they can be easily found. They are linked directly here:

* * *

Articles of Related Interest
Recommended PSUs
Power Supply Fundamentals
Power Distribution within Six PCs
Desktop CPU Power Survey, April 2006


Discuss this article in the forums.

Previous 1 2 3 4 5

Reference|Recommended - Article Index
Help support this site, buy from one of our affiliate retailers!