Power Supply Fundamentals

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

or How PSU Power Ratings Mean Almost Nothing

A frustrating fact about PSUs is that there does not appear to be a stringent or regulated standard for reporting, advertising and labeling rated power. This is despite the existence of standards like ATX2.03 or Intel ATX12V.

There are well-established standards for measuring and rating HDD capacity, an engine's horsepower, or the heat generated by a furnace... but not one for how much power a PSU can deliver. There are so many cases of people with "450W" PSUs having power stability issues running a system that can't possibly draw more than 150W. And "300W" units that keep running where the "450W" units are faltering.

It's not just about bad PSUs vs better ones. It's a dumb situation caused by uncontrolled marketing competition. Real regulation would bring PSUs out of snake oil territory and into a more sensible consumer-friendly terrain.

There are many ways PSU makers fudge to make their units seem more powerful.

1) Out and out lying. You add up the power on all the lines in many PSUs and they fall short of the rated power by 10, 20 30W or even more.

There are more sophisticated ways:

2) Limit the AC input voltage to a very narrow tolerance. The best PSUs are able to deliver their rated power given a decent range of AC input power, say 90~130V for a 120V unit. It's much more demanding to produce 300W w/90VAC input than with 120VAC, so what some PSU makers will detail in their tech specs (usually not in their consumer brochures) is to specify 115-120VAC for input power. A PSU specified this way will not deliver full power if the AC voltage sags, if there is a brown-out. Surely it causes instability more often than a PSU rated to deliver full power with 90-130VAC.

3) Specify a low operating temperature for rated output. This is quite common, but again not often seen in consumer brochures, but rather tech spec sheets provided usually only on demand by engineers or corp buyers. A typical PSU operating temp statement is somthing like this:

0°C~25°C for full rating of load, decrease to zero Watts O/P at 70°C

Examine what that says. Full power (let's say 400W) is available when the unit is at 0°C~25°C. Hmmm. Think about this.

Have you ever felt air blown out of a PSU in a PC running absolutely full tilt (which it would have to do to get anywhere near 400W output) that felt cool to the fingers? 25°C airflow would feel exactly that: Cool, given that normal body temperature is 37°C.

So this PSU cannot deliver full rated power when its temperature goes over 25°C. OK, what happens to the max power output capacity above that temp? It decreases gradually so that by the time the PSU temp reaches 70°C, the PSU cannot deliver any power at all. So if you assume that this power drop as temp rises is linear, then max power capacity will drop by ~9W for every degree over 25°C.

Now having examined as many PSUs as I have over the last 2~3 years, I have to say there's not a single PSU in ANY PC I have ever used or examined that would not measure at least 30~35°C almost anywhere inside the PSU under almost any kind of load. And if/when it is pushed, 45°C is nothing at all, especially for or near hot running components like voltage regulators.

So let's say 40?C is a fairly typical temp inside a PSU. This 400W rated unit would actually be able to deliver a max of just 220W at that temp. Hmmm. Interesting, isn't it? At 50°C, the available power would drop to just 130W. No wonder some PSUs have 3 fans each capable of 50 cfm!!

Here's a simple fact: Really high quality PSUs are actually rated for full power output at as high as 50°C. The trick is get a hold of the spec sheets that tell such information so you can compare apples to apples. Or ask.


300W models have replaced 230W and 250W models as baseline units since the introduction of the AMD Athlon. They feature a fan (or two) rated for 35~40 cubic feet per minute (CFM) airflow. Presumably, this level of airflow is required for adequate cooling at full power output to pass safety approvals under UL, CSA, CE and other regulations. In early 2005, retail PSU models rated for increasingly higher power, as much >600W, are being introduced by many brands.

Our own experience indicates that despite all the new power hungry components such as >75W video cards and >120W CPUs, it is still rare to find a desktop computer than draws much more than 200W DC under typical demanding applications. Around 300W DC looks to be about the highest power draw from a single CPU full-bore high end system at this time (Feb 2005). Although some headroom is always good to have, there seems little question that consumers are being persuaded to pay for power capacity that is never used. One of the nasty side effects is the fan noise of the high airflow required to keep the PSU adequately cooled when delivering maximum power. High speed fans generally make more noise than slower ones even when they are slowed by undervolting.

Why this state of affairs exists is a matter of marketing and technical obfuscation, probably more by accident than any massive conspiracy. With relatively low current requirements prior to the AMD Athlon processor, the aforementioned 230W and 250W were perfectly adequate for PC systems, even if the power supplies didn't deliver full rated performance. That changed with the Athlon and then the P4. PSU makers were quick to introduce higher rated models said to be required for the new power hungry processors. It was a good marketing opportunity. Rather than "Our 250W PSU is better than theirs," it is easier to sell the message "Our 300W PSU is better than their 250W PSU." Bigger is always better, isn't it? It also allowed higher prices to be charged.

A counterpoint is AMD's system builder's guide, which suggests higher numbers: up to ~180W DC for a typical system and ~250W DC for a high performance system, but these numbers are obtained by adding the maximum power rating for each component, then taking 20% off to account for real-world conditions. It is almost impossible for any application to demand 80% of maximum power draw from each component simultaneously. Intel's PSU recommendations are similar.

Suffice it to say that as manufacturers, both AMD and Intel are looking at worst-case secenarios. As custom builders, enthusiasts and system integrators can make choices based on real needs and applications.

Even so, Is Higher Power Better?

Without getting into technical details, the nature of a switching power supply is that it delivers as much power as is demanded by the components. This means that when installed in a PC whose components require 200W, a 400W PSU and a 250W PSU will each deliver 200W. Does this mean the 400W is coasting while the 250W is struggling? Not if they are both rated honestly and if they have the same efficiency. If one has lower efficiency than the other, then it will consume more AC to deliver the same power to the components, and in the process, generate more heat within itself. As long as there is adequate power, higher efficiency is the key to cooler, quieter PSU operation.

The main benefit of higher power PSUs is when the airflow in the PSU is deliberately set very low in order to minimize noise. This usually means the PSU components will run hotter. If all other things are equal, a higher rated PSU may be a better choice in such an application because its parts are generally rated for higher current and heat than a lower rated model.

What are the Key Aspects to Good PSU Performance?

There is a great deal of fuzzy and unclear thinking about what constitutes a good power supply. The obsfucation caused by competitive marketing is certainly one cause of this confusion. Another is the proliferation of computer hardware web sites that publish "reviews" of PSUs without much notion of what should be examined or how or why.

These parameters are the keys to good PSU performance:

  1. Stable power delivery under load
  2. High efficiency
  3. Good cooling
  4. Low noise operation
  5. Long term reliability

The truth is that a computer power supply is a complex electronic device with a complex role that is little appreciated by most hardware reviewers. Most system integrators don't really appreciate it either, either. This is due partly to the assemble-and-sell nature of the PC industry, where manufacturers build components in accordance to an accepted standard specification for "universal" compatibility with other components. Such piecemeal component manufacturing does not nurture or reward system thinking, which has been much more the norm for Apple.


Version 2.0 of Intel's ATX12V Power Supply Design Guide began recommending dual 12V lines for PSUs that can deliver more than 18A at 12V. In the latest guide, this recommendation has been reworded in section 3.5.7:

The 12 V rail on the 2x2 power connector should be a separate current limited output to meet the requirements of UL and EN 60950.

The requirements of UL and EM 60950 are related to safety. It stipulates that not more than 240VA is carried on any wires or exposed traces.

What is the safety reason for the 240VA maximum? It's the maximum recommended for an electronic device that a consumer will have reasonable likelihood of access. In plain terms, it might be to keep people from zapping themselves inside a PC, or more likely, accidentally creating a fire risk. This safety "rule" does not apply to any electronic or electrical devices where the chance of consumer exposure is low, such as a TV or CRT monitor, for example.

In PSUs that conform strictly to ATX12V v2.xx, it's important to know that even though there are two "independent" 12V lines, they still draw from the same main source. It's highly unlikely that there are two separate 120VAC:12VDC power conversion devices in a PSU; this would be too costly and inefficient. There is only one 12VDC source, and each of the two lines draw from the same 12VDC source, but through its own "controlled gateway".

PSU makers' specs are misleading in that they rate the current capacity of each 12V rail independently. What really matters is the total 12V current: Generally, up to 20A is available on any one 12V line assuming the total 12V current capacity of the PSU is not exceed.

What the above means is that you don't need to worry about imbalances in power draw on the 12V lines — as long as no single line is asked to deliver more than 20A. PSU makers seem to mark each line for max current on a purely arbitrary basis, probably more for marketing reasons than any other. A PSU rated for 32A max on the 12V lines can be labelled many different ways:

  • 12V1: 18A, 12V2: 14A
  • 12V1: 17A, 12V2: 15A
  • 12V1: 16A, 12V2: 16A
  • 12V1: 15A, 12V2: 17A
  • 12V1: 14A, 12V2: 18A

It could be marked 20A + 12A, but being a cautious bunch, the engineers will probably not specify more than 18A on any one line. This gives 2A headroom to allow some room for error for the current limiting circuit.


Note that 12V2 is supposed to supply only the AUX12V (2x12V) 4-pin plug, which feeds only the CPU. With PSUs that adhere strictly to the ATX 12V v2.xx Guide, 12V1 then must supply 12V to all the other components that require it. This might lead to a problem with very high power gaming systems that utilize two high power video cards in SLI or Crossfire mode. Current high end VGA cards by themselves can draw >90VA each. Much of this comes from the 12V line via the 6-pin PCIe connector for the VGA card. If you add several hard drives and optical drives, the 240VA limit may be too low.

The current ATX12V v2.2 spec was created before dual VGA card gaming configurations for Intel boards were announced. SLI, being an nVidia feature on nForce 4 chips for AMD CPU motherboards that came many months earlier, may have been ignored by Intel's PSU design guide team.

Not all PSUs with 6-pin PCIe connectors follow ATX12V v2.xx to the letter. In fact, they can't, as the guide does not cover the 6-pin 12V PCIe outputs. This connector and its current delivery capacity was specified by nVidia, the originator of the SLI concept. nVidia maintains a list of power supplies that they have certified as being suitable for SLI systems. The question is, Where should this 12V come from? More to the point, which line DOES it come from?

I interviewed a number of engineers from several power supply manufacturers to pose this very question. The answers were surprising. All of the engineers I spoke with wished to remain anonymous. This is a summary of what they told me:

  • Some PSU makers are using 12V2 to supply more than just the 2x12V or 4x12V connectors. It is often used to power the 6-pin 12V PCIe outputs as well.
  • Many PSUs marked as having dual (or more) 12V lines actually have only a single 12V line — they do not feature two 240VA current limiters specified by ATX12V v2.xx; they have only one Over Current Protection (OCP - current limiter) for the single 12V line.
  • The 240VA current limit is considered a high cost, useless annoyance by most PSU makers. If multiple 12V lines are used, because the vast majority of components now use mostly 12V, the 18~20A limit for any line means that the precise power distribution to the various 12V output connectors can become critically important in some cases.
  • The engineers point to the many high power pre-V2.xx ATX12V PSUs that had as much as 30A on a single 12V line. As a product class, those have not proven to be any more dangerous in any way than other ATX12V PSUs. Even if exceeding 240VA in a single wire run was dangerous, this is extremely unlikely to occur in a PC because 12V is distributed to many different components on many different wire runs.

What's really interesting is that Intel has tacitly waived the 240VA limit requirement in its PSU validation program for the better part of a year. Intel maintains a web page listing all the ATX12V they have tested that "meet MINIMUM electrical, mechanical fit and functional compatibility" with Intel desktop boards and processors. For the 32 ATX12V v2.2 PSUs tested in 2005 that are on this list, 17 models are identified as having at least one output line that exceeds 240VA. And yet, these 17 models are on Intel's approved list.

According to the engineers I spoke with, the majority of these 17 models have just one 12V line. They also point out that there are another 20 or so ATX12V v2.0 PSU models on the Intel list, and none of them were tested for the 240VA current limit conformance. My sources say that if these models had been tested, more than half would not conform to the 240VA current limit because they have only one 12V line.

In the last couple of months, my PS engineering sources report, Intel has verbally informed them that the 240VA limit has been removed. A single 12V line is now "officially" approved, never mind what ATX12V v2.2 specifies.

What does all this mean? The safety benefit of dual 12V lines is questioned by the engineers I spoke with. There are many downsides to multiple 12V lines, including higher cost and the extra headache of ensuring adequate 12V current for all the components in complex, high power systems. For the consumer who is trying to make a choice among the myriad of PSUs available on the retail market today, the most practical approach regarding dual 12V lines and power capacity is to consider only the combined 12V current capacity.

This is not to say that there are no advantages of multiple, independent 12V lines. The fact is that the 12V line is where most of the power is delivered in a modern PC. In a high power system where many components are pulling on the 12V line simultaneously (for example, a high power dual video card gaming rig), independent 12V lines could help improve stability under certain conditions. The current limits on each 12V line then become important to consider when wiring up the system. If no specific guidlines are given by the manufacturer regarding which components should be connected to which connectors and/or cables (especially with detachable output cable models), then it is probably safe to assume that the PSU does not have really independent, separate 12V lines.

Previous 1 2 3 4 5 Next

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