Power Supply Fundamentals

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THE PUSH TO HIGHER EFFICIENCY

Those who followed the evolution of the ATX12V Power Supply Design Guide (now v2.2) authored by Intel's power division may recall that it was the release of v1.3 in April 2003 that added efficiency guidelines at typical (50%) and light (20%) loads, and saw the minimum efficiency increased from 68% to 70%. Details for Energy Star and standby efficiency were also added in that version.

Version 2.0 of the ATX12V guide released February 2004 underwent a few more significant changes. Aside from those related to higher power requirements for more power hungry systems, section 3.2.5. for Efficiency listed not only the required minimum efficiency, but for the first time, the recommended minimum efficiency. Those numbers are substantially higher. The v2.01 spec of June 2004 remained unchanged from v2.0, but the current v2.2 spec of March 2005 saw substantial increases in both Required and Recommended efficiency.

AXT12V v2.01: "Minimum PSU Efficiency Vs Load"
Loading
Full
Typical
Light
Required Min
70%
70%
60%
Recommended Min
75%
80%
68%

AXT12V v2.2: "Minimum PSU Efficiency Vs Load"
Loading
Full
Typical
Light
Required Min
70%
72%
65%
Recommended Min
77%
80%
75%

In the current PS Design Guide for Desktop Platform Factors v1.0, efficiency guidelines have been raised even higher, and power factor correction has finally been introduced. The Optional Minimum Efficiency numbers come straight from the 80 Plus program guidelines. (More on PFC and 80 Plus below)

"Efficiency Vs Load" for all Desktop PS types
in PS Design Guide for Desktop Platform Factors v1.0
Loading
Full
Typical
Light
PFC
Required Min
70%
72%
65%
-
Recommended Min
74%
77%
72%
-
Optional Min
80%
80%
80%
0.9

What is efficiency in a power supply? It is defined as the power loss in AC-to-DC conversion, expressed as a percentage of total AC input power. For example, a power supply that requires 100W AC input to produce 70W DC output has an efficiency of 70%. In this example, 30W is lost as heat within the PSU. A power supply that requires 100W AC input to produce 80W DC output has an efficiency of 80%. 20W is lost as heat. The 10W difference in these examples seems trivial. However, at higher output levels, differences in efficiency becomes quite significant in terms of how much energy is lost as heat, as the table below shows. The ideal efficiency is 100%, where AC input and DC output are the same, and there is no loss to heat in the power supply.

THE EFFECTS OF EFFICIENCY
Efficiency
Power Lost as Heat
@100W output
@200W output
@400W output
85%
18W
35W
70W
80%
25W
50W
100W
75%
33W
67W
133W
70%
43W
86W
174W
65%
54W
108W
216W

The above table show clearly how much more heat is generated at higher power levels with even just a five percentage point drop in efficiency. When you consider the 10 percentage point differences, you can see nearly a doubling of power loss or heat generation. Cooling a power supply quietly becomes progressively easier as the power supply efficiency is improved.

Part of the push for higher efficiency comes from sheer necessaity. Intel (and AMD) systems are drawing so much more power now than they were even a few scant years ago that the 65% typical efficiency is no longer viable from a thermal management point of view. When the total AC power consumed was 100W, the heat generated by the PSU at 65% efficiency was just 35W. With some powerful systems drawing over 400W DC at high loads, the heat from a 65% efficient PSU would amount to >200W. This is a lot of extra heat heat to eliminate from computers. The generally shrinking size and tight internal spacing of PC cases makes cooling even tougher. An 80% efficient PSU in the same >400W DC system would generate less than one third of the heat, 100W. With system integrators everywhere concerned about the cost and complexity of adequate CPU and component cooling, reduction in heat had to start in the obvious places.

There is a certain confluence of factors leading to similar efficiency targets for PC power supplies. There's the aformentioned need to curb heat, being recognized and addressed by Intel's power team. Then there are increasing concerns about energy efficiency in light of world ecology and the challenges of electricity delivery. Here are several relevent orginizations and programs that are all pushing the efficiency envelope:

The Energy Star program of the US Environmental Protection Agency is proposing a new tough low target for AC consumption of computers in idle (50~60W for desktops) effective 2007. We first covered this story in March 2005, A New Energy Star... in 2007, and included an update on page 3 of the article, The State of the Industry, March 2006: Through Silent Eyes.

The 80 Plus Program run by Ecos Consulting is certifying high efficiency power supplies for eligibility in a national buy-down rebate program that returns $5 and $10 to system integrators who incorporate these PSUs in complete systems.

The related web site Efficient Power Supplies aims to "initiate a global dialogue about energy efficient power supplies." Run jointly by EPRI Solutions and Ecos Consulting, the site is an influential central clearing house for all things related to efficient power supplies, and even sponsors international competitions for high efficient PSU designs.

There's no question that higher efficiency is the trend in computer power supplies. In the past 24 months, the number of PSU samples that have been tested by SPCR to have > 80% efficiency has risen dramatically. As one commentator in the SPCR forums noted recently, the last three decades were a complete waste regarding PSU efficiency; there was almost no improvement. Then, in the last two years, it jumped from an average of <70% up to >80% for higher quality PSUs. Looked at in reverse — from the poiint of how much electricity is wasted as heat — going from >30% to <20% is a huge 33% improvement!

The big payoff for those who loathe PC noise is that a PSU that generates less heat needs less forced airflow to stay cool. This means lower fan noise. It also means less heat leaking out from the PSU into the rest of the system, especially to the already hot CPU, which is usually located right next to the PSU, within a couple inches. Lowering the heat radiating from the PSU can help to lower the airflow requirement for cooling the CPU as well, leading again, to less fan noise.

HIGH 12V LINE RELIANCE

The high reliance of current systems on the 12V is dramatic compared to even just a couple of years ago, and the evolution of the ATX12V spec reflects this change. Almost any system assembled from current components will draw the vast majority of current from 12V, in some cases, as much as 90% at load. This is one of the factors in increasing efficiency. It is generally easier to obtain higher efficiency in the conversion from 120VAC to 12VDC rather than 5VDC or 3.3VDC.

We recently studied the power distribution in half a dozen systems of varied configuration to confirm the high 12V reliance first hand and reported our finding in the article, Power Distribution within Six PCs. In all the systems the current draw on the 5V and 3.3V lines was a maximum of just 5A! Note that in dual 12V line models, 12V2 is supposed to only supply the AUX12V (2x12V) 4-pin plug, which feeds only the CPU. 12V1 is supposed to supply 12V to all the other components that require it. This can be a potential problem in some high end gaming systems; see the sections on Dual 12V Lines on the next page.

BTX FORM FACTOR

BTX prototype, mATX equivalent sizeThe various BTX interface, system design and case specifications and studies released by Intel since September 2003 are major departures from the ATX form factor, but they are only just trickiling into the market now from a few vendors. FormFactors.org describes the BTX as follows:

"The BTX form factor specification gives developers options to balance thermal management, acoustics, system performance, and size in the system form factors and stylish designs that are desired in today's products. The BTX form factor is a clear break from previous ATX form factor layouts and was developed with emerging technologies such as Serial ATA, USB 2.0, and PCI Express*.

"Thermal improvements come primarily from taking advantage of in-line airflow. The BTX defined in-line airflow layout allows many of the main board components (i.e.: processor, chipset, and graphics controller) to utilize the same primary fan airflow, thereby reducing the need for, and noise from, additional system fans. In some cases this also allows fewer and/or less expensive heat sinks to be used when compared to ATX solutions. The system level acoustics are also improved by the reduced air turbulence within the in-line airflow system. The BTX layout supports better component placement for back panel I/O controllers — important as the signal speed of external devices continues to increase. In addition to smaller than microATX system sizes, BTX was designed to scale up to tower size systems using the same core layout by increasing the number of system slots included."

We will provide analysis of BTX cases, PSUs and systems as they become more widely available.



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