Power Supply Fundamentals

Power | Reference|Recommended
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by Mike Chin

  • April 4, 2007 - Added a new high power system and an A64X2 system to section on Real System Power Requirements (page 4).
  • Dec 1, 2006 - The Lists of Recommended PSUs have been separated from this article. This article was just getting too long and cumbersome. Separating them means it's a bit easier to update them more frequently.
  • Sept 2, 2006 - Added section on how to identify the PSU maker and a counterpoint to correct PSU sizing.
  • Aug 31, 2006 - Updated much of the text on Intel PSU guidelines based on latest Power Supply Design Guide for Desktop Platform Form Factors (Revision 1.0 - June 2006). Also expanded discussion on multiple 12V lines, efficiency, power factor correction, etc.
  • Aug 24, 2006 - Added discussion about efficient PSUs that don't start with some motherboards.
  • Aug 14, 2006 - Seasonic S12-330 review added. PicoPSU and Silentmaxx Fanless 400W MX460-PFL01 added. Antec NeoHE430 reinstated. Nexus 4090 & Zalman 400B retired.
  • Jan 3, 2006 - Zalman ZM460-APS, Fortron Green 400W and Seasonic SS300-SFD 80 Plus added. Enermax Noisetaker 325 & 475 retired; Antec NeoHE430 retired until proven reliable.
  • Oct 28, 2005 - Antec NeoHE430 added, adjustments in recommended list comments made to reflect new efficiency findings reported in Corrected Efficiency Results from PSU Test Rig V.3.
  • Major Update Sept 8, 2005 - Revisions in selected portions of the text, update version info on PSU design guides, added info on 12V reliance, dual 12V lines, more system power examples, added new models and retired some old ones.
  • April 12, 2005 - Numeric ranking system of PSUs revised.
  • Major Update April 5, 2005 - Mass retirement of many older PSUs. Added Seasonic S12, Enermax Noisetaker 701, Nexus 4090. Major revisions of the entire article.
  • Update February 4, 2005 - Added detailed information about real desktrop system power requirements. Minor adjustments in comments to PSU tables; removed two PSUs no longer available.
  • Update October 17, 2004 - Added three fanless PSUs: Antec Phantom, Silverstone ST30NF and CoolMax Taurus CF-300.
  • Update June 5, 2004 - Raised Seasonic Super series ratings after Rev.03 reviews, added other Enermax Noisetaker and Seasonic Super series models.
  • Update May 31, 2004 - Added Zalman ZM400B, links to useful sticky posts in the PSU forum.
  • Update April 4, 2004 - Added Enermax NoiseTaker 475. RSG Electronics PSUs removed: Not enough validation from firsthand or trusted sources about these models.
  • Update March 10, 2004 - Added section on Power Factor Correction.
  • Updated Feb 1, 2004 - Major changes to text content, inlcuding information about BTX Form Factor, and PSU output power ratings. Added Nexus NX3500. Seasonic Silencer rev. 02 inconsistency noted.
  • Updated Dec 8, 2003 - Seasonic Tornado downgraded for inconsistency; Fortron 350 Aurora added.
  • Updated Sept 2, 2003 - Added new Seasonic "Super" models, refined comments.
  • Updated July 10, 2003 - Quoted section "5.7 Acoustics" from ATX12V 1.3 guide.
  • Updated June 26, 2003 - proSilence PCS-350 fanless PSU added.
  • Updated June 19, 2003 - Reorganized ranking tables into 4 categories, refined comments, reshuffled rankings based on latest info and results.
  • Updated June 17, 2003 - Added Nexus NX-4000, info on new Intel PSU standards and microATX system design guide.
  • Updated May 11, 2003 - Added SilenX 14dB 400.
  • Updated April 7, 2003 - Added Verax 300.
  • Updated Dec 10, 2002 - Minor adjustments in table comments, note to ATX spec.
  • Updated Dec 8, 2002 - Antec TrueControl 550 added.
  • Updated November 29, 2002 - Nexus NX-3000 PSU added.
  • Updated November 15, 2002 - Enermax and SH models removed; quieter alternatives now plentiful.

* * *

Often one of the biggest noise makers in a PC, the Power Supply Unit delivers regulated DC voltages to various components. Computer PSUs are switching mode types, which provide relatively high efficiency at low cost. They utilize forced air cooling, usually an 80mm fan, and sometimes incorporate a second fan. The fan is the primary source of noise in a PSU. Coils in a PSU can buzz and hum, especially when pushed under high loads, but usually fan noise masks coil noise. Typically, the fan is rated for higher than the maximum airflow needed to keep the PSU cool. It's cheap engineering insurance for the manufacturer and also fulfills the case cooling role of the PSU fan.

In the ATX case specification, cool air is drawn into a typical case from vents in the front panel. The incoming air helps cool components as it moves through the case, becoming warm in the process. It is evacuated through the PSU and out the rear by the PSU fan. So the loud, fast fans do help to keep a case cooler. Manually varying a high airflow PSU fan can cause CPU temperature to be affected as much as 5-6 degrees C.

The ATX specification was created some years ago at a time when desktop CPUs generated no more than ~30W. Now, they are up over 130W. The airflow arrangement of exhausting hot case air out through the PSU no longer makes as much sense as it did in the past. The PSU has to handle both its own self-generated heat, which is naturally higher than before, as well as the heat generated by the other components.

Fast-spinning fans make a lot of noise, especially when confined in a small space with nearby airflow obstructions. It is not unusual for the noise of a PSU to be 12-15 dB higher than the rated noise of its fan in free air. The noise is further exacerbated by the way a PSU is mounted in a typical tower case: the typical 4~5 pound weight "hangs" off the top of the back panel on 4 screws. In this mounting configuration, excitation of case panel resonances by direct transfer of PSU fan vibrations is almost unavoidable. The end result is more noise, especially as a droning type of hum in the lower frequencies.

The quietest PSUs on our list (on page six if you want to jump straight there) feature either no fan at all or a fan that spins at low speed under most conditions. Keep in mind that components will tend to run a bit hotter than usual as a result of reduced airflow. This can be a concern if the normal ambient room temperature is high or if very hot components are used. The best fan-cooled models have low normal fan speed, and allow the fan to ramp up to full speed only when really necessary.

There are many different motherboard/case form factors, such as Mini-ATX, LTX, Flex-ATX, AT, Mini-ITX, etc. There are also many proprietary cases that don't conform to any general form factor. ATX power supplies can often be used, but some cases require different PSU form factors, such as STX for Flex-ATX cases. In time, we will expand our list to include different form factor PSUs.


Intel's power department has been very busy in the last few years as rapid deloyment of higher power and different form factor devices keep adding to the PC mix. They have made frequent updates and additions their design guidelines for PSUs in the ATX12V, SFX12V, TFX12V, LFX12V and CFX12V PSU Design Guides on their site Desktop Form Factors. These guides are not standards that must be adhered to by regulation, but specifications that almost every PSU maker in the industry follows very closely in order to ensure compatibility between their products and new motherboards, graphics cards and other peripherals.

In summary:

  • NEW! Power Supply Design Guide for Desktop Platform Form Factors (Revision 1.0 - June 2006) Consolidated guide for CFX12V, LFX12V, ATX12V, SFX12V, and TFX12V PSU form factors
  • ATX12V (Revision 2.2 - March 2005) is for standard tower and desktop form factor systems.
  • SFX12V (Revision 3.1 - March 2005) is primarily intended for use with small form factor microATX and FlexATX systems.
  • TFX12V (Revision 2.01 - June 2004) - Thin Form Factor - for SFF system designs (9-15 liters in total system volume).
  • LFX12V(Revision 1.0 - April 2004) - Lowprofile Form Factor - for Balanced Technology Extended (BTX) form factor systems with ultra small form factor system designs (6-9 liters in total system volume).
  • CFX12V (Revision 1.2 - May 2004) - Compact Form Factor - for BTX system designs with ultra small form factor system designs (10-15 liters in total system volume).

Common themes in most of the PSU Guides are:

  • Increased +12 VDC output capability
  • Dual 12V rails for PSUs that have >20A current capability on the 12V line
  • 2x12-pin main power connector for additional 75W PCI Express requirements (Most v2.2 PSUs are coming with 20/24 pin ATX connectors for backward compatibility)
  • Minimum efficiency for typical and light load.
  • Higher recommended efficiency targets for typical and light load.
  • Details on an S-ATA power connectors
  • Definitions of the new 24-pin ATX connector
  • Acoustic guidance to support low noise systems.

The Recommended Acoustics has been revised since ATX 12V version 2.0. Section 5.1, page 33 of Power Supply Design Guide for Desktop Platform Form Factors, v.1.0 states:

5.7. Acoustics - RECOMMENDED

It is recommended that the power supply be designed with an appropriate fan, internal impedance, and fan speed control circuitry capable of meeting the acoustic targets listed in Table23.

The power supply assembly shall not produce and prominent discrete tone determined according to ISO 7779, Annex D.

Sound power determination is to be performed at 43 C, at 50% of the maximum rated load, at sea level. This test point is chosen to represent the environment seen inside a typical system at the idle acoustic test condition, with the 43 C being derived from the standard ambient assumption of 23°C, with 20°C added for the temperature rise within the system (what is typically seen by the inlet fan). The declared sound power shall be measured according to ISO 7779 and reported according to ISO 9296.

Table 23. Recommended Power Supply Acoustic Targets

Typical - 50% load
3.5 BA
4.0 BA
5.0 BA
3.0 BA
3.8 BA
4.5 BA

The acoustic targets recommended here are not particularly low (quiet). As far as I know, no power supply maker is testing or reporting acoustics in the way recommended above.


The latest PSU Guide consolidated all the desktop form factors into one: Power Supply Design Guide for Desktop Platform Form Factors revision 0.5 was released in January 2006. It was updated as Revision 1.0 in June 2006. This guide combines design guidelines for the ATX12V, CFX12V, LFX12V, TFX12V and SFX12V power supply form factors into one comprehensive power supply design guide.

The highlights of revision 0.5 were:

  • Combined CFX12V, LFX12V, ATX12V, SFX12V, and TFX12V content into one desktop power supply design guide
  • CFX12V content derived from revision 1.2
    - Updated 12V1 current for 300 W configuration
    - Updated efficiency loading for 300 W configuration
  • LFX12V content derived from revision 1.1
  • ATX12V content derived from revision 2.2
  • SFX12V content derived from revision 3.1
  • TFX12V content derived from revision 2.1
    - Updated 12V1 current for 300 W configuration
    - Updated efficiency loading for 300 W configuration
  • Updated Capacitive Load section to use standard capacitor values
  • Updated 5 VSB efficiency recommendations for Digital Office platforms
  • Removed power-down warning from power supply timing diagram
  • Marked sections with labels to indicate REQUIRED, RECOMMENDED, or OPTIONAL items

Changes in in revision 1.0:

  • Added 12V2 Current for Processor Configurations table
  • Added revision numbers to form factor specific chapters
  • Changed Input Line Current Harmonic Content to OPTIONAL to better reflect geographical requirements

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