Power Supply 101: A Reference Of Specifications

Modern Form Factors: EPS, TFX, CFX, LFX, And Flex ATX

EPS/EPS12V

In 1998, a group of companies including Intel, Hewlett-Packard, NEC, Dell, Data General, Micron, and Compaq created the Server System Infrastructure (SSI), an industry initiative to promote industry-standard form factors covering common server hardware elements such as chassis, power supplies, motherboards, and other components. The idea was to be able to design network servers that could use industry-standard interchangeable parts. You can find out more about SSI at www.ssiforum.org. Although this book does not cover network servers, in many ways a low-end server is a high-end PC, and many high-end components that were once found only on servers have trickled down to standard PCs. This trickle-down theory is especially true when it comes to power supplies.

In 1998, the SSI created the entry-level power supply (EPS) specification, which defines an industry-standard power supply form factor for entry-level pedestal (standalone tower chassis) servers. The initial EPS standard was based on ATX, but with several enhancements. The first major enhancement was the use of a 24-pin main power connector, which eventually trickled down to the ATX12V as well as other power supply form factor specifications in 2003. EPS also originally called for the use of HCS terminals in the Molex Mini-Fit Jr.–based power supply connectors, which became standard in ATX12V in March 2005. In addition, the (now-obsolete) auxiliary six-pin power connector, the four-pin +12 V power connector, and a variation of the six-pin graphics power connector all appeared in the EPS specifications before ending up in ATX.

The EPS specification originally used a mechanical form factor identical to ATX, but the EPS form factor was later extended to support higher power outputs by allowing the body of the supply to be deeper if necessary. The ATX and the original EPS standards call for a supply that is 86 mm tall by 150 mm wide by 140 mm deep, the same dimensions as the LPX or PS/2 form factors. EPS later added optional extended depths of 180 mm and 230 mm total. Most power supplies with true ratings of 500 watts or more are made in the EPS12V form factor, because it isn’t really possible to fit more power than that into the standard ATX size. You may think these would require a custom EPS chassis, but in fact many (if not most) full-sized ATX tower chassis can handle these greater depths without interference, especially when using one of the newer breed of shorter-length optical drives (because one or more of the optical drives are often inline with the power supply).

With the improvements in EPS/EPS12V power supplies trickling down to ATX/ATX12V, I have studied the SSI EPS specifications to see what potential improvements might come to ATX. The main difference today between ATX and EPS with respect to connectors is the use of an eight-pin dual +12 V connector in EPS12V instead of a four-pin +12 V connector in ATX12V. The eight-pin dual +12 V connector is essentially the equivalent of two four-pin connectors mated together and is used by entry-level servers to power multiple processors. Because of the way the connectors are designed, an eight-pin +12 V connector can plug into a four-pin +12 V connector on a motherboard, with the unused pins simply hanging unused, offset to one side or the other.

The only other major difference between EPS12V and ATX12V is that EPS power supplies can be up to 180 mm or 230 mm deep, whereas ATX supplies are technically limited to 140 mm depth according to the specification. An example of an EPS12V type supply from PC Power and Cooling is shown below.

This power supply is basically a 230 mm-deep EPS12V supply that works in place of or as an upgrade to an ATX12V supply as long as the chassis can accommodate the additional depth. EPS12V supplies are sometimes called extended ATX power supplies because of their optional extended length. If you plan to use one of these EPS12V power supplies in a standard ATX chassis, it’s important that you measure the available space in your chassis to ensure you have the room behind the supply for the additional depth.

Connector compatibility isn’t generally a problem because, by virtue of the Molex Mini-Fit Jr. connector design, you can plug a 24-pin main power connector into a 20-pin socket, as well as an eight-pin dual +12 V connector into a four-pin +12 V socket.

If you have the room, an EPS12V power supply can be used with most ATX chassis and motherboards for the ultimate in high-output capabilities.

TFX12V

The TFX12V (thin form factor) power supply was originally introduced by Intel in April 2002 and is designed for small form factor (SFF) systems of about 9–15 liters in volume, primarily those using low-profile SFF chassis and microATX, FlexATX, or Mini-ITX motherboards. The basic shape of TFX12V is longer and narrower than the ATX- or SFX-based form factors, allowing it to more easily fit into low-profile systems. The dimensions of the TFX12V form factor are shown in the figure below.

TFX12V power supplies are designed to deliver nominal power output capabilities of 180–300 watts, which is more than adequate for the smaller systems for which they are designed. TFX12V supplies include a side-mounted internal 80 mm fan that is usually thermostatically controlled, so as to run both coolly and quietly. A symmetrically designed mounting system allows the fan to be oriented facing either side inside the system for optimum cooling and flexibility in accommodating different chassis layouts.

Unlike SFX-based supplies, only one standard mechanical form factor exists for TFX12V supplies. TFX12V supplies have also always included the four-pin +12 V connector since the standard appeared in April 2002, well after the +12 V connector had been included in other power supply form factors. TFX12V 1.2 (April 2003) added the Serial ATA power connector as an option, whereas the TFX12V 2.0 release (February 2004) made them mandatory and changed the main power connector from 20 pins to 24 pins. Revision 2.1 (July 2005) includes only minor updates and changes from the previous version.

CFX12V

The CFX12V (compact form factor) power supply was originally introduced by Intel in November 2003 and is designed for mid-sized balanced technology extended (BTX) systems of about 10–15 liters in volume, primarily using microBTX or picoBTX motherboards.

CFX12V power supplies are designed to deliver nominal power output capabilities of 220–300 watts, which is more than adequate for the mid-sized systems for which they are designed. CFX12V supplies include a rear-mounted internal 80 mm fan that is typically thermostatically controlled, which enables it to run both coolly and quietly. The shape of the supply includes a ledge such that part of the supply can extend over the motherboard, reducing the overall size of the system. The dimensions of the CFX12V form factor are shown in the image below.

CFX12V supplies have included the four-pin +12 V connector since the standard first appeared in November 2003, well after the +12 V connector had been included in other power supply form factors. TFX12V also included the 24-pin main power connector and Serial ATA power connectors as mandatory since its inception. The current CFX12V 1.2 release dates from 2005 and has only minor revisions over previous versions, including a change to HCS terminals in the connectors.

LFX12V

Intel originally introduced the LFX12V (low profile form factor) power supply in April 2004. It’s designed for ultra-small BTX systems of about 6–9 liters in volume, primarily using picoBTX or nanoBTX motherboards.

LFX12V power supplies are designed to deliver nominal power output capabilities of 180–260 watts, which is ideal for the tiny systems for which they are designed. LFX12V supplies include an internal 60 mm fan, which is 20 mm smaller than that of the CFX12V design. Similar to the CFX12V fan, it is usually thermostatically controlled to ensure quiet operation while still providing adequate cooling. The shape of the supply includes a ledge such that part of the supply can extend over the motherboard, reducing the overall size of the system. The dimensions of the LFX12V form factor are shown below.

All LFX12V supplies include a 24-pin main motherboard power connector, a four-pin +12 V connector, and Serial ATA connectors. The current LFX12V 1.1 release dates from April 2005 and has only minor revisions over the previous version.

Flex ATX

A power supply company called FSP (Fortron Source Power) originally introduced variations of what was to become the Flex ATX power supply form factor in 2001 in the form of proprietary designs for SFF desktop and thin (1U) server systems. These power supplies became popular in systems from Shuttle, but they have also been used by HP/Compaq, IBM, SuperMicro, and others. In an effort to make this form factor an official standard, Intel introduced the Flex ATX power supply form factor in March 2007 as part of the 1.1 and later revisions of the “Power Supply Design Guide for Desktop Platform Form Factors” document, which is available on the www.formfactors.org site. These are also sometimes called 1U (one unit) power supplies because they are used in many 1U server chassis.

Flex ATX power supplies, like the one shown below, are designed to deliver nominal power output capabilities of between 180 and 270 watts, which is ideal for the small systems for which they are designed. Flex ATX supplies usually include one or two internal 40 mm fans; however, larger fans can be mounted horizontally, and even fanless models exist.

Flex ATX power supplies include either a 20-pin or 24-pin main motherboard power connector and a four-pin +12 V connector for the motherboard. They also usually include standard peripheral and floppy power connectors, with newer units having Serial ATA power connectors as well.

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33 comments
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  • joytech22
    Quote:
    On the other hand, if you plug into a 240 V outlet and have the switch set for 120 V, you can cause damage.


    Did that when unboxing a computer, must have flipped the small red switch on the supply and boom, at the Windows XP loading bar the PSU exploded. lol.
  • cmcghee358
    Did I miss them covering efficiency and the whole 80 PLUS thing?

    I can't imagine as detailed as it is, omitting something like that...
  • cangelini
    cmcghee358Did I miss them covering efficiency and the whole 80 PLUS thing?I can't imagine as detailed as it is, omitting something like that...


    There's still one last part to go!
  • cmcghee358
    But the last part isn't for PSUs. It's just the last part in the series of PC components.
  • nikorr
    Thanx ...
  • neiroatopelcc
    I wonder how much the power_good signal prevents? is it just the powering of the cpu ?
    I recall once using two power supplies to power a sli board and accidently use a molex from the second supply to power a sli power connector on the motherboard - resulting in fans powering up if you powered the second psu even when the first wasn't on (and if you didn't, the geforces would screech due to lack of power)..... maybe that was just the creative yet rubbish asrock board design, but it certainly didn't need a power_good to power up the fans.

    ps. "Note: If you find that a system consistently fails to boot up properly the first time you turn on the switch, but that it subsequently boots up if you press the reset or Ctrl+Alt+Delete warm boot command, you likely have a problem with the Power_Good timing. You should install a new, higher-quality power supply and see whether that solves the problem."
    Could this explain why I only have 4-6GB memory at post, but 10GB after a quick power off and back on (didn't bother with a reset switch when designing case). Note that 10GB is still 2 short. It used to initialize 10GB - then power off and back on would provide the full amount. Running less than 6GB memory doesn't cause the error.
    Someone said I'd have to reseat the cpu, but maybe it's just that rubbish coolermaster power supply?
  • chesteracorgi
    Very informative and interesting. The part about single 12V vs. multiple 12V rails is important reading for system builders who opt for "safer" multiple 12V PSUs. With the current state of design of PSUs anyone planning a sli or Xfire rig is well advised to opt for the single 12V design rather than risk an imbalanced PSU that overvolts a component.
  • JohnnyLucky
    Great article. It's not just for beginners.
  • Reynod
    Compatibility Issues was a useful section.

    Overall very well written.

    Cheers,
  • kd0frg
    awesome information! nice work!
  • elbert
    cmcghee358But the last part isn't for PSUs. It's just the last part in the series of PC components.
    Quote:
    covering efficiency and the whole 80 PLUS

    If you picked one of these books up you would want the efficiency to move them. Edition 17 was huge and very heavy. These books are already to thick for many to pick up with one hand. Scott Mueller's has published 20 editions of this book and most come with CD/DVD which may guide you to online information about the subject.

    Here is a link to his online forum.
    http://forum.scottmueller.com/
  • digiex
    I'm just wondering what is the use of the floppy connector...

    Until unexpected glitch ruined the flashing if my motherboard, beyond this, I think the floppy connector is useless.
  • mayankleoboy1
    PSU: the most overlooked and underrated component
  • I fixed one of those non-compatible Dells way back with a standard PSU. Dell wanted £120 for a new PSU, I was suspicious, "how could they get away with that?". Checked online, found the incompatibility, dodged the bullet bought a PSU for £20 and an adapter for £5. Never bought Dell again nor recommend them.
  • A Bad Day
    This reminded me of a friend who bought a $5 no-name "600 watt" PSU for a +$900 rig.

    As soon as he turned on the computer, the PSU failed so badly that it exploded into flames and took out everything: motherboard, RAM, CPU, GPU, hard drive, CD drive, you name it.
  • grantmcconnaughey
    I've been reading this book lately. To me, this is absolutely the bible of PC hardware.
  • newbie_mcnoob
    I remember working on an old Dell Dimension 4100 series with the proprietary power supply and RIMM memory. I'm glad those got phased out.
  • hunter315
    Quote:
    In other words, it is far better to have a single 12 V rail that can supply 40 amps than two 12 V rails supplying 20 amps each because with the single rail you don’t have to worry which connectors derive power from which rail and then try to ensure that you don’t overload one or the other.


    Im quite disappointed to see tom's fell for the marketing BS of "a single rail is better than multiple rails". On a well designed unit it does not matter one bit, the design engineers already split the connectors so the rails were reasonably balanced, and the OCP threshold is set such that added together their theoretical current limit is more than the total limit of the 12 V source so you don't have to have your rails perfectly balanced to get the full power out of your unit.

    I wrote up a post on this a while ago, if anyone has any questions or anything they think should be added to it let me know.
    Single 12V rail or multiple 12V rails? The eternal question answered


    Also, you guys left the CPU off the +12 V part of your chart of what requires what voltages.
  • PreferLinux
    ChesteracorgiVery informative and interesting. The part about single 12V vs. multiple 12V rails is important reading for system builders who opt for "safer" multiple 12V PSUs. With the current state of design of PSUs anyone planning a sli or Xfire rig is well advised to opt for the single 12V design rather than risk an imbalanced PSU that overvolts a component.

    I guess it is better to be able to use the 12 V rail as an arc welder then? Because you could if you have a >1000 W single-rail PSU. Not to mention that it won't overvolt anything – how does a high power draw cause high voltages? It generally causes low voltages. And if the PSU is a decent one, the rails will be pretty well balanced, especially for SLI or Crossfire.
  • iam2thecrowe
    ChesteracorgiVery informative and interesting. The part about single 12V vs. multiple 12V rails is important reading for system builders who opt for "safer" multiple 12V PSUs. With the current state of design of PSUs anyone planning a sli or Xfire rig is well advised to opt for the single 12V design rather than risk an imbalanced PSU that overvolts a component.

    you couldn't be more wrong.
  • Onus
    Thank you, Hunter315. Maybe that was an issue 6-8 years ago, maybe? It no longer is; I cannot imagine any possibility of creating sufficiently unbalanced +12V rail loading creating the least difficulty for any reasonable PSU.
    Anyway, there were some interesting tidbits in there, like the specifics on the capacities of individual connectors. The connector type used is another area where I'd be certain the cheap PSUs skimp.
    Some of it was a trip down memory lane; I remember those load resistors in the hard drive cages of the IBM PC-AT.

    Edit: Oh, and your answer to the Eternal Question was very good, as was the short discussion which followed, touching on the safety issues associated with overly large +12V rails. Although I have said many of those things, I doubt I've expressed myself as clearly. Those who have read this chapter and this far into the comments would probably also like to click on your link and read that thread.
  • aidynphoenix
    It really doesn't matter weather you have a psu with a single 12v rail or several.
    what matters is the quality of the power supply and how well it performs.
    viewing a powesupply's voltages on a graph and seeing how far the voltages fluctuate and how smooth the ripple currents are is the most important thing.
    even if your powersupply is 1200w id rather have a psu that can get the job done, but stays as close as possible to the 12v specification, and the power is clean and has little spikes.
    i am a firm believer that poorly designed powersupply's is the #1 cause of premature hardisk failure aside from shipping damage and excessive heat.
    you know that when company's calculate their MTBF they are useing the cleanest power possible.
    I have had alot of experience with poor powersupply's and many HDD deaths.
    now im running a powersupply with a very smooth 12v ripple and high tolorance (thanks corsair)
    and have not had a hardisk die on me since.

    i order recommend 1 seasonic, 2 corsair, 3 antec.. i do not recommend cooler-master.. or any other brand.
  • neiroatopelcc
    neiroatopelcc"Note: If you find that a system consistently fails to boot up properly the first time you turn on the switch, but that it subsequently boots up if you press the reset or Ctrl+Alt+Delete warm boot command, you likely have a problem with the Power_Good timing. You should install a new, higher-quality power supply and see whether that solves the problem." Could this explain why I only have 4-6GB memory at post, but 10GB after a quick power off and back on (didn't bother with a reset switch when designing case). Note that 10GB is still 2 short.


    To answer my own question, as nobody else probably will, I tested it. Installed a reset switch yesterday - well two wires I could shortcircuit anyway. Only effect was that my mainboard thought overclocking had failed. It didn't fix the memory issue. So this is a fat NO. PSU power good timing isn't my issue.
  • ojas
    Chris/Don,

    Any idea when the paperback comes out??? Looking to buy it...