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

Power Supply 101: A Reference Of Specifications
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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.

EPS12V form factor power supply (www.pcpower.com).EPS12V form factor power supply (www.pcpower.com).

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 supply form factor dimensions.TFX12V power supply form factor dimensions.

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.

TFX12V power supplies are symmetrical and can be mounted with the fan facing either left or right.TFX12V power supplies are symmetrical and can be mounted with the fan facing either left or right.

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 supply.CFX12V power supply.

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 power supply dimensions.CFX12V power supply dimensions.

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.

Figure 18.12  LFX12V power supply.Figure 18.12  LFX12V power supply.

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.

LFX12V power supply dimensions.LFX12V power supply dimensions.

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 supply.Flex ATX power supply.

Flex ATX power supply dimensions.Flex ATX power supply dimensions.

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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