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[ The PC Guide | Systems and Components Reference Guide | Power | The Power Supply | Power Supply Form Factors ]

SFX Form Factor

As part of the continuing trend towards smaller and smaller PCs, Intel in 1997 introduced the new microATX form factor, based upon the original ATX form factor. In 1999, Intel produced the FlexATX addendum to the microATX specification, detailing plans for an even smaller motherboard and case standard. Neither of these form factors include specifications for a power supply. Instead, Intel created the SFX power supply form factor, which they may optionally use. The "S" in "SFX" is for "small" of course! microATX and FlexATX systems can also use the ATX power supply, though since miniaturization is the key with these systems, the SFX power supply makes much more sense. You can find detailed specifications on the SFX power supply form factor at the Platform Development Support Web Site.

The SFX specification actually calls for a default configuration, and several options. The "regular" SFX power supply is nominally 100 mm wide, 125 mm deep and 63.5 mm in height. It includes a 60 mm power supply fan for cooling. An optional configuration calls for the placement of a larger fan on the top of the power supply. This fan option is 80 mm and is very often selected by manufacturers as it provides for improved system cooling. It increases the height of the supply by about 10 mm. Another option is for an extra-small power supply with dimensions of only 100 x 125 x 50, and a 40 mm power supply fan. This configuration however requires an additional fan for system cooling, because the small 40 mm fan is only sufficient to cool the power supply itself.

Clockwise from upper left: side, top and rear view diagrams of the SFX standard
design power supply, with the optional top-mounted fan included.

Original image PC Power & Cooling, Inc.
Image used with permission.

In many ways the SFX form factor could be considered a "little brother" to ATX. It is mostly interchangeable with the ATX power supply. The main SFX motherboard connector is 20 pins, in the same shape and size as the ATX connector, and 19 of the pins are the same as those of ATX. The one difference here is that the SFX power supply specification does not call for providing the -5 V compatibility voltage. The reason is that -5 V is only required for ISA bus compatibility, and since Intel wants to move new systems away from ISA (to PCI and AGP only) it intentionally left -5 V off the specification (presumably to save on the cost of the power supply). Systems that need -5 V and want to use the SFX power supply must generate it on the motherboard. The SFX power supply specification calls for the power supply fan to be internally thermally speed-controlled, but an additional "Fan On/Off" signal is included on the SFX optional motherboard connector. Another issue with ATX exchangeability is that an SFX power supply equipped only with the standard 60 mm fan may have considerable trouble cooling a large ATX system case.

View of the top and side of an SFX power supply,
showing the top-mounted cooling fan.

Original image Enlight Corporation
Image used with permission.

The specified output rating of the SFX power supply is 90 W. This is sufficient to run rather small systems with low-powered CPUs and few peripherals, but makes things a bit tight and leaves little room for expansion. Fortunately, some manufacturers are producing SFX power supplies with much higher output ratings.

Next: WTX Form Factor


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