Power Supply 101: A Reference Of Specifications

Modern Form Factors: ATX And SFX

The power supply form factors detailed in the following sections are the standards used in current systems. ATX is far and away the most common of these, but if you work on a variety of PC types, you are likely to encounter the other types listed here.


In 1995, Intel saw that the existing power supply designs were literally running out of power. The problem was that the existing standards used two connectors with a total of only 12 pins providing power to the motherboard. In addition, the connectors used were difficult to properly key, and plugging them in improperly resulted in short-circuiting and damage to both the motherboard and the power supply. To solve these problems, in 1995 Intel took the existing popular LPX (PS/2) design and simply changed the internal circuitry and connectors (while leaving the mechanical shape the same), giving birth to the ATX power supply form factor.

Intel released the ATX specification in 1995, and in 1996, it started to become increasingly popular in Pentium and Pentium Pro–based PCs, capturing 18% of the market within the first year. Since 1996, ATX variants have become both the dominant motherboard and power supply form factors, replacing the previously popular Baby-AT/LPX designs. ATX12V power supplies are also used with newer motherboard form factors such as BTX, ensuring that ATX and its derivatives will remain the most popular power supply form factors for several years to come. The ATX12V specification defines the physical or mechanical form as well as the electrical connectors for the power supply.

From 1995 through early 2000, the ATX power supply form factor was defined as part of the ATX motherboard specification. However, in February 2000, Intel took the power supply specification out of the then-current ATX 2.03 motherboard/chassis specification and created the ATX/ATX12V power supply specification 1.0, adding an optional four-pin +12 V connector at the same time (those with the +12 V connector were called ATX12V supplies). The +12 V connector was made a requirement in version 1.3 (April 2002), whereupon the specification became only ATX12V. The ATX12V 2.0 specification (February 2003) dropped the six-pin auxiliary connector, changed the main power connector to 24 pins, and made Serial ATA power connectors a requirement. The current version is ATX12V 2.2, which was released in March 2005 and contains only minor changes from the previous releases, such as the use of Molex High Current System (HCS) terminals in the connectors.

As the ATX power supply specification has evolved, there have been some changes in the cooling fan orientation and design. The ATX specification originally called for an 80 mm fan to be mounted along the inner side of the supply, where it could draw air in from the rear of the chassis and blow it inside across the motherboard. This kind of airflow runs in the opposite direction than most standard supplies, which exhaust air out the back of the supply through a hole in the case where the fan protrudes. The idea was that the reverse-flow design could cool the system more efficiently with only a single fan, eliminating the need for a fan (active) heatsink on the CPU.

Another benefit of the reverse-flow cooling is that the system would run cleaner, freer from dust and dirt. The case would be pressurized, so air would be continuously forced out of the cracks in the case—the opposite of what happens with a negative pressure design. For this reason, the reverse-flow cooling design is often referred to as a positive-pressure-ventilation design. On an ATX system with reverse-flow cooling, the air is blown out away from the drive because the only air intake is the single fan vent on the power supply at the rear. For systems that operate in extremely harsh environments, you can add a filter to the fan intake vent to further ensure that all the air entering the system is clean and free of dust.

Although this sounds like a good way to ventilate a system, the positive-pressure design needs to use a more powerful fan to pull the required amount of air through a filter and pressurize the case. Also, if a filter is used, it must be serviced periodically; depending on operating conditions, it could need changing or cleaning as often as every week. In addition, the heat load from the power supply on a fully loaded system heats the air being ingested, blowing warm air over the CPU and reducing the overall cooling capability.

As CPUs evolved to generate more and more heat, the cooling capability of the system became more critical and the positive-pressure design was simply not up to the task. Therefore, subsequent versions of the ATX specification were rewritten to allow both positive- and negative-pressure designs, but they emphasized the standard negative-pressure system with an exhaust fan on the power supply and an additional high-quality cooling fan blowing cool air right on the CPU as the best solution.

Because a standard negative-pressure system offers the greatest cooling capacity for a given fan’s airspeed and flow, virtually all recent ATX-style power supplies use a negative-pressure design, in which air flows out the back of the power supply. Most use an 80 mm fan mounted on the rear of the unit blowing outward, but some use an 80 mm, a 92 mm, or a 120 mm fan mounted on the inside upper or lower surface, with open vents on the rear of the system. In either example, the flow of air is such that air is always exhausted out of the system through the rear of the supply.

The ATX power supply form factor addressed several problems with the previous PC/XT, AT, and LPX-type supplies. One is that the power supplies used with PC/XT/AT boards had only two connectors that plugged into the motherboard. If you inserted these connectors backward or out of their normal sequence, you would usually fry both the motherboard and the power supply! Most responsible system manufacturers tried to “key” the motherboard and power supply connectors so you couldn’t install them backward or out of sequence. However, most vendors of cheaper systems did not feature this keying on the boards or supplies they used. The ATX form factor includes intelligently designed and keyed power plugs to prevent users from incorrectly plugging in their power supplies. The ATX connectors also supply +3.3 V, reducing the need for voltage regulators on the motherboard to power +3.3 V-based circuits.

Besides the new +3.3 V outputs, ATX power supplies furnish another set of outputs that is not typically seen on standard power supplies. The set consists of the Power_On (PS_ON) and 5V_Standby (5VSB) outputs mentioned earlier, known collectively as Soft Power. This enables features to be implemented, such as Wake on Ring or Wake on LAN, in which a signal from a modem or network adapter can actually cause a PC to wake up and power on. Many such systems also have the option of setting a wakeup time, at which the PC can automatically turn itself on to perform scheduled tasks. These signals also can enable the optional use of the keyboard to power the system on—an option you can set on some systems. These features are possible because the +5 V Standby power is always active, giving the motherboard a limited source of power even when off. Check your BIOS Setup for control over these types of features.


Intel released the smaller microATX motherboard form factor in December 1997. At the same time, it released the small form factor (SFX) power supply design to go with it. Even so, most microATX chassis continued to use the standard ATX power supply instead. Then in March 1999, Intel released the FlexATX addendum to the microATX specification, which was a small board designed for low-end PCs or PC-based appliances. Since then, the SFX supply has found use in many new compact system designs. Unlike most of the power supply form factor specifications in which a single mechanical or physical outline is defined, the SFX standard actually defines five different physical shapes, some of which are not directly interchangeable. In addition, there have been changes to the connectors required as the specification has evolved. Therefore, when replacing an SFX/SFX12V-type supply, you need to ensure you are purchasing the correct type—which is to say the type that will physically install in your chassis—as well as have the correct connectors for your motherboard.

The number and types of connectors have changed over the life of the specification. The original SFX power supply specification included a single 20-pin motherboard connector. The four-pin +12 V connector to provide independent CPU power was added as an option in the 2.0 revision (May 2001) and was made a requirement in revision 2.3 (April 2003), causing the spec to be renamed as SFX12V in the process. SFX12V version 3.0 changed the main motherboard power connector from 20 pins to 24 pins and made Serial ATA power connectors a requirement. The current SFX12V version 3.1 was released in March 2005 and contains a few additional minor revisions, including a change to High Current System (HCS) terminals in the connectors. SFX12V includes several physical designs, including one called the PS3 form factor.

On a standard SFX/SFX12V power supply, a 60 mm diameter cooling fan is located inside the power supply housing, facing the inside of the computer’s case. The fan draws the air into the power supply housing from the system cavity and expels it through a port at the rear of the system. Internalizing the fan in this way reduces system noise and results in a standard negative-pressure design. The system can also use additional processor and chassis cooling fans, which are separate from the power supply.

For systems that require more cooling capability, a version that allows for a larger, 80 mm top-mounted cooling fan also is available. The larger fan provides more cooling capability and airflow for systems that need it.

Another variation of SFX12V also uses a recessed top-mounted 80 mm cooling fan, but it has the body of the power supply rotated for greater width but reduced depth, as shown in the following image.

A special low-profile version of SFX12V designed for a slim chassis is only 50 mm tall with an internal 40 mm cooling fan, as shown in the image below that.

Finally, a more recent variation on SFX is called the PS3 form factor, defined in Appendix E of the SFX12V specification. Although defined as part of SFX12V, the PS3 form factor is actually a shortened version of ATX12V and is generally used in systems with microATX chassis and motherboards that require higher power output than the smaller SFX variants can supply.

SFX12V power supplies are specifically designed for use in small systems containing a limited hardware and limited upgradeability. Most SFX supplies are designed to provide 80–300 watts of continuous power in four voltages (+5 V, +12 V, –12 V, and +3.3 V). This amount of power has proven to be sufficient for a small system with a processor, an AGP or PCI Express x16 interface, up to four expansion slots, and three peripheral devices—such as hard drives and optical drives.

Although Intel designed the SFX12V power supply specification with the microATX and FlexATX motherboard form factors in mind, SFX is a wholly separate standard that is compliant with other motherboards as well. For example, the PS3 variant of SFX12V can replace standard ATX12V power supplies as long as the output capabilities and provided connectors match the system requirements. SFX power supplies use the same 20-pin or 24-pin connectors defined in the ATX/ATX12V standards and include both the Power_On and 5V_Standby outputs. SFX12V power supplies add the four-pin +12 V connector for CPU power, just as ATX12V supplies do. Whether you will use an ATX- or SFX-based power supply in a given system depends more on the case or chassis than the motherboard. Each has the same basic electrical connectors; the main difference is which type of power supply the case is physically designed to accept.

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