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Power Supply Form Factors

The shape and general physical layout of a component is called the form factor. Items that share a form factor are generally interchangeable, at least as far as their sizes and fits are concerned. When designing a PC, the engineers can choose to use one of the popular standard PSU (power supply unit) form factors, or they can elect to build their own. Choosing the former means that a virtually inexhaustible supply of inexpensive replacement parts will be available in a variety of quality and power output levels. Going the custom route means additional time and expense for development. In addition, the power supply is unique to the system and available only from the original manufacturer.

If you can't tell already, I am a fan of the industry-standard form factors! Having standards and then following them allows us to upgrade and repair our systems by easily replacing physically (and electrically) interchangeable components. Having interchangeable parts means that we have a better range of choices for replacement items, and the competition makes for better pricing, too.

In the PC market, IBM originally defined the standards, and everybody else copied them. This included power supplies. All the popular PC power supply form factors up through 1995 were based on one of three IBM models, including the PC/XT, AT, and PS/2 Model 30. The interesting thing is that these three power supply definitions all had the same motherboard connectors and pinouts; where they differed was mainly in shape, maximum power output, the number of peripheral power connectors, and switch mounting. PC systems using knock-offs of one of those three designs were popular through 1996 and beyond, and some systems still use them today.

Intel gave the power supply a new definition in 1995 with the introduction of the ATX form factor. ATX became popular in 1996 and started a shift away from the previous IBM-based standards. ATX and the related standards that followed have different connectors with additional voltages and signals that allow systems with greater power consumption and additional features that would otherwise not be possible with the AT style supplies.

Technically, the power supply in your PC is described as a constant voltage half-bridge forward converting switching power supply:

  • Constant voltage means that the power supply puts out the same voltage to the computer's internal components, no matter what the voltage of AC current running it or the capacity (wattage) of the power supply.

  • Half-bridge forward converting switching refers to the design and power regulation technique used by most suppliers. This design is commonly referred to as a switching supply. Compared to other types of power supplies, this design provides an efficient and inexpensive power source and generates a minimum amount of heat. It also maintains a small size and a low price.


Although two power supplies can share the same basic design and form factor, they can differ greatly in quality and efficiency. Later in this chapter, you'll learn about some of the features and specifications to look for when evaluating PC power supplies.

Seven main power supply physical form factors have existed that can be called industry standard. Five of these are based on IBM designs, whereas two are based on Intel designs. Of these, only three are used in most modern systems; the others are pretty much obsolete.

Note that although the names of the power supply form factors seem to be the same as those of motherboard form factors, the power supply form factor is more related to the system chassis (case) than the motherboard. That is because all the form factors use one of only two types of connector designs, either AT or ATX.

For example, all PC/XT, AT, and LPX form factor supplies use the same pair of six-pin connectors to plug into the motherboard and will therefore power any board having the same type of power connections. Plugging into the motherboard is one thing, but for the power supply to physically work in the system, it must fit the case. The bottom line is that it is up to you to ensure the power supply you purchase not only plugs into your motherboard but also fits into the chassis or case you plan to use.

Table 3.2 shows modern power supply form factors, their connection types, and associated motherboards.

Table 3.2 Power Supply Connector Types and Form Factors

Modern PS Form Factors

Originated From

Connector Type

Associated MB Form Factors

LPX style*

IBM PS/2 Model 30 (1987)


Baby-AT, Mini-AT, LPX

ATX style

Intel ATX, ATX12V (1985/2000)



SFX style

Intel SFX (1997)


Flex-ATX, Micro-ATX

*Note: LPX is also sometimes called Slimline or PS/2.

Each of these power supply form factors is available in numerous configurations and power output levels. The LPX form factor supply had been the standard used on most systems from the late 1980s to mid-1996, when the ATX form factor started to gain in popularity. Since then, ATX has become by far the dominant form factor for power supplies, with the new SFX style being added as an ATX derivative for use in very compact systems that mainly use Flex-ATX–sized boards. The earlier IBM-derived form factors have been largely obsolete for some time now.

→ See "Motherboard Form Factors," p. 84

LPX Style

The next power supply form factor to gain popularity was the LPX style, also called the PS/2 type, Slimline, or slim style (see Figure 3.1). The LPX-style power supply has the exact same motherboard and disk drive connectors as the previous standard power supply form factors; it differs mainly in the shape. LPX systems were designed to have a smaller footprint and lower height than AT-sized systems. These computers used a different motherboard configuration that mounts the expansion bus slots on a "riser" card that plugs into the motherboard. The expansion cards plug into this riser and are mounted sideways in the system, parallel to the motherboard. Because of its smaller case, an LPX system needed a smaller power supply. The power supply designed for LPX systems is smaller than the Baby-AT style in every dimension and takes up less than half the space of its predecessor.


IBM used this type of power supply in some of its PS/2 systems in the late 1980s; hence it is sometimes called a PS/2-type supply.

Figure 3.1Figure 3.1 LPX form factor power supply.

As with the Baby-AT design in its time, the LPX power supply does the same job as its predecessor but comes in a smaller package. The LPX power supply quickly found its way into many manufacturers' systems, soon becoming a de facto standard. This style of power supply became the staple of the industry for many years, coming in everything from low-profile systems using actual LPX motherboards to full-size towers using Baby-AT or even full-size AT motherboards. It still is used in some PCs produced today; however, since 1996 the popularity of LPX has been overshadowed by the increasing popularity of the ATX design.

ATX Style

One of the newer standards in the industry today is the ATX form factor (see Figure 3.2). The ATX specification, now in version 2.03, defines a new motherboard shape, as well as a new case and power supply form factor.

Figure 3.2Figure 3.2 ATX form factor power supply, used with both ATX and NLX systems.

The shape of the ATX power supply is based on the LPX design, but some important differences are worth noting.

One difference is that the ATX specification originally called for the 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 as 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, more free 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 would be blown out away from the drive because the only air intake would be 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 to pressurize the case. Also, if a filter is used, it must be serviced on a periodic basis—depending on operating conditions, it can need changing or cleaning as often as every week. In addition, the heat load from the power supply on a fully loaded system heats up the air being ingested, blowing warm air over the CPU, reducing overall cooling capability. As newer CPUs create more and more heat, the cooling capability of the system becomes more critical. In common practice, it was found that using a 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 is the best solution. For this reason, the ATX power supply specification has been amended to allow for either positive- or negative-pressure ventilation.

Because a standard negative-pressure system offers the most cooling capacity for a given fan airspeed and flow, most of the newer ATX-style power supplies use the negative-pressure cooling system.

The ATX specification was first released by Intel in 1995. In 1996, it became increasingly popular in Pentium and Pentium Pro–based PCs, capturing 18% of the motherboard market. Since 1996, ATX has become the dominant motherboard form factor, displacing the previously popular Baby-AT. ATX and its derivatives are likely to remain the most popular form factor for several years to come.

The ATX form factor addressed several problems with the power supplies used with Baby-AT and mini-AT form factors. One is that the power supplies used with Baby-AT boards have two connectors that plug into the motherboard. If you insert these connectors backward or out of their normal sequence, you will fry the motherboard! Most responsible system manufacturers "key" the motherboard and power supply connectors so that you cannot install them backward or out of sequence. However, some vendors of cheaper systems do not feature this keying on the boards or supplies they use. The ATX form factor includes different power plugs for the motherboard to prevent users from plugging in their power supplies incorrectly. The ATX design features up to three motherboard power connectors that are definitively keyed, making plugging them in backward virtually impossible. The new ATX connectors also supply +3.3v, reducing the need for voltage regulators on the motherboard to power the chipset, DIMMs, and other +3.3v circuits.

Besides the new +3.3v outputs, another set of outputs is furnished by an ATX power supply that is not normally 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 wake-up 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—exactly like Apple systems. Users can enable these features because the 5v Standby power is always active, giving the motherboard a limited source of power even when off. Check your BIOS Setup for control over these features.

NLX Style

The NLX specification, also developed by Intel, defines a low-profile case and motherboard design with many of the same attributes as the ATX. In fact, for interchangeability, NLX systems were designed to use ATX power supplies, even though the case and motherboard dimensions are different.

As in previous LPX systems, the NLX motherboard uses a riser board for the expansion bus slots. Where NLX differs is that it is a true (and not proprietary) standard. See Chapter 4, "Motherboards and Buses," for more information on the NLX form factor.

For the purposes of this discussion, NLX systems use ATX power supplies. The only real difference is that the supply plugs into the riser card and not the motherboard, enabling NLX motherboards to be more quickly and easily removed from their chassis for service.

SFX Style

Intel released the smaller Micro-ATX motherboard form factor in December of 1997, and at the same time also released a new smaller SFX (Small form factor) power supply design to go with it (see Figure 3.3). Even so, most Micro-ATX chassis used the standard ATX power supply instead. Then in March 1999, Intel released the Flex-ATX addendum to the Micro-ATX specification, which was a very small board designed for low-end PCs or PC-based appliances. At this point, the SFX supply has found use in many new compact system designs.

Figure 3.3Figure 3.3 SFX style power supply (with 90mm top-mounted cooling fan).

The SFX power supply is specifically designed for use in small systems containing a limited amount of hardware and limited upgradability. Most SFX supplies can provide 90 watts of continuous power (135 watts at its peak) in four voltages (+5, +12, –12, and +3.3v). This amount of power has proved to be sufficient for a small system with a processor, an AGP interface, up to four expansion slots, and three peripheral devices—such as hard drives and CD-ROMs.

Although Intel designed the SFX power supply specification with the Micro-ATX and Flex-ATX motherboard form factors in mind, SFX is a wholly separate standard that is compliant with other motherboards as well. SFX power supplies use the same 20-pin connector defined in the ATX standard and include both the Power_On and 5v_Standby outputs. Whether you will use an ATX or SFX power supply in a given system is dependent 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.

One limiting factor on the SFX design is that it lacks the –5v and so shouldn't be used with motherboards that have ISA slots (most Micro-ATX and Flex-ATX boards do NOT have ISA slots). SFX power supplies also won't have the Auxiliary (3.3v and 5v) or ATX12V power connectors, and therefore shouldn't be used with full-size ATX boards that require those connections.

On a standard model SFX power supply, a 60mm diameter cooling fan is located on the surface of the 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. In many cases, additional fans might be needed in the system to cool the processor (see Figure 3.4).

Figure 3.4Figure 3.4 SFX form factor power supply dimensions with a standard internal 60mm fan.

For systems that require more cooling capability, a version that allows for a larger 90mm top-mounted cooling fan also is available. The larger fan provides more cooling capability and airflow for systems that need it (see Figure 3.5).

Figure 3.5Figure 3.5 SFX form factor power supply dimensions with an internal 90mm top-mounted fan.

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