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 power supply unit (PSU) 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 most PCs is described as a constant voltage switching power supply, which is defined as follows:

• 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.

• Switching refers to the design and power regulation technique that most suppliers use. 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 low price.

Seven 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 the two Intel-based designs 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 closely 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 that the power supply you purchase not only plugs into your motherboard, but also fits into the chassis or case you plan to use. Each of these power supply form factors is available in numerous configurations and power output levels.

The LPX form factor supply was 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. Any power supply that does not conform to one of these standards is considered proprietary. Any systems that use proprietary designs should generally be avoided because replacements are difficult to obtain and upgrades are generally not available.

PC/XT Style

IBM's PC and XT systems (circa 1981 and 1983, respectively) used the same power supply form factor; the only difference was that the XT supply had more than double the power output capability. Because they were identical in external appearance and the type of connectors they used, you easily could install the higher-output XT supply as an upgrade for a PC system; thus, the idea of upgrading the power supply was born.

The tremendous popularity of the original PC and XT systems led several manufacturers to begin building systems that mimicked their shapes and layouts.

These clones or compatibles, as they have been called, could interchange virtually all components with the IBM systems, including the power supply. Numerous manufacturers then began producing components that followed the form factor of these systems. This form factor, however, is not used in modern systems today.

AT/Desk Style

The AT desktop system that IBM introduced in August 1984 had a larger power supply and a form factor different from the original PC/XT. Other manufacturers rapidly cloned this system, which represented the basis for many subsequent IBM-compatible designs. The power supply used in these systems was called the AT/Desktop-style power supply.

Hundreds of manufacturers began making motherboards, power supplies, cases, and other components that were physically interchangeable with the original IBM AT. This form factor is no longer used today.

AT/Tower Style

The AT/Tower configuration was basically a full-sized, AT-style desktop system running on its side. The tower configuration was not new; in fact, even IBM's original AT had a specially mounted logo that could be rotated when you ran the system on its side in the tower configuration.

The type of power supply used in most of the AT tower systems was identical to that used in a desktop system, except for the location of the power switch. On the original AT/Desktop systems, the power switch was built in to the side of the power supply (usually in the form of a large toggle switch).

AT/Tower systems instead used an external switch attached to the power supply through a four-wire cable. A full-sized AT power supply with a remote switch is called an AT/Tower form factor power supply and is identical to the AT/Desktop supply in size and dimensions. The only difference is the use of an external switch. This form factor is still used today in large server chassis that run AT form factor motherboards.

Baby-AT Style

Another type of AT-based form factor is the so-called Baby-AT, which is a shortened version of the full-size AT form factor. The power supply in these systems is shortened in one dimension but matches the AT design in all other respects. Baby-AT-style power supplies could fit in place of the larger AT/Desktop-style supply.

However, the full-size AT/Tower supply would not fit in the Baby-AT chassis. Because the Baby-AT PSU performed the same functions as the AT-style power supply but was in a smaller package, it became a common form factor until it was overtaken by later designs. This form factor is rarely used today.

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. 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 a lower height than AT-size 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 needs 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. 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 and soon became 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

The current standard in the industry is the ATX form factor. The ATX specification, now in version 2.03, defines a new motherboard shape from its predecessors, as well as a new case and power supply form factor.

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, 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.

NLX Style

The NLX specification—one that Intel also developed—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.

SFX Style

Intel released the smaller Micro-ATX motherboard form factor in December 1997, and at the same time also released a new smaller SFX power supply design to go with it. 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. Now, the SFX supply has found use in many new compact system designs.