The form factor of a power supply is a description of its size and connection to the motherboard. Size is important; the power supply must fit into a specially designed area in the case and leave room for other internal components. The connection must also fit the motherboard type. Since most power supplies are already installed in the cases, "form factor" occasionally, and incorrectly, refers to the case.
The first personal computer designs made widely available were the IBM PC/XT, using the XT form factor motherboard; the first power supply form factor was the XT. This was an invention of International Business Machines (IBM), but the company wisely decided to leave the hardware standards open so other developers could create their own and expand on them.
In 1984, the Advanced Technology (AT) form factor motherboard was released. It used the same pair of six-conductor motherboard connectors and four-conductor hard drive connectors. The AT form factor tripled the wattage on the power supplies to 192 watts and was the first form factor to be included in tower-style computer cases.
The Baby AT was a decrease in size of both the motherboard and power supply. This power supply fit in either the smaller Baby AT cases or the full-size AT case. This meant more flexibility at a time when PCs were becoming more popular. Many different companies capitalized on this, and the Baby AT was used as the primary form factor for many years.
The LPX form factor is also called the slimline or PS2 power supply, after the popular computer model. The LP in LPX stands for Low Profile. This power supply allows for great flexibility in case size and allows cases to be customized to fit many different areas in the modern office.
The ATX was, at the time of its release, the most radical departure from all previous types of power supplies. It no longer had the extra power port pass-through for the monitor like previous ones, since most modern monitors were sold with their own power cords. Also, it included several revolutionary functions that are standard today: this was the first power supply to offer +3.3 volts, so there was no longer a need for a power converter on the motherboard. It also offered the first software-controlled power switch that was controlled by the motherboard, instead of a large hardware-wired switch on the back of the machine.
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