Power supply unit (computer)
A power supply unit converts mains AC to low-voltage regulated DC power for the internal components of a desktop computer. Modern personal computers universally use switched-mode power supplies. Some power supplies have a manual switch for selecting input voltage, while others automatically adapt to the main voltage.
Most modern desktop personal computer power supplies conform to the ATX specification, which includes form factor and voltage tolerances. While an ATX power supply is connected to the mains supply, it always provides a 5-volt standby power so that the standby functions on the computer and certain peripherals are powered. ATX power supplies are turned on and off by a signal from the motherboard. They also provide a signal to the motherboard to indicate when the DC voltages are in spec, so that the computer is able to safely power up and boot. The most recent ATX PSU standard is version 3.1 as of mid 2025.
Functions
The desktop computer power supply converts the alternating current from a wall socket of mains electricity to a low-voltage direct current to operate the motherboard, processor and peripheral devices. Several direct-current voltages are required, and they must be regulated with some accuracy to provide stable operation of the computer. A power supply rail or voltage rail refers to a single voltage provided by a PSU.Some PSUs can also supply a standby voltage, so that most of the computer system can be powered off after preparing for hibernation or shutdown, and powered back on by an event. Standby power allows a computer to be started remotely via wake-on-LAN and Wake-on-ring or locally via Keyboard Power ON if the motherboard supports it. This standby voltage may be generated by a small linear power supply inside the unit or a switching power supply, sharing some components with the main unit to save cost and energy.
History
First-generation microcomputer and home computer power supply units used a heavy step-down transformer and a linear power supply, as used, in for example, the Commodore PET introduced in 1977. The Apple II, also introduced in 1977, was noted for its switched-mode power supply, which was lighter and smaller than an equivalent linear power supply would have been, and which had no cooling fan. The switched-mode supply uses a ferrite-cored high frequency transformer and power transistors that switch thousands of times per second. By adjusting the switching time of the transistor, the output voltage can be closely controlled without dissipating energy as heat in a linear regulator. The development of high-power and high-voltage transistors at economical prices made it practical to introduce switched-mode supplies that had been used in aerospace, mainframes, minicomputers and color television, into desktop personal computers. The Apple II design by Atari engineer Rod Holt was awarded a patent, and was in the vanguard of modern computer power supply design. Now all modern computers use switched-mode power supplies, which are lighter, less costly, and more efficient than equivalent linear power supplies.Computer power supplies may have short circuit protection, overpower protection, over-voltage protection, under-voltage protection, over-current protection, and over-temperature protection.
Input voltage switch
Power supplies designed for worldwide use were once equipped with an input voltage selector switch that allowed the user to configure the unit for use on local power grid. In the lower voltage range, around 115 V, this switch is turned on changing the power grid voltage rectifier into a voltage doubler in Delon circuit design. As a result, the large primary filter capacitor behind that rectifier was split up into two capacitors wired in series, balanced with bleeder resistors and varistors that were necessary in the upper input voltage range, around 230 V. Connecting the unit configured for the lower range to a higher-voltage grid usually resulted in immediate permanent damage. When a power-factor correction was required, those filter capacitors were replaced with higher-capacity ones, together with a coil installed in series to delay the inrush current. This is the simple design of a passive PFC.Active PFC is more complex and can achieve higher PF, up to 99%. The first active PFC circuits just delayed the inrush. Newer ones work as an input and output condition-controlled step-up converter, supplying a single 400 V filter capacitor from a wide-range input source, usually between 80 and 240 V. Newer PFC circuits also replace the NTC-based inrush current limiter, which is an expensive part previously located next to the fuse.
Development
Original IBM PC, XT and AT standard
The first IBM PC power supply unit supplied two main voltages: +5 V and +12 V. It supplied two other voltages, −5 V and −12 V, but with limited amounts of power. Most microchips of the time operated on 5 V power. Of the 63.5 W these PSUs could deliver, most of it was on this +5 V rail.The +12 V supply was used primarily to operate motors such as in disk drives and cooling fans. As more peripherals were added, more power was delivered on the 12 V rail. However, since most of the power is consumed by chips, the 5 V rail still delivered most of the power. The −12 V rail was used primarily to provide the negative supply voltage to the RS-232 serial ports. A −5 V rail was provided for peripherals on the ISA bus, but was not used by any motherboard other than the original IBM PC motherboard.
An additional wire referred to as 'Power Good' is used to prevent digital circuitry operation during the initial milliseconds of power supply turn-on, where output voltages and currents are rising but not yet sufficient or stable for proper device operation. Once the output power is ready to use, the Power Good signal tells the digital circuitry that it can begin to operate.
Original IBM power supplies for the PC, XT and AT included a line-voltage power switch that extended through the side of the computer case. In a common variant found in tower cases, the line-voltage switch was connected to the power supply with a short cable, allowing it to be mounted apart from the power supply.
An early microcomputer power supply was either fully on or off, controlled by the mechanical line-voltage switch, and energy saving low-power idle modes were not a design consideration of early computer power supplies. These power supplies were generally not capable of power saving modes such as standby or "soft off", or scheduled turn-on power controls.
Due to the always-on design, in the event of a short circuit, either a fuse would blow, or a switched-mode supply would repeatedly cut the power, wait a brief period of time, and attempt to restart. For some power supplies the repeated restarting is audible as a quiet rapid chirping or ticking emitted from the device.
ATX standard
When Intel developed the ATX standard power supply connector, microchips operating on 3.3 V were becoming more popular, beginning with the Intel 80486DX4 microprocessor in 1994, and the ATX standard supplies three positive rails: +3.3 V, +5 V, and +12 V. Earlier computers requiring 3.3 V typically derived that from a simple but inefficient linear regulator connected to the +5 V rail.The ATX connector provides multiple wires and power connections for the 3.3 V supply, because it is most sensitive to voltage drop in the supply connections. Another ATX addition was the +5 V SB rail for providing a small amount of standby power, even when the computer was nominally "off".
When a computer is in ACPI S3 sleep mode, only +5 V SB rail is used.
There are two basic differences between AT and ATX power supplies: the connectors that provide power to the motherboard, and the soft switch. In ATX-style systems, the front-panel power switch provides only a control signal to the power supply and does not switch the mains AC voltage. This low-voltage control allows other computer hardware or software to turn the system on and off.
Since ATX power supplies share both the same dimensions and the same mounting layout, with the prior format, there is no major physical difference preventing an AT case from accepting an ATX PSU, provided that the specific PSU is not too long for the specific case.
ATX12V standard
As transistors become smaller on chips, it becomes preferable to operate them on lower supply voltages, and the lowest supply voltage is often desired by the densest chip, the central processing unit. In order to supply large amounts of low-voltage power to the Pentium and subsequent microprocessors, a special power supply, the voltage regulator module began to be included on motherboards. Newer processors may require over 200 A at 1.5 V or less, which is impractical to deliver from off-board power supplies.Initially, this was supplied by the main +5 V supply, but as power demands increased, the high currents required to supply sufficient power became problematic. To reduce the power losses in the 5 V supply, with the introduction of the Pentium 4 microprocessor, Intel changed the processor power supply to operate on +12 V, and added the separate four-pin P4 connector to the new ATX12V 1.0 standard to supply that power.
Modern high-powered graphics processing units do the same thing, resulting in most of the power requirement of a modern personal computer being on the +12 V rail. When high-powered GPUs were first introduced, typical ATX power supplies were "5 V-heavy", and could only supply 50–60% of their output in the form of 12 V power. Thus, GPU manufacturers, to ensure 200–250 W of 12 V power, recommended power supplies of 500–600 W or higher. More modern ATX power supplies can deliver almost all of their total rated capacity in the form of +12 V power.
Because of this change, it is important to consider the +12 V supply capacity, rather than the overall power capacity, when using an older ATX power supply with a more recent computer.
Low-quality power supply manufacturers sometimes take advantage of this overspecification by assigning unrealistically high power supply ratings, knowing that very few customers fully understand power supply ratings.