Power supply unit (computer)

ATX power supply unit with top cover removed

For electronic power supplies in general, not just used in desktop computer devices or computer server devices, see Power supply.

A power supply unit (PSU) converts mains AC to low-voltage regulated DC power for the internal components of the computer. Modern personal computers universally use a switched-mode power supply. Some power supplies have a manual selector for input voltage, while others automatically adapt to the supply voltage.

Most modern desktop personal computer power supplies conform to the ATX form factor. 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 power lines are correct so that the computer is able to boot up. While an ATX power supply is connected to the mains supply it provides a 5 V stand-by (5VSB) line so that the standby functions on the computer and certain peripherals are powered. The most recent ATX PSU standard is version 2.31 of mid-2008.

Functions

The desktop computer power supply changes alternating current from a wall socket to low-voltage direct current to operate the 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 power supply unit (PSU). Although the term is generally used in electronic engineering, many people, especially computer enthusiasts, encounter it in the context of personal computer power supplies.

First-generation microcomputer and home computer power supply units used a heavy step-down transformer and a linear power supply. Modern computers use switched-mode power supplies (SMPS) with a ferrite-cored high frequency transformer. The switched-mode supply is much lighter and less costly, and is more efficient, than an equivalent linear power supply.

Computer power supplies may have short circuit protection, overpower (overload) protection, overvoltage protection, undervoltage protection, overcurrent protection, and over temperature protection.

Recent power supplies have a standby voltage available, to allow most of the computer system to be powered off. When the computer is powered down but the power supply is still on, it can be started remotely via Wake-on-LAN and Wake-on-ring or locally via Keyboard Power ON (KBPO) if the motherboard supports it.

Power supplies may have passive or active power factor correction (PFC). Passive PFC is a simple way of increasing the power factor by putting a coil in series with the primary filter capacitors. Active PFC is more complex and can achieve higher PF, up to 99%.

Development

Original IBM PC, XT, AT standard

The original IBM PC power supply unit (PSU) 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 watts 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 rail was provided for peripherals on the ISA bus, but was not used by the 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", scheduled turn-on, or "last state" power controls, as these concepts didn't exist yet.

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

Main article: ATX

Voltage converter for 80486DX4 processors (5 V to 3.3 V). Note the heat sink on the linear regulator, required to dissipate the wasted power.

When Intel developed the ATX standard power supply connector (published in 1995), 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 which wished to operate on 3.3 V typically used a simple but inefficient linear regulator to generate it from 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 +5sb rail for providing a small amount of standby power, even when the computer was nominally "off".

A typical installation of an ATX form factor computer power supply.

There are two basic differences between AT and ATX power supplies: The connectors that provide power to the motherboard, and the soft switch. On ATX power supplies, 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 hardware or software to turn the system on and off.

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 require up to 100 amperes at 2 volts 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 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 watts of 12 V power (peak load, CPU+GPU), recommended power supplies of 500–600 W or higher. More modern ATX power supplies can deliver almost all (typically 80–90%) 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.

+3.3 V and +5 V rails

These voltage supplies are rarely a limiting factor; generally any supply with a sufficient +12 V rating will have adequate capacity at lower voltages. However, a large quantity of hard drives or PCI cards will create a greater load on the +5 V rail. A linear regulator could be used to convert the +12 V rail into a +5 V rail for each hard drive if the +5 V rail is overloaded.

Entry-Level Power Supply Specification

Entry-Level Power Supply Specification' (EPS) is a power supply unit meant for high-power-consumption computers and entry-level servers. Developed by the Server System Infrastructure (SSI) forum, a group of companies including Intel, Dell, Hewlett-Packard and others, that works on server standards, the EPS form factor is a derivative of the ATX form factor. The EPS standard provides a more powerful and stable environment for critical server-based systems and applications. EPS power supplies have a 24-pin motherboard power connector and an 8-pin +12V connector. The standard also specifies two additional 4-pin 12V connectors for more power hungry boards (one required on 700–800W PSUs, both required on 850W+ PSUs). EPS power supplies are in principle compatible with standard ATX or ATX12V motherboards found in homes and offices but there may be mechanical issues where the 12V connector and in the case of older boards the main connector overhang the sockets. Many PSU vendors use connectors where the extra sections can be unclipped to avoid this issue. As with later versions of the ATX PSU standard there is also no -5V rail. The latest specification is v2.93.

Multiple +12 V rails

As power supply capacity increased, the ATX power supply standard was amended (beginning with version 2.0) to include:

3.2.4. Power Limit / Hazardous Energy Levels

Under normal or overload conditions, no output shall continuously provide more than 240

VA under any conditions of load including output short circuit, per the requirement of UL 1950/​CSA 950/​EN 60950/​IEC 950.

—ATX12V Power Supply Design Guide, version 2.2

This is a safety limit on the amount of power that may pass, in case of a fault, through any one wire. That much power can significantly overheat a wire, and would be more likely to melt the insulation and possibly start a fire. Each wire must be current-limited to no more than 20 A; typical supplies guarantee 18 A without triggering the current limit. Power supplies capable of delivering more than 18 A at 12 V connect wires in groups to two or more current sensors which will shut down the supply if excess current flows. Unlike a fuse or circuit breaker, these limits reset as soon as the overload is removed.

Ideally, there would be one current limit per wire, but that would be prohibitively expensive. Since the limit is far larger than the reasonable current draw through a single wire, manufacturers typically group several wires together and apply the current limit to the entire group. Obviously, if the group is limited to 240 VA, so is each wire in it. Typically, a power supply will guarantee at least 17 A at 12 V by having a current limit of 18.5 A, plus or minus 8%. Thus, it is guaranteed to supply at least 17 A, and guaranteed to cut off before 20 A.

These groups are the so-called "multiple power supply rails". They are not fully independent; they are all connected to a single high-current 12 V source inside the power supply, but have separate current limit circuitry. The current limit groups are documented so the user can avoid placing too many high-current loads in the same group. Originally, a power supply featuring "multiple +12 V rails" implied one able to deliver more than 20  A of +12 V power, and was seen as a good thing. However, people found the need to balance loads across many +12 V rails inconvenient. When the assignment of connectors to rails is done at manufacturing time it is not always possible to move a given load to a different rail.

Rather than add more current limit circuits, many manufacturers have chosen to ignore the requirement and increase the current limits above 20 A per rail, or provide "single-rail" power supplies that omit the current limit circuitry. (In some cases, in violation of their own advertising claims to include it. For one example of many, see ) The requirement was deleted from version 2.3 (March 2007) of the ATX12V power supply specifications.

Because of the above standards, almost all high-power supplies claim to implement separate rails, however this claim is often false; many omit the necessary current-limit circuitry, both for cost reasons and because it is an irritation to customers. (The lack is sometimes advertised as a feature under names like "rail fusion" or "current sharing".)

12V-only supplies

Since 2011, Fujitsu and other Tier 1 manufacturers have been manufacturing systems containing motherboard variants which require only a 12V supply from a custom made PSU (typically rated at 250–300W). DC-DC conversion, providing 5V and 3.3V, is done on the motherboard; the proposal is that 5V and 12V supply for other devices, such as HDDs, will be picked up at the motherboard rather than from the PSU itself (though this does not appear to be fully implemented as of January 2012). The reasons given for this approach to power supply are that it eliminates cross-load problems, simplifies and reduces internal wiring which can affect airflow and cooling, reduces costs, increases power supply efficiency and reduces noise by bringing the power supply fan speed under the control of the motherboard.

Power rating

As all of the rails come from one transformer and primary-side switching components, there is an overall maximum power limit. Power requirements for a modern desktop personal computer may range from 300 watts to more than 1000 watts for a high-performance computer with multiple discrete graphics cards. The power rating of a PC power is rated by the manufacturer. Simple, general purpose computers rarely require more than 300–350 watts maximum.

It is possible to overload one voltage from a power supply well below the total rating of the power supply. For example, most PSUs create their 3.3 V output by regulating down their 5 V rail. As such, 3.3 V and 5 V typically have a combined limit as well. A 3.3 V rail may have a 10 A rating by itself (33 W), and the 5 V rail may have a 20 A rating (100  W) by itself, but the two together may only be able to output 110 W. In this case, loading the 3.3 V rail to maximum (33 W), would leave the 5 V rail only be able to output 77 W.

Since supplies are self-certified, a manufacturer's claimed output may be double or more what is actually provided. Although a too-large power supply will have an extra margin of safety as far as not over-loading, a larger unit is often less efficient at lower loads (under 20% of its total capability) and therefore will waste more electricity than a more appropriately sized unit. Computer power supplies generally may shut down or malfunction if they are loaded too lightly, less than about 15% of rated total load. Some power supplies have no-load protection.

The most important factor for suitability for certain graphics cards is the PSUs total 12V output. If the total 12V output stated on the PSU is higher than the suggested minimum of the card, then that PSU can fully supply the card. However a system will have other loads on the 12 volt supply.

Power supplies are usually sized so that the typical calculated system consumption is about 60% of the rated capacity, and the maximum system demand does not exceed the rated capacity of the supply. The power supply ratings often given by the manufacturer of single component, typically graphics cards, should be treated with skepticism. These manufacturers want to minimize support issues due to under rating of the power supply specifications and advise customers to use a more powerful power supply to avoid these issues.^{[citation needed]}

Efficiency

See also: Green computing

Computer power supplies are generally about 70–75% efficient. For a 75% efficient power supply to produce 75 W of DC output it would require 100 W of AC input and dissipate the remaining 25 W in heat. Higher-quality power supplies can be over 80% efficient; energy efficient PSU's waste less energy in heat, and requires less airflow to cool, and as a result will be quieter. Google's server power supplies are more than 90% efficient. HP's server power supplies have reached 94% efficiency. Standard PSUs sold for server workstations have around 90% efficiency, as of 2010.

It is important to match the capacity of a power supply to the power needs of the computer. The energy efficiency of power supplies drops significantly at low loads. Efficiency generally peaks at about 50–75% load. The curve varies from model to model (examples of how this curve looks can be seen on test reports of energy efficient models found on the 80 PLUS website).

Various initiatives are underway to improve the efficiency of computer power supplies. Climate savers computing initiative promotes energy saving and reduction of greenhouse gas emissions by encouraging development and use of more efficient power supplies. 80 PLUS certifies power supplies that meet certain efficiency criteria, and encourages their use via financial incentives. On top of that the businesses end up using less electricity to cool the PSU and the computer's themselves and thus save an initially large sum(i.e. incentive + saved electricity = higher profit).

Appearance

Most desktop personal computer power supplies are a square metal box, and have a large bundle of wires emerging from one end. Opposite the wire bundle is the back face of the power supply, with an air vent and an IEC 60320 C14 connector to supply AC power. There may be a power switch or a voltage selector switch or both.

A label on one side of the box lists technical information about the power supply, including safety certifications and maximum output power. Common certification marks for safety are the UL mark, GS mark, TÜV, NEMKO, SEMKO, DEMKO, FIMKO, CCC, CSA, VDE, GOST R and BSMI. Common certificate marks for EMI/RFI are the CE mark, FCC and C-tick. The CE mark is required for power supplies sold in Europe and India. A RoHS or 80 PLUS can also sometimes be seen.

Dimensions of an ATX power supply are 150 mm width, 86 mm height, and typically 140 mm depth, although the depth can vary from brand to brand.

Some power supplies come with sleeved cables, which are aesthetically nicer, makes wiring easier and cleaner and have less detrimental effect on airflow.

Connectors

Various connectors from a computer PSU.

Typically, power supplies have the following connectors (all are Molex (USA) Inc Mini-Fit Jr, unless otherwise indicated):

• PC Main power connector (usually called P1): This is the connector that goes to the motherboard to provide it with power. The connector has 20 or 24 pins. One of the pins belongs to the PS-ON wire (it is usually green). This connector is the largest of all the connectors. In older AT power supplies, this connector was split in two: P8 and P9. A power supply with a 24-pin connector can be used on a motherboard with a 20-pin connector. In cases where the motherboard has a 24-pin connector, some power supplies come with two connectors (one with 20-pin and other with 4-pin) which can be used together to form the 24-pin connector.
• 12V only power connector (labelled P1, though it is not compatible with the ATX 20 or 24 pin connector): This is a 16-pin Molex connector supplying the motherboard with six 12V lines with common returns, a 'supply OK' signal, a 'PSU ON' signal and an 11V auxiliary supply. One pin is left unused.
• 12V only System monitoring (P10): This is a 171822-8 AMP or equivalent connector carrying a supply to the PSU fan and sense returns.
• ATX12V 4-pin power connector (also called the P4 power connector). A second connector that goes to the motherboard (in addition to the main 24-pin connector) to supply dedicated power for the processor. For high-end motherboards and processors, more power is required, therefore EPS12V has an 8-pin connector.
• 4-pin Peripheral power connectors: These are the other, smaller connectors that go to the various disk drives of the computer. Most of them have four wires: two black, one red, and one yellow. Unlike the standard mains electrical wire color-coding, each black wire is a ground, the red wire is +5 V, and the yellow wire is +12 V. In some cases these are also used to provide additional power to PCI cards such as FireWire 800 cards.
• 4-pin Molex (Japan) Ltd power connectors (usually called Mini-connector or "mini-Molex"): This is one of the smallest connectors that supplies a 3 1/2 inch floppy drive with power. In some cases, it can be used as an auxiliary connector for AGP video cards. Its cable configuration is similar to the Peripheral connector.
• Auxiliary power connectors: There are several types of auxiliary connectors designed to provide additional power if it is needed.
• Serial ATA power connectors: a 15-pin connector for components which use SATA power plugs. This connector supplies power at three different voltages: +3.3, +5, and +12 volts.
• 6-pin Most modern computer power supplies include 6-pin connectors which are generally used for PCI Express graphics cards, but a newly introduced 8-pin connector should be seen on the latest model power supplies. Each PCI Express 6-pin connector can output a maximum of 75 W.
• 6+2 pin For the purpose of backwards compatibility, some connectors designed for use with high end PCI Express graphics cards feature this kind of pin configuration. It allows either a 6-pin card or an 8-pin card to be connected by using two separate connection modules wired into the same sheath: one with 6 pins and another with 2 pins.
• A IEC 60320 C14 connector with an appropriate C13 cord is used to attach the power supply to the local power grid.

Modular power supplies

See also: Modular power supply unit

A modular power supply to the left and a non-modular power supply to the right.

A modular power has some permanent multiwire cables with connectors at the ends such as PC main and 4-pin molex but also has connectors mounted directly on it allowing for unused cables to be removed, and cables to be matched in length and type to the system layout. This reduces clutter and removes the risk of dangling cables interfering with other components. A small amount of extra resistance is introduced by the additional connector. Airflow within a case may also be improved by eliminating superfluous cables.

Other form factors

The Thin Form Factor with 12 Volt connector (TFX12V) configuration has been optimized for small and low profile microATX and FlexATX system layouts. The long narrow profile of the power supply (shown in Figure 1) fits easily into low profile systems. The fan placement can be used to efficiently exhaust air from the processor and core area of the motherboard, making possible smaller, more efficient systems using common industry ingredients.

Most portable computers have power supplies that provide 25 to 200 watts. In portable computers (such as laptops) there is usually an external power supply (sometimes referred to as a "power brick" due to its similarity, in size, shape and weight, to a real brick) which converts AC power to one DC voltage (most commonly 19 V), and further DC-DC conversion occurs within the laptop to supply the various DC voltages required by the other components of the portable computer.

Some web servers use a single-voltage 12 volt power supply. All other voltages are generated by voltage regulator modules on the motherboard.

Life span

Life span is usually measured in mean time between failures (MTBF). Higher MTBF ratings are preferable for longer device life and reliability. Quality construction consisting of industrial grade electrical components or a larger or higher speed fan can help to contribute to a higher MTBF rating by keeping critical components cool. Overheating is a major cause of PSU failure. Calculated MTBF value of 100,000 hours (about 11 years continuous operation) is not uncommon.

Power supplies for servers, industrial control, or other places where reliability is important may be hot swappable, and may incorporate N+1 redundancy; if N power supplies are required to meet the load requirement, one extra is installed to allow for swaps.

Wiring diagrams

AT power connector (Used on older AT style mainboards) Color Pin Signal P8.1 Power Good P8.2 +5 V P8.3 +12 V P8.4 −12 V P8.5 Ground P8.6 Ground P9.1 Ground P9.2 Ground P9.3 −5 V P9.4 +5 V P9.5 +5 V P9.6 +5 V

24-pin ATX12V 2.x power supply connector (20-pin omits the last four: 11, 12, 23 and 24) Color Signal Pin Pin Signal Color Orange +3.3 V 1 13 +3.3 V Orange +3.3 V sense Brown Orange +3.3 V 2 14 −12 V Blue Black Ground 3 15 Ground Black Red +5 V 4 16 Power on Green Black Ground 5 17 Ground Black Red +5 V 6 18 Ground Black Black Ground 7 19 Ground Black Grey Power good 8 20 Reserved N/C Purple +5 V standby 9 21 +5 V Red Yellow +12 V 10 22 +5 V Red Yellow +12 V 11 23 +5 V Red Orange +3.3 V 12 24 Ground Black Pins 8, and 16 (shaded) are control signals, not power: Power on is pulled up to +5 V by the PSU, and must be driven low to turn on the PSU. Power good is low when other outputs have not yet reached, or are about to leave, correct voltages. Pin 13 supplies +3.3 V power and also has a second thinner wire for remote sensing. Pin 20 (formerly −5 V, white wire) is absent in current power supplies; it was optional in ATX and ATX12V ver. 1.2, and deleted as of ver. 1.3. The right-hand pins are numbered 11–20 in the 20-pin version.

Testing

A power supply tester is a tool used to test the functionality of a computer's power supply. Testers can confirm the presence of the correct voltages at each power supply connector. Testing under load is recommended for the most accurate readings.

See also

• Hybrid voltage system
• List of manufacturers
• Power management
• Power supply
• Quiet PC

External links

• How PC Power Supplies Work
• Website with Information & Research on Active Mode Power Supply Efficiency
• How to Buy an Energy-Efficient Power Supply
• Computer Power Supply - Schematics, Reviews, Repair Guides
• PC Repair and Maintenance: In-depth Look at Power Supply
• Specifications for various PSUs
• ATX12V Power Supply Design Guide v2.2
• Power Supply Design Guide for Desktop Platform Form Factors Revision 1.2
• How to Discover Your Power Supply Real Manufacturer
• How Much Power Can a Generic 500 W Power Supply Really Deliver?
• Everything You Need To Know About Power Supplies
• PC Power Supply Connections and Pinouts
• What is power supply for computers?

Computer power supply calculators

• eXtreme Power Supply Calculator (frequently updated)
• SnooP and goodone's Power Supply Calculator; provides rails' amperage distribution
• Computer Power Supply Calculator (outdated)
• Journey Systems' online power supply calculator