Power supply unit (computer)
From Wikipedia, the free encyclopedia
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 a 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.
Contents |
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
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".
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 4-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 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.[1]
+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.[2]
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[3]) 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[4]
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 [5]) The requirement was deleted from version 2.3 (March 2007) of the ATX12V power supply specifications.[6]
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,[7] both for cost reasons and because it is an irritation to customers.[8] (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[9]
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. Other advantages it offers is the
potential ability to power a PC off a sealed lead-acid 12 volt battery, or from automotive power without using a power inverter.
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
300–500 watts maximum.[8]
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.[10][11]
Although a too-large power supply will have an extra margin of safety
against overloading, a larger unit is often less efficient at lower
loads and therefore wastes more electricity than a more appropriately
sized unit. For instance, an 80 PLUS, 520 watt supply could still be 70%
or less efficient at 60 watts (a typical idle power for a desktop
computer).[12] Some power supplies have no-overload 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
A test in 2005 revealed computer power supplies are generally about 70–80% efficient.[13]
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.
As of 2012 some high-end consumer PSUs can exceed 90% efficiency at
optimal load levels, though will fall to 87-89% efficiency during heavy
or light loads. Google's server power supplies are more than 90%
efficient.[14] HP's server power supplies have reached 94% efficiency.[15] Standard PSUs sold for server workstations have around 90% efficiency, as of 2010.
The energy efficiency of a power supply drops significantly at low
loads. Therefore it is important to match the capacity of a power supply
to the power needs of the computer. 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
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.[16]
- 12V only System monitoring (P10): This is a 171822-8 AMP or equivalent connector carrying a supply to the PSU fan and sense returns.[17]
- 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 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. [18] 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.[19]
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.[14]
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.
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