Power supply unit (computer)
Most modern desktop personal computer power supplies conform to the ATX specification, which includes form factor and voltage tolerances.
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 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.
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 (PFC) was required, those filter capacitors were replaced with higher-capacity ones, together with a coil installed in series to delay the inrush current.
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.
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.
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 requiring 3.3 V typically derived that from a simple but inefficient linear regulator connected to the +5 V rail.
Since ATX power supplies share both the same dimensions (150 × 86 mm (5.9 × 3.4 in)) and the same mounting layout (four screws arranged on the back side of the unit), with the prior format, there is no major physical difference preventing an AT case from accepting an ATX PSU (or vice versa, if the case can host the power switch needed by an AT PSU), provided that the specific PSU is not too long for the specific case.
Rarely, a linear regulator generated these, supplied from the 5 V and converting the product of voltage drop and current to heat.
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.
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.The requirement was later deleted from version 2.3 (March 2007) of the ATX12V power supply specifications,[8] but led to a distinction in modern ATX power supplies between single and multiple rails.
The rule was intended to set a safe limit on the current able to pass through any single output wire.
Power supplies capable of delivering more than 18 A at 12 V would provide their output in groups of cables (called "rails").
However, people found the need to balance loads across many +12 V rails inconvenient, especially as higher-end PSUs began to deliver far greater currents up to around 2000 W, or more than 150 A at 12 V (compared to the 240 or 500 W of earlier times).
At least two of Dell's business PCs introduced in 2013, the OptiPlex 9020 and Precision T1700, ship with 12 V–only power supplies and implement 5 V and 3.3 V conversion exclusively on the motherboard.
[15] The overall power draw on a PSU is limited by the fact that all of the supply rails come through one transformer and any of its primary side circuitry, like switching components.
Climate Savers Computing Initiative promotes energy saving and reduction of greenhouse gas emissions by encouraging development and use of more efficient power supplies.
[23] 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.
Most desktop personal computer power supplies are a square metal box, and have a large bundle of wires emerging from one end.
Some power supplies come with sleeved cables, which besides being more aesthetically pleasing, also make wiring easier and have a less detrimental effect on airflow.
Typically, power supplies have the following connectors (all are Molex (USA) Inc Mini-Fit Jr, unless otherwise indicated): A modular power supply provides a detachable cable system, offering the ability to remove unused connections at the expense of a small amount of extra electrical resistance introduced by the additional connector.
[33] This reduces clutter, removes the risk of dangling cables interfering with other components, and can improve case airflow.
The SFX-L configuration is a minor variation of SFX12V that solely increases the allowed depth by 30% (to 1 cm shallower than the maximum of regular ATX12V).
For example, genuine Dell power source uses 1-Wire protocol to send data by third wire to the laptop.
[24] Rack mount servers may utilise redundant power supply based on CRPS standard.
