070 - What is Computer Power Supply

A Power Supply Unit (PSU) is an electronic device used to supply the components in a computer with power. PSU main function is to converting potentially lethal 110-115 or 220-230 volt Alternating Current (AC) into a steady low-voltage Direct Current (DC) usable by the computer.

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 specification, which includes form factor and voltage tolerances. While an ATX power supply is connected to the mains supply, it always provides a 5 V standby (5VSB) voltage 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 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 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.

This standby voltage is generated by a smaller power supply inside the unit. Originally, it was used to supply the voltage regulator, located on the low-voltage side of the transformer, allowing the regulator to measure output voltages. 

The regulator controls the switching transistors insulated by optocoupplers or pulse transfomers. The standby power source was a small linear power supply with conventional transformer, which was later changed to a switching power supply, sharing some components of the main unit due to cost- and energy-saving requirements.

PFC: Power Factor Correction
Power supplies may have passive or active Power Factor Correction

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%.

Caution: Do not open the power supply, it contains capacitors that are capable of holding hold electricity even if the computer is off and unplugged for a week, if not longer.

Power Supply is also known as: PSU or power supply unit.

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