Power supply unit (computer)
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. Power supplies, often referred to as "switching power supplies", use switcher technology to convert the AC input to lower DC voltages. The typical voltages supplied are:
· 3.3 volts
· 5 volts
· 12 volts
The 3.3- and 5-volts are typically used by digital circuits, while the 12-volt is used to run motors in disk drives and fans.
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 5V 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. Themost 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).
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, under voltage protection, over current 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.
Development:-
Original IBM PC, XT and AT standard
The first 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 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 the motherboard.
ATX standard
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 +5 V SB (standby) rail for providing a small amount of standby power, even when the computer was nominally "off".
Working of Power Supply Unit (Computer):-
a. Transient Filter
A computer power supply depends on stable power coming in to do its job. That's why the better built units employ a transient filter between the power connector and the rest of the circuitry. These use a combination of metal oxide varistors (used to control potentially damage-causing power surges), capacitors, and coils to take as much interference out of the incoming AC power as possible. They also have the benefit of acting in reverse, keeping the PSU from adding its own interference to the line
b. Input Rectifier
Located immediately after the transient filter (if present), the input rectifier is responsible for converting AC to DC power. Self-contained bridge rectifiers are most commonly used, sometimes in pairs in higher watt designs, but some cheaper units get by with four diodes in a bridge arrangement as well. Most power supplies lacking active PFC will have a switch on the back to go from 120V input to 240V input. This switch is part of the input rectifier, doubling the input voltage when closed and allowing the circuit to operate as a straight rectifier when open. If an active PFC circuit is employed, the voltage doubler is omitted and the PFC circuitry takes over its duties instead
c. Primary Switchers
Once the incoming power has been converted to DC, it now needs to be converted into high frequency AC power. This, being the heart of a SMPS design, is done to allow lots of power to be rectified using far less space than an old linear power supply would need. There are several ways to do this, but the most common arrangements use a pair of power transistors for the main voltages and a single smaller transistor to handle standby voltages.
d. Primary Transformers
Now that the incoming power has been regulated to DC and then converted to high frequency AC, step down transformers are needed to drop the voltage to the point where they can be regulated back to DC for the output. Usually, this means one large transformer for 3.3V, 5V, 12V, -12V, and -5V; and one smaller transformer connected to the standby switcher for the standby voltage. But, sometimes an extra transformer will make its way in there as well, most often for extra 12V capacity these days.
e. PWM Control Circuit This is the circuit used to control the switching frequency in a power supply. Often, these can be based on an all in one integrated circuit that can also handle various protection modes or APFC duties. These circuits are connected to the primary switching transistors using a small transformer or by optocouplers to isolate the controller from the transistors
f. Secondary Stage
Once the high frequency AC power has been reduced to the proper voltage, it must be converted back into DC. This is done using a combination of rectifiers (usually two diodes back to back in one package) and filters. Often, the 3.3V output will be taken either from the 5V regulated output or will be taken from the 5V output at the transformer. However, in some very high end designs the 3.3V will get its own transformer output as well.
Connectors
Typically, power supplies have the following connectors :
- 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 12 V lines with common returns, a 'supply OK' signal, a 'PSU ON' signal and an 11 V 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.
Four-pin power 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, mini-Molex, or Berg connector): This is one of the smallest connectors that supplies a 3.5-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 V.
- 6-pin Most modern computer power supplies include six-pin connectors that are generally used for PCI Express graphics cards, but a newly introduced eight-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 six-pin card or an eight-pin card to be connected by using two separate connection modules wired into the same sheath: one with six pins and another with two pins. Each PCI Express eight-pin connector can output a maximum of 150 W.
- An IEC 60320 C14 connector with an appropriate C13 cord is used to attach the power supply to the local power grid
