ATX Power Supply

The ATX specification requires the power supply to produce three main outputs, +3.3 V, +5 V and +12 V. Low-power -12 V and 5 VSB (standby) supplies are also required. A -5 V output was originally required because it was supplied on the ISA bus, but it became obsolete with the removal of the ISA bus in modern PCs and has been removed in later versions of the ATX standard.

Originally the motherboard was powered by one 20-pin connector. An ATX power supply provides a number of peripheral power connectors, and (in modern systems) two connectors for the motherboard: a 4-pin auxiliary connector providing additional power to the CPU, and a main 24-pin power supply connector, an extension of the original 20-pin version.

Four wires have special functions:

• PS_ON# stands for "Power Signal On" means that there is a signal coming from the motherboard to the power supply. When this is connected to GND (“ground” by the motherboard) the PS_ON is active or in the ON mode. It is pulled up internally to about five volts (+5V) in the power supply.
• PWR_OK stands for “Power Good” means that there is a stabilized and ready for consumption output from the power supply. For a short period of about 100-500 ms (milliseconds or 1/1000 of a second), the output signal remains low after the PS_ON# or “Power Signal On” is pulled low.
• +5 VSB stands for “+5 Volts Standby” provides power even though all the rest of the lines of the power supply are in the off state. This is particularly useful to power the circuitry which controls the PS_ON# or “Power On” signal.
• +3.3V or “+3.3 Volts” sense must be connected to the specified +3.3 V located on the motherboard or its specific connector. This is intended to allow the sensing of the voltage drop in the power supply wiring remotely.

Supply voltages, generally and at all times, should be within ±5% of their nominal values. The tolerance of the seldom, if not, little-used negative voltages of the power supply, is ±10%. Hereinbelow is the specification for ripple voltage in the 10-20 MHz frequency range.

Main changes from AT design

Power switch

AT- (Advanced Technology) style computer cases normally had a power switch or button which was connected directly to the PSU or Power Supply Unit of the computer system. This configuration was a double-pole which latches or locks main voltage switch with the four (4) pins that are connected to the wires from a four- (4) core cable. Those wires were soldered to the power switch or push button or sometimes blade receptacles were employed. The soldered wires to the power button makes it difficult to replace the power supply if the unit malfunctions.

ATX (Advanced Technology Extended) power supply, unlike the AT (Advanced Technology), is not directly connected to the power button or switch of the computer system. This enables the user to turn off or shut down the computer thru a software or program without physically pushing the power button. However, most ATX power supply manufacturers have included a manual switch located at the rear or back panel of the unit to make sure that the system is indeed turned off and that no signal voltage is sent to the peripherals and components. This is because when the said manual switch is on, there is still power flowing to the computer components even though the system appears to be in the “off” state. This state is known as “soft-off” or standby mode and may be utilized in order to remotely power on or “wake up” the computer system through Wake-on-Ring or Wake-on-LAN or other modes, aside from the general purpose of booting the system through pushing the power switch located at the front panel.

Power connection to the motherboard

With the new ATX (Advanced Technology Extended) power supply, there has been a considerable change with the way power supply is connected to the motherboard. While older AT power supplies have connectors which are similar and identical which may be switched or interchanged accidentally that could lead to irreversible damage to the motherboard and short-circuits. ATX has one (1) large, keyed connector as compared to the older AT. This feature of the ATX makes a reversed connection difficult thus avoiding the problems and damages in the system from short-circuits. The new connector has also a 3.3V (volts) output thus eliminating the necessity for motherboards to fetch this voltage source from one of the power rails. Some motherboards which were manufactured, particularly the models succeeding from the AT-model from factor offerings, supports both AT and ATX power supply units. When these motherboards does not boot with an ATX, the remedy is to fully turn it on (as it is always on or in standby mode) by simply shorting from pin 16 (marked as the green wire) on the ATX connector to a ground wire (marked as black wire).

Airflow

As with the ventilation, the original ATX specification requires a power supply to be placed near the central processing unit (CPU) with the power supply exhaust fan to suck in cool air into the chassis and feed it directly to the processor.

ATX12V vs. ATX Power Supplies

AT or Advanced Technology original design was developed with the IBM systems and its compatible during the early stages of the evolution of the personal computers. With the new layouts and designs, calls for new power requirements, thus the development of the Advanced Technology Extended or the ATX. A new format called ATX12V has been developed and this standard is now referred to as the ATX v2.0 (officially known as version 2.0 of ATX) and has been continuously developed today.

The primary differences with the latest ATX v2.2 and ATX v1.3 are:

There are primary distinctions with the latest ATX v2.2 and the ATX v1.3. These are enumerated hereinbelow:

1. The utilization of a 24-pin Main Power Connector over the 20-pin Connector for PCI Express Support;
2. There is no need for a 6-pin Auxiliary Power Connector;
3. Dual 12V Rails are used if current rating is greater than 18A;
4. Requires Serial ATA Power Connectors; and
5. Maximum Power Conversion Efficiency

The most noticeable change for the ATX12V standard is the 24-pin main power. The older 20-pin connector could not handle the 75-watt requirement of the PCI Express. In order to remedy this trouble, additional four (4) pins were included to the connector in order to supply additional signals through the 12V rails. The new pin layout is designed in such a way that the 24-pin power connector are compatible with the older ATX motherboards or those using the 20-pin connector.

Dual 12V Rails

The required amount of power has increased due to the new power requirements of processors, hard and/or disk drives, and fans on the system. The capacity of the PSU to provide voltage signals which are stable was difficult to achieve even though now at higher levels of current. In order to address this issue, the new standard requires that any power supply that provides current over 18A for the 12V rail to be split into two 12V rails which are separate in order to increase stability.

Serial ATA Connectors

Now that SATA drives are rapidly used and replacing the PATA, there is a complementary need for the connectors of the power supplies. This forced a new standard requiring a minimum number of connectors on new power supplies which are manufactured. ATX v2.0+ supplies four connectors while the older ATX v1.3 provides only two.

Power Efficiency

Power efficiency rating determines the ratio of the input and output power supply, that is how much power is derived from the standard socket or source and how much is fed to the computer. This is because the current from the standard outlet is converted to low levels of voltages which is needed by the system. With regard to the power efficiency rating of new power supplies, the new standards now requires a minimum of 70% efficiency and 80% rating as the recommended efficiency.