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http://en.wikipedia.org/wiki/ATX
ATX power supply revisions
[edit] Original ATX
ATX, introduced in late 1995, defined three types of power connectors:
4-pin "Molex connector" — transferred directly from AT standard: +5 V and +12 V for P-ATA hard disks, CD-ROMs, 5.25 inch floppy drives and other peripherals.[11]
4-pin Berg floppy connector — transferred directly from AT standard: +5 V and +12 V for 3.5 inch floppy drives and other peripherals.[12]
20-pin Molex Mini-fit Jr. main motherboard connector — new to the ATX standard.
A supplemental 6-pin AUX connector providing additional 3.3 V and 5 V supplies to the motherboard, if needed. This was used to power the CPU in motherboards with CPU voltage regulator modules which required 3.3 volt and/or 5 volt rails and could not get enough power through the regular 20-pin header.
The power distribution specification defined that most of the PSU's power should be provided on 5 V and 3.3 V rails, because most of the electronic components (CPU, RAM, chipset, PCI, AGP and ISA cards) used 5 V or 3.3 V for power supply. The 12 V rail was only used by fans and motors of peripheral devices (HDD, FDD, CD-ROM, etc.).
The original ATX power supply specification was little revised until 2000.
[edit] ATX12V 1.x
While designing the Pentium 4 platform in 1999/2000, the standard 20-pin ATX power connector was found insufficient to meet increasing power-line requirements; the standard was significantly revised into ATX12V 1.0 (ATX12V 1.x is sometimes inaccurately called ATX-P4). ATX12V 1.x was also adopted by AMD Athlon XP and Athlon 64 systems. However, some early model Athlon XP and MP boards (including some server boards) and later model lower-end motherboards do not have the 4-pin connector as described below.
[edit] ATX12V 1.0
The main changes and additions in ATX12V 1.0 (released in February 2000) were:
Increased the power on the 12 V rail (power on 5 V and 3.3 V rails remained mostly the same).
An extra 4-pin mini fit JR (Molex 39-01-2040), 12-volt connector to power the CPU.[13] Formally called the +12 V Power Connector, this is commonly referred to as the P4 connector because this was first needed to support the Pentium 4 processor.
Before the Pentium 4, processors were generally powered from the 5V rail. Later processors operate at much lower voltages, typically around 1 V, and some draw over 100 A. It is infeasible to provide power at such low voltages and high currents from a standard system power supply, so the Pentium 4 established the practice of generating it with a DC-to-DC converter on the motherboard next to the processor, powered by the 4-pin 12V connector.
[edit] ATX12V 1.1
This is a minor revision from August 2000. The power on the 3.3 V rail was slightly increased, and other smaller changes were made.
[edit] ATX12V 1.2
A relatively minor revision from January 2002. The only significant change was that the −5 V rail was no longer required (it became optional). This voltage was used only on some old systems with certain ISA add-on cards.
[edit] ATX12V 1.3
Introduced in April 2003 (a month after 2.0). This standard introduced some changes, mostly minor. Some of them are:
Slightly increased the power on 12 V rail.
Defined minimal required PSU efficiencies for light and normal load.
Defined acoustic levels.
Introduction of Serial ATA power connector (but defined as optional).
The −5 V rail is prohibited.
[edit] ATX12V 2.x
ATX12V 2.x brought a very significant design change regarding power distribution. On analyzing the then-current PC architecture's power demands, it was determined that it would be much cheaper and more practical to power most PC components from 12 V rails, instead of from 3.3 V and 5 V rails.
[edit] ATX12V 2.0
ATX 450 PHF.
The above conclusion was incorporated in ATX12V 2.0 (introduced in February 2003), which defined quite different power distribution from ATX12V 1.x:
The main ATX power connector was extended to 24 pins. The extra four pins provide one additional 3.3 V, 5 V and 12 V circuit.
The 6-pin AUX connector from ATX12V 1.x was removed because the extra 3.3 V and 5 V circuits which it provided are now incorporated in the 24-pin main connector.
Most power is now provided on 12 V rails. The standard specifies that two independent 12 V rails (12 V2 for the 4 pin connector and 12 V1 for everything else) with independent overcurrent protection are needed to meet the power requirements safely (some very high power PSUs have more than two rails, recommendations for such large PSUs are not given by the standard).
The power on 3.3 V and 5 V rails was significantly reduced.
The power supply is required to include a Serial ATA power cable.
Many other specification changes and additions.
[edit] ATX12V v2.01
This is a minor revision from June 2004. An errant reference for the -5V rail was removed. Other minor changes were introduced.
[edit] ATX12V v2.1
This is a minor revision from March 2005. The power was slightly increased on all rails. Efficiency requirements changed. Added 6-pin connector for PCIe graphics cards that aids the PCIe slot in the motherboard, delivering 75 watts.
[edit] ATX12V v2.2
Another minor revision. Added 8-pin connector for PCIe graphics cards that delivers another 150 watts.
[edit] ATX12V v2.3
Effective March 2007 and current as of 2012. Recommended efficiency was increased to 80% (with at least 70% required), and the 12 V minimum load requirement was lowered. Higher efficiency generally results in less power consumption (and less waste heat), and the 80% recommendation brings supplies in line with new Energy Star 4.0 mandates.[14] The reduced load requirement allows compatibility with processors that draw very little power during startup.[15] The absolute over-current limit of 240VA per rail was removed, allowing 12V lines to provide more than 20A per rail.
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