Hardware Reference
In-Depth Information
with the +12V connector were called ATX12V supplies.) The +12V connector was made a
requirement in version 1.3 (April 2002), whereupon the specification became only ATX12V. The
ATX12V 2.0 specification (February 2003) dropped the 6-pin auxiliary connector, changed the main
power connector to 24 pins, and made Serial ATA power connectors a requirement. The current
version is ATX12V 2.2, which was released in March 2005 and contains only minor changes from
the previous releases, such as the use of Molex High Current System (HCS) terminals in the
connectors.
As the ATX power supply specification has evolved, there have been some changes in the cooling fan
orientation and design. The ATX specification originally called for an 80mm fan to be mounted along
the inner side of the supply, where it could draw air in from the rear of the chassis and blow it inside
across the motherboard. This kind of airflow runs in the opposite direction from most standard
supplies, which exhaust air out the back of the supply through a hole in the case where the fan
protrudes. The idea was that the reverse-flow design could cool the system more efficiently with only
a single fan, eliminating the need for a fan (active) heatsink on the CPU.
Another benefit of the reverse-flow cooling is that the system would run cleaner, freer from dust and
dirt. The case would be pressurized, so air would be continuously forced out of the cracks in the case
—the opposite of what happens with a negative pressure design. For this reason, the reverse-flow
cooling design is often referred to as a positive-pressure-ventilation design . On an ATX system with
reverse-flow cooling, the air is blown out away from the drive because the only air intake is the
single fan vent on the power supply at the rear. For systems that operate in extremely harsh
environments, you can add a filter to the fan intake vent to further ensure that all the air entering the
system is clean and free of dust.
Although this sounds like a good way to ventilate a system, the positive-pressure design needs to use
a more powerful fan to pull the required amount of air through a filter and pressurize the case. Also, if
a filter is used, it must be serviced periodically; depending on operating conditions, it could need
changing or cleaning as often as every week. In addition, the heat load from the power supply on a
fully loaded system heats the air being ingested, blowing warm air over the CPU and reducing the
overall cooling capability.
As CPUs evolved to generate more and more heat, the cooling capability of the system became more
critical and the positive-pressure design was simply not up to the task. Therefore, subsequent
versions of the ATX specification were rewritten to allow both positive- and negative-pressure
designs, but they emphasized the standard negative-pressure system with an exhaust fan on the power
supply and an additional high-quality cooling fan blowing cool air right on the CPU as the best
solution.
Because a standard negative-pressure system offers the greatest cooling capacity for a given fan's
airspeed and flow, virtually all recent ATX-style power supplies use a negative-pressure design, in
which air flows out the back of the power supply. Most use an 80mm fan mounted on the rear of the
unit blowing outward, but some use an 80mm, a 92mm, or a 120mm fan mounted on the inside upper
or lower surface, with open vents on the rear of the system. In either example, the flow of air is such
that air is always exhausted out of the system through the rear of the supply.
The ATX power supply form factor addressed several problems with the previous PC/XT, AT, and
LPX-type supplies. One is that the power supplies used with PC/XT/AT boards had only two
connectors that plugged into the motherboard. If you inserted these connectors backward or out of
their normal sequence, you would usually fry both the motherboard and the power supply! Most
 
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