Hardware Reference
In-Depth Information
AsCPUsevolvedtogeneratemoreandmoreheat,thecoolingcapabilityofthesystembe-
came more critical and the positive-pressure design was simply not up to the task. There-
fore, subsequent versions of the ATX specification were rewritten to allow both positive-
and negative-pressure designs, but they emphasized the standard negative-pressure sys-
tem 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.
Becauseastandardnegative-pressuresystemoffersthegreatestcoolingcapacityforagiv-
en 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
hadonlytwoconnectors that pluggedintothemotherboard. Ifyouinserted these connect-
ors backward or out of their normal sequence, you would usually fry both the mother-
board and the power supply! Most responsible system manufacturers tried to “key” the
motherboard and power supply connectors so you couldn't install them backward or out
of sequence. However, most vendors of cheaper systems did not feature this keying on
theboardsorsupplies theyused.TheATXformfactorincludes intelligently designed and
keyed power plugs to prevent users from incorrectly plugging in their power supplies.
The ATX connectors also supply +3.3V, reducing the need for voltage regulators on the
motherboard to power +3.3V-based circuits.
Besides the new +3.3V outputs, ATX power supplies furnish another set of outputs that is
nottypicallyseenonstandardpowersupplies.ThesetconsistsofthePower_On(PS_ON)
and 5V_Standby (5VSB) outputs mentioned earlier, known collectively as
Soft Power
.
This enables features to be implemented, such as Wake on Ring or Wake on LAN, in
which a signal from a modem or network adapter can actually cause a PC to wake up and
power on. Many such systems also have the option of setting a wakeup time, at which the
PC can automatically turn itself on to perform scheduled tasks. These signals also can en-
able the optional use of the keyboard to power the system on—an option you can set on
some systems. These features are possible because the +5V Standby power is always act-
ive, giving the motherboard a limited source of power even when off. Check your BIOS
Setup for control over these types of features.
