3) Airflow Direction
Computer OEMs and power supply manufactures have indicated that the ATX specification did not state which direction air should flow through the chassis. While many different cooling methods exists for ATX,
the most prevalent solution in the market has the power supply fan sucking air from outside the chassis and
blowing it across the processor. See power supply section 4.0 for details on airflow.
4. ATX power supply
The standard PS/2 power supply unit can be modified to better support the ATX form-factor. These
modifications include adding a 3.3V supply rail, repositioning the fan to move air across the processor, and
consolidating the baseboard connectors into one 20 pin header. Initial units of ATX power supplies will ship
with externally mounted fans that pull air from outside the chassis and blow air across the processor. Over time, as integration improves, power supply versions will be developed where the fan is moved inside the PSU casing. Although pulling air through the power supply from outside the chassis is the most common airflow solution, other airflow solutions maybe implemented to meet the specific cooling requirements of different ATX chassis. For example, one alternative cooling solution would be to use a standard PS/2 power supply, modified with the 20 pin power connector, without repositioning the fan, but use an active heatsink (heatsink with small fan mounted on top) to cool the microprocessor.