Airflow / Fan | Intel ATX12V instruction

ATX12V Power Supply Design Guide

Version 2.0

4.3. Airflow / Fan

The ATX Specification allows for numerous (and often confusing) possibilities for power supply fan location, direction, speed, and venting. The designer’s choice of a power supply cooling solution depends in part on the targeted end-use system application(s). At a minimum, the power supply design must ensure its own reliable and safe operation.

Fan location/direction. In general, exhausting air from the system chassis enclosure via a power supply fan at the rear panel is the preferred, most common, and most widely applicable system-level airflow solution. Other solutions are permitted, including fans on the topside of figure 5 and the Wire harness side of figure 4 or 5. Some system/chassis designers may choose to use other solutions to meet specific system cooling requirements.

Fan size/speed. An 80 mm or larger axial fan is typically needed to provide enough cooling airflow through an average ATX system. Exact CFM requirements vary by application and end-use environment, but 25-35 CFM is typical for the fan itself.

For consumer or other noise-sensitive applications, it is recommended that a thermally sensitive fan speed control circuit be used to balance system-level thermal and acoustic performance. The circuit typically senses the temperature of an internal heatsink and/or incoming ambient air and adjusts the fan speed as necessary to keep power supply and system component temperatures within specification. Both the power supply and system designers should be aware of the dependencies of the power supply and system temperatures on the control circuit response curve and fan size and should specify them very carefully.

The power supply fan should be turned off when PS_ON# is de-asserted (high). In this state, any remaining active power supply circuitry must rely only on passive convection for cooling.

Venting. In general, more venting in a power supply case yields reduced airflow impedance and improved cooling performance. Intake and exhaust vents should be as large, open, and unobstructed as possible so as not to impede airflow or generate excessive acoustic noise. In particular, avoid placing objects within 0.5 inches of the intake or exhaust of the fan itself. A flush-mount wire fan grill can be used instead of a stamped metal vent for improved airflow and reduced acoustic noise.

There are three caveats to the venting guidelines above:

•Openings must be sufficiently designed to meet the safety requirements described in Section 8.

•Larger openings yield decreased EMI-shielding performance (see Section 6).

•Venting in inappropriate locations can detrimentally allow airflow to bypass those areas where it is needed.