ATX Version 3.0 Multi Rail Desktop Platform Power Supply

Design Guide

ID 336521
Date 02/01/2023

A newer version of this document is available. Customers should click here to go to the newest version.

Document Table of Contents

Fan Size and Speed

A thermally sensitive fan speed control circuit is recommended to balance system-level thermal and acoustic performance. The circuit typically senses the temperature of the secondary 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 carefully.

Fan should not turn on at the same time as PS_​ON# is Asserted. This is because of power optimization at low levels and Alternative Low Power Modes. Two options to consider.

  1. Wait for at least 2 seconds before the fan turns on.
  2. Fan needs to be only turned on when the PSU needs the thermal cooling.

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.