ATX Version 3 Multi Rail Desktop Platform Power Supply

Design Guide

ID 336521
Date 11/01/2023
Document Table of Contents

Overall Power Supply Efficiency Levels

The efficiency of the power supply should be tested at nominal input voltage of 115 VAC input and 230 VAC input, under the load conditions defined in the Generalized Test Protocol for Calculating the Energy Efficiency of Internal Ac-Dc and Dc-Dc Power Supplies document. This document defines how to determine full load criteria based on the label of each rail of the power supply. The loading condition for testing efficiency represents fully loaded systems, typical (50%) loaded systems, and light (20%) loaded systems.

The Efficiency requirements listed below are applicable to AC Input voltage of 115V.

Table 4-14: Efficiency versus Load Minimum Requirements


Full Load


Typical Load


Light Load


REQUIRED Minimum Efficiency




Low Load Efficiency

Computers have decreased Idle power greatly since 2005, to where PSU loss is a big concern for overall AC power of a computer in Idle Mode. The lowest DC load for computers at this Idle Mode is determined to be 10 Watts for mainstream computers and could go lower. Computers with PSU larger than 500 watts are also expected to have more components and therefore the Idle Mode will be at a higher DC Load. A PSU above 500 watts will use the Low Load Efficiency set at the 2% level. The Low Load Efficiency requirements are shown in Table 4-15: Low Load Efficiency Requirements.

Table 4-15: Low Load Efficiency Requirements

Low Load Efficiency

10W / 2%





The 10-Watt load conditions are defined in the Test Plan (#338448).