Server Power Supply Loading Guideline
Server Power Supply Loading Guideline

Loading guidelines have been developed based on proportional allocation of load current for each DC bus rail based on the MAXIMUM CURRENT rating of each DC bus rail.

For 100% load level the maximum rating for each DC bus rail is decreased proportionately to ensure that the total combined loading for all the output busses is equal to the overall rating of the power supply AND the maximum combined rating for a subset of the output busses (e.g., +5V and +3.3V) is not exceeded.

The purposes of this Guideline are to ensure that the loading for each DC bus for a multi output server power supply is consistent when measuring power supply efficiency and to allow consistency in comparing server power supply efficiency reported by vendors as part of the electrical specification.

The graph in Figure A depicts a typical server power supply efficiency pattern, omitting the efficiency percentages, which can vary by power supply. As you can see in Figure A, at below 40% to 50% utilization, power supplies aren’t fully efficient. In fact, at a load of 20%, power supply efficiency can be as much as 10% lower than efficiency at the 50% load mark and even less efficient at lower levels. From about 60% usage to 100% usage, there is a noticeable efficiency drop-off as well, although this drop-off appears to be much less noticeable for server-based power supplies. In my research for this post, I saw some desktop power supplies that, at 100% usage, would drop to very poor efficiency levels, so there is a vast variety of operating reality out there. In other words, one size does not fit all, so your results may vary.
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