Power Supply Derating Guide Lines

Derating Factor: General recommendations that should be followed to increase the reliability of a power supply when used in a design. Use the numbers provided as a guide and not a hard and fast rule. Some engineering firms may have numbers that are higher or lower than the ones provided. The point is not to operate the power supply at the maximum output provided by in the data sheet at the maximum indicated temperature.
Operating Temperature range is normally 0 to 50oC, derate 20% for each additional 10oC increase above 50oC.
Operating Temperature range; 0oC to +70oC. Full power to +50oC then derate 2.5%/oC to +70oC. Or, derate linearly from 100% load @ 50oC to 75% @ 60, derated at 10% per oC to a Maximum of +65oC
Forced air cooling [20CFM nominal]: a power supply may require a forced air flow to operate at its rated maximum temperature rating. Derate 20% with out cooling
Open Frame supplies: may require an additional 15% power de-rating when covered by a frame, or placed in a case.
About: The power supply specification should detail its operation, use the above as a rule of thumb. Normal operational temperature range is defined as ambient [Maximum] in the data sheet.

Although a completely separate issue, it's assumed that the power supply was correctly sized to begin with. That is a power supply that is chosen that just barely meets the system requirements would never be able to meet the requirement once the derating has been accounted for. So the power supply needs to be sized for worst case operation, the maximum current the supply is expected to delivery and the maximum temperature needs to be accounted for.

Of course when designing an equipment chassis the decision has to be made as to how many expansion slots will be provided and how many will be powered. If a 12 slot backplane is used in a equipment chassis, should the power supply be sized to power all of the slots, even though only 10 would be used. Should two of the slots be blocked from use, or should the power supply be sized to power the slots in the event of an up-grade.

A list of Power Supply manufacturers.
How to Derate; Guideline for Derating Electronic Components

PC motherboard

Distributor rolodex Electronic Components Electronic Equipment EDA CDROM Software Engineering Standards, BOB card Cabled Computer Bus Electronic Engineering Design Table Conversion DB9-to-DB25.
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Modified 12/27/11
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