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What is Output Power Derating?

Output derating curve

All power supplies have a specified “Operating temperature range”. For example, a particular 500W switching power supply has an operating temperature range from “-20°C to +70°C”. However, the specification may also state “Output derating linearly above +50°C”. So what does this mean?

Most power supply manufacturers provide output derating curve (see figure below) to make it easier for the end user to determine the maximum output power that can be provided by a power supply at various operating temperatures. Operating or ambient temperature is the air temperature surrounding the power supply. By comparing the “Operating Temperature Range” specification listed above to the derating curve, the following information can be seen:

  • The power supply can deliver 100% of its rated output power load (500W) from -20°C to +50°C ambient temperatures
  • Above 50°C ambient, the power supply will deliver a reduced amount of power
  • At 60°C ambient, the power supply can provide about 75% of its maximum rated power (0.75 x 500 = 375W)
  • At 70°C ambient, the power supply can provide about 50% of its maximum rated power load (0.50 x 500 = 250W)

Output derating curve

In addition to the power supply’s normal “operating temperature range” and output derating-curve, some power supplies may have a specified low-temperature “start-up” capability (i.e., -40°C). This means that the power supply can “start-up” or be “turned-on” with an ambient temperature as low as -40°C (below the -20°C spec) and deliver 100% of its rated power, however, the power supply’s output regulation, hold-up time, ripple & noise, and other specifications cannot be fully guaranteed until the power supply warms up to at least -20°C. This cold temperature start-up is a nice feature to have, especially for outdoor-mounted applications. Once the power supply is turned-on it will usually self-heat due to the heat generated by its internal electronic power components.

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