Wednesday, December 5, 2007

What do they mean by Output Power Derating?

All power supplies have a specified “Operating Temperature Range”. For example, TDK-Lambda’s AC-DC switch-mode SWS600L series of 600 watt, single output power supplies have an operating temperature range from “-20°C to +74°C”. However, the spec also states: “…derating linearly to 50% load above 50°C”. What does this mean?

Please refer to Figure 1 below. Most power supply manufacturers provide this type of curve to make it easier for the end user to determine the maximum output power that can provided by a power supply at various operating or ambient temperatures. Ta = Temperature of the Ambient Air, or, the temperature of the air surrounding the power supply, especially the air at the intake of a fan-cooled supply. By comparing the “Operating Temperature Range” specification listed above to the derating curve, the following information can be seen:
  • The supply can deliver 100% of its rated output power load (600 watts) from -20°C to +50°C ambient temperatures
  • Above 50°C ambient, the supply can deliver a reduced amount of power
  • At 60°C ambient, the supply can provide about 80% of its max. rated power (0.80 x 600 = 480 watts)
  • At 74°C ambient, the supply can provide 50% of its max. rated power load (0.50 x 600 = 300 watts)

Figure 1: SWS600L Output Power Derating Curve

In addition to the supply’s normal “operating temperature range” and output derating-curve, some supplies like this one, have a specified low-temperature “start-up” capability (i.e., -40°C). This means that the 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 supply’s output regulation, hold-up time, ripple & noise, and other specifications cannot be fully guaranteed until the 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 supply is turned-on it will usually self-heat due to the heat generated by its internal electronic power components.


Anonymous said...

One thing I want to comment on is that de-rating is often misunderstood. All power supplies can provide 100% of the rated power no matter the temperature. However, at higher temperature, components are degraded if the power supply load isn't reduced. Therefore, life is removed.

Anonymous said...

The last comment is true in that the life is degraded but depending upon the design margin some power supplies will shut down pretty quickly due to thermal trips if operating at 100% rated power at say 70 centigrade .

Anonymous said...

Hi All,
I'm a power supply engineer. Sorry, but regarding the first post, it is not true that all power supplies can provide 100% of their rated power no matter the temperature. If you try to operate a power supply beyond its specified temperature range, and power derating curve, it will go into a thermal shutdown, and you may damage the supply. Also, the expected field life of a power supply is reduced by operating it near its maximum rated temperature.

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