Due to the increasing market pressures on power supply size and power density, as well as cost, an increasing number of AC/DC power supplies are being released that rely on derated specifications in order to increase their headline power rating.

The de-rating may not be readily apparent, and it is usually located at the very end of the datasheet for the product. This information is often far away from the headline. This level of detail may not be included in the catalog or short form of the data. In these cases, it is important to carefully select the product for the application.

What is a de-rating specification?

The de-rating specification is based on reducing output power ratings of power supplies during high-temperature operation or when the line input voltage is low. It reduces the temperature increase of components, prolongs their life, and ensures safety-critical isolation parts do not exceed thermal limits. The input voltage and operating temperature are two of the most common derated specifications.

Temperature de-rating

See Figure 1 for an example. This de-rating curve is typically applied to products that are intended for integration into equipment. The power supply can be used to maintain the full power rating even if the temperature in the equipment increases. At 70degC, the output power rating is usually reduced to 50%. A small number of manufacturers also de-rate their products below 0degC because they can start them at low temperatures.

Some manufacturers have introduced open-frame power supply units that limit the maximum ambient temperatures for full power operation at 40degC. The output power is reduced to 50% when the ambient temperature reaches 60degC. The reason for this is that component temperatures are too high at 50degC to operate the full power. This is because component specifications, product safety, and lifetime requirements limit component temperature increases.

This “specmanship”, while it may appear to have a higher headline rating, or to be smaller or cheaper at first glance, is actually a reduction of up to 25% in the output power when used with end equipment that must operate within an ambient temperature of 40degC. This means that a 100W headline product is in reality a 75W unit and therefore cannot be compared to other units with the same power rated for operation at 50degC.

Input voltage de-rating

Universal input ranges are typically 90-264VAC for products designed to operate worldwide. A product with universal input should offer its full power across this input range. Some products may de-rate their output to operate in areas with AC power that is prone to brownout.
This post was written by Justin Tidd, Director at Swartz Engineering. For nearly half a century, Swartz Engineering has been at the forefront of industry safety. They are a family-owned company specializing in power distribution for the electrical industry. Our design ensures maximum flexibility for excellent reliability and a high return on investment.


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