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Understanding power supply de-rating in practice

Posted: 02 Mar 2015 ?? ?Print Version ?Bookmark and Share

Keywords:AC-DC? power supplies? de-rating? DC/DC converters? PFC?

With escalating market pressure on power supply size, power density and cost, there is an increasing number of AC-DC power supplies released that rely on de-rating specifications to improve their headline power ratings.

This de-rating information may not be immediately apparent and is typically located at the end of the product data sheet, well away from the headline data. In some cases the short form or catalogue version of the data does not include this level of detail, so care must be taken when selecting the product to ensure that it is truly suitable for a given application.

De-rating specifications are based on reducing the specified output power rating of the power supply during high temperature operation or low line input voltage operation to mitigate excessive component temperature rises and ensure that safety critical isolation components do not exceed their thermal limits.

Temperature de-rating
Virtually all power supplies have a de-rating curve based on ambient temperature (figure 1). For products designed for integration into end equipment, this de-rating typically starts at ambient temperatures in excess of 50C. This allows for temperature rises within the end equipment, whilst maintaining the full specified power rating of the power supply. The output power rating will usually fall to 50% at a maximum ambient temperature of 70C. There are also a small number of manufacturers who de-rate products below 0C based on their ability to start at low temperature.

Figure 1: Output power derating curve based on ambient temperature.

For external power supplies the de-rating normally starts at 40C as these products are not exposed to the temperature rises within the end equipment.

In recent times, open frame power supplies are being introduced by some manufacturers which limit the maximum ambient temperature for full power operation to 40C with the output power reducing to 50% at a maximum ambient of 60C. This is due to component temperature rises which are too high to allow full headline power operation at 50C limited by component specifications, lifetime and product safety requirements.

While such "specmanship" provides a higher headline power rating and, at first sight, appears to offer smaller size or lower cost, when integrated into end equipment which needs to operate in an ambient of 40C the available output power is reduced immediately by 25% or more. Put another way this means that such a product with a head line power rating of 100W is actually a 75W product in practical terms and cannot be considered comparable to other units of the same power that are rated for 50C operation.

Input voltage de-rating
Products designed for world-wide operation have a universal input range typically covering 90 C 264VAC. Conventionally a product with universal input is expected to offer its full power rating across this input range with some products offering a de-rated output for lower input voltages down to 85 or 80VAC to cover operation in areas where the AC supply is prone to brownout.

Figure 2: Output power de-rating curve based on input voltage.

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