With mounting market pressure on power supply size, power density and cost there are an increasing number of AC-DC power supplies released which 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 catalog 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.
Virtually all power supplies have a de-rating curve based on ambient temperature – see example Figure 1. For products designed for integration into end equipment, this de-rating typically starts at ambient temperatures in excess of 50°C. 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 70°C. There are also a small number of manufacturers who de-rate products below 0°C 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 40°C 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 40°C with the output power reducing to 50% at a maximum ambient of 60°C. This is due to component temperature rises which