Understanding AC-DC power supply de-rating specifications

Understanding AC-DC power supply de-rating specifications

Feature articles |
By Nick Flaherty

An increasing number of AC-DC power supplies rely on de-rating specifications to improve their headline power ratings. This is when a power system or component is operated at a level below its headline rating to meet safety, thermal & reliability requirements.

Two of the most common de-rating specifications are linked to temperature and input voltage, says Steve Dodson, Engineering Services Manager for European custom power supplies at XP Power.

De-rating specifications are based on reducing the specified output power rating of the power supply during higher temperature operation or low line input voltage operation. This mitigates excessive component temperature rises, preserves component lifetime, and ensures that safety-critical isolation components do not exceed their thermal limits. Two of the most common de-rating specifications are linked to operating temperature and input voltage.

Temperature de-rating

Virtually all power supplies have a de-rating curve based on ambient temperature. 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 temperatures.

Open frame power supplies being introduced 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 that are too high to allow full headline power operation at 50°C, limited by component specifications, lifetime, and product safety requirements.

This might provide a smaller size or lower cost when integrated into end equipment which needs to operate in an ambient of 40°C, the available output power is reduced immediately by 25% or more. This means that such a product with a headline 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 50°C operation, says Dodson.

Voltage de-rating

Products designed for world-wide operation have a universal input range typically covering 90 – 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.

It has become commonplace to increase the power rating of the product and specify de-rating for input voltages less than 100VAC or even as high as 120VAC. This input de-rating can be as much as 20% when operating at 90VAC.

Using specification de-rating in this way makes the product appear to offer increased power density and lower cost. However, if the application is required to operate globally, a higher power version of the product would be required. Put another way a power supply with a headline rating of 100W can only practically be rated at 80W. 

Input voltage de-rating is employed to mitigate overheating in the input filter, bridge rectifier & PFC boost converter as the input current increases. Some losses increase proportionally to the current but resistive losses, such as those found in EMC chokes, increase by the square of the current.

If the end equipment is intended for sale on a worldwide basis, care must be taken to ensure that the power supply rating is adequate at low line voltages. Exceeding the de-rating curves may result in reliability and lifetime problems.

Combined de-rating

When combined, thermal and low line de-rating result in a 40 °C rated 100W power supply being reduced to a rating as low as 60W if used in an ambient of 50°C with a line voltage of 90VAC says Dodson.

There will also be de-rating rules applicable to the complete final system. For example, all the components may be specified at 80% of their rated power, to ensure long life and enhanced reliability. If the finer details of the de-rating curves have been missed, then there may be a situation where the system is completed and signed off, whilst infringing these design rules, with the detrimental effects on lifetime and reliability that will bring.

While the product specification requires de-rating under certain line voltage and ambient temperature conditions, the power supply will not limit the available power at the output and will continue to operate. If the product is operated outside of the de-rating curves there are serious consequences in terms of reliability, product lifetime, and potentially safety if the thermal limits of the isolation barriers are exceeded.

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