Thermal design and derating in enclosed power supplies

Every power supply turns part of its input into heat. At 90% efficiency, a 200 W supply dissipates over 20 W inside its enclosure. That heat raises the temperature of components, and temperature, more than any other factor, decides how long a power supply lasts. Thermal design is therefore part of selecting a supply, not an afterthought once it is mounted.

Where the heat goes

Heat leaves an enclosed power supply by conduction through its mounting surface, by convection to the surrounding air and by radiation. A unit rated for free-air convection relies on air moving freely around it. The same unit, mounted inside a sealed luminaire or a crowded control box, sees a much higher local ambient and may lose most of its convection path.

The rating on a datasheet is valid only under the cooling conditions it assumes. Those conditions should be stated: orientation, mounting surface, free space around the unit and the ambient temperature.

Reading a derating curve

A derating curve shows the output a supply can deliver against ambient temperature. A typical curve is flat up to a stated temperature, commonly in the range of 40–50 °C, and then falls in a straight line. Many enclosed supplies derate to about 50% of rated output at 60–70 °C.

Two points are easy to miss. The ambient on the curve is the air next to the supply, not the room temperature. And some products state the curve against case temperature instead of ambient, which is a different measurement.

The case temperature point

Many drivers and supplies carry a marked case temperature point, often labelled Tc. The manufacturer states a maximum temperature at that point and, often, an expected life at a lower temperature. Measuring Tc in the finished product, at the highest expected ambient and the actual load, is the most direct check that the supply is used within its limits.

Temperature and life

The component that most often limits life is the electrolytic capacitor. A widely used rule of thumb is that its life roughly halves for every 10 °C rise in its operating temperature. A supply that runs 10 °C cooler can therefore last substantially longer, all else being equal.

This is why derating is often applied deliberately, not only when the curve requires it. Running a supply at 70–80% of its rating, in a cooler position, is a common design choice where long life matters.

Practical design checks

1. Establish the real ambient. Measure the air temperature next to the supply inside the product at its worst operating condition, with the sun, the enclosure and neighbouring heat sources included.
2. Check the cooling assumption. If the datasheet rating assumes mounting on a metal surface, provide that surface.
3. Leave space. Keep the clearances the datasheet asks for and avoid placing the supply above other heat sources.
4. Measure Tc. Run the product at full load and highest ambient until temperatures settle, then compare Tc with the limit and with the life data.
5. Allow for the enclosure. Sealing for ingress protection reduces convection. A sealed driver is designed for this; an open-frame supply inside a sealed box is not, unless the box is designed to carry the heat away.

Enclosure and ingress protection

Outdoor and damp installations need ingress protection, stated as an IP rating. Potted, sealed drivers carry heat to their case through the potting compound and rely on the case to shed it. They are rated for this duty, but they still need a mounting position that lets the case cool.

When the measurement fails

If Tc or the local ambient is over the limit, the options are a larger supply running at a lower share of its rating, a better thermal path to a metal surface, more ventilation, or moving the supply away from other heat sources. Each option is checked by measuring again.

What to share when selecting a supply

• Required output and the normal operating load.
• Maximum ambient temperature at the supply's location inside the product.
• Mounting arrangement: metal plate, free air or enclosed space.
• Ingress protection required.
• Expected operating hours and design life.

Where to go next

• CC/CV & Custom Power Supplies
• LED Drivers
• Questions for a power-supply supplier before validation
• Discuss your requirement: share the load, ambient and mounting details.