To properly interpret thermal-derating curves provided by power-module manufacturers, engineers must determine the test conditions under which the curves were derived.

The bulleted items on the front page of a dc-dc power module's data sheet often highlight electrical performance that the product cannot actually deliver in system. This presents a challenge to system designers, who must compare the electrical/thermal performance of different manufacturers' modules. System designers must ensure the dc-dc power modules chosen for their end equipment offer the requisite electrical/thermal performance across the application's full temperature range. Estimating the minimum and maximum load currents a module can supply in an actual system environment can be the most important factor for determining the cost and reliability of a power supply. This helps designers choose a module that is capable of the required output current at the most economical cost.

The electrical/thermal performance of a power module is characterized by its thermal-derating curves. The curves are the best and most commonly used metric for judging the overall performance of a module. Power-module manufacturers conduct extensive thermal testing to generate the curves, which are published in data sheets. The thermal-derating curves in Fig. 1 show the maximum current a module can deliver under various airflow velocities and ambient temperatures. This defines the device's safe operating area (SOA) — the operating condition where the maximum electrical output can be achieved without exceeding the recommended thermal design limits.

Each point on a thermal-derating curve represents a combination of output current and an environmental condition that causes the temperature of some component within the module to reach a predetermined limit. In the example above, an application may require 30-A load current. Environmental conditions include 50°C ambient temperature with air velocities as low as 1 m/sec (200 lfm). Upon consulting the module's data sheet, the SOA curves (Fig. 1) may reveal that a module with a 30-A maximum-output current rating can reliably deliver only 23 A continuously under these conditions.

Thermal-derating curves tell the designer if a chosen module will deliver the desired current at the desired ambient temperature, if additional airflow is needed, and how much margin or reserve is available in case of clogged filters or cooling-fan failure in an enclosure. Moreover, thermal data tells the designer if he must either derate (operate the module below its maximum-output rating), or supply increased amounts of cooling air or, in some instances, attach a heatsink.

In actual applications, many dc-dc power modules do not achieve the output current rating on the front of their data sheets. One reason for this is power-module manufacturers' specsmanship. Another is the fact that the power-module industry has no standard thermal-derating characterization process for isolated and nonisolated dc-dc power modules.

System designers face the challenge of selecting modules from a large number of suppliers. As a result, the dc-dc power-module business is highly competitive. An aspect of the intense competition is specsmanship, which has led power-supply manufacturers to describe product performance in increasingly creative ways.

Unfortunately, when it comes to interpretation of manufacturers' thermal-derating data, a true comparison is not so simple. The system designer should take into consideration differences in derating test details. Differences such as airflow and ambient-temperature measurement method and location, the maximum component temperature allowed, board pitch and test fixtures have significant influence on the derating curves. Because of these differences, the derating curves published by different manufacturers cannot be easily compared without first understanding their measurement method.

Thermal-Derating Measurement

There is no industry standard for measuring thermal performance. Two of the traditional approaches use an air velocity measurement inside a wind tunnel. This setup replicates the typical thermal environments in most modern electronic systems with distributed power architectures. The electronic equipment in networking, telecom, wireless and advanced computer systems operates in similar environments and uses vertically mounted pc boards or circuit cards in cabinet racks.