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Installing Open-Frame and U-Channel AC-DC Power Supplies

Power supplies are often provided in an open frame, relying on the end-application to provide a suitable enclosure to provide additional safety and EMC protection for the PCB.

Another common format for power supplies for integration into end equipment is the U channel, where the power-supply PCB is installed in a U-shaped, usually aluminum, chassis that’s often used as a part of the thermal management of the power semiconductors (Fig. 2). It also provides multiple fixing options for the equipment manufacturer to install the supply into the final assembly.

The alternative to an open-frame power supply is the U channel, supplied either without a cover (a) or with one (b). This type of enclosure can provide some thermal conductivity, as well as additional EMC protection and increased mounting options.

There are several considerations when installing open-frame and U-channel power supplies. Principally, they’re related to safety, electromagnetic compatibility (EMC), and thermal management, all of which are discussed in this article.

Another important consideration is the detailed specification of the power supply, especially with regard temperature and input voltage derating, when compared to the datasheet headline power rating. The best products maintain the rated power to 50°C ambient temperature and down to 90-V ac input. Some products advertise a headline power rating with derating of up to 20% at low line and derate the available power at ambient temperatures as low as 40°C, which may make these products unsuitable for the end application.

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