A New Category of Rugged, Fanless Power Supply

New approach meets large-load requirements at high ambient temperatures for medical, clean room or outdoor signage applications.

By Joel Zaens, Emerson Network Power

There is a category of application which has been poorly served by makers of standard AC-DC power supplies. These applications combine high ambient or operating temperatures with a requirement for a rugged, reliable power supply serving loads up to 250W. Particularly when fanless operation is required, original equipment manufacturers have struggled to find a cost-effective solution.

This problem particularly affects designers of outdoor applications, such as exterior lighting and billboards, communications towers and remote monitoring devices, which require a high power output. Here, there is a benefit to avoiding the use of equipment that contains a fan. This is because a fan can significantly lower the equipment’s reliability (usually measured as a mean time between failure, or MTBF), and some applications cannot bear the risk of premature failure. This might be because unexpected downtime causes large financial losses. For end products such as advertising billboards and communications masts, it is also because the cost of repairing faults in remote or awkward locations is high.

Fanless operation at high power is also required in other categories of application: in clean rooms and hospital equipment, for instance, the movement of air has to be kept to a minimum in order to avoid the circulation of particles or microbes. In other applications, such as airport signage, silent operation is a key requirement. Here again, therefore, power supplies must operate at high power with no fan.

Most system designers facing this problem are working in highly competitive markets and with small- to mid-volume production runs: for them, the luxury of specifying a custom power supply is not financially realistic. To keep materials costs low, these systems must use a reasonably priced standard power supply.

So for designers of products used outdoors, which require a certain amount of protection from the elements, the typical approach today is to ruggedize an open-frame standard power supply unit by mounting it within an enclosure which resists the ingress of liquid. Enclosing an open-frame unit does of course provide protection against moisture and contaminants, but technically this approach entails many undesirable trade-offs.

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Figure 1. A new category of AC-DC power supplies, such as Emerson’s LCC250 series, is optimized for enclosed and fanless operation up to 85°C baseplate temperature

Crucially, open-frame units are designed to operate in the presence of airflow; an enclosure stops the free movement of air, which means that the power supply’s components are likely to operate near the top end of their operating temperature range. But ordinary open-frame units are not specially designed for high- or low-temperature operation. At extremes of temperature, standard open-frame power supply units have to be de-rated. So, depending on the ambient temperature in which the device is to be used, the designer might then find that the power supply cannot support the load under all operating conditions.

Designers of applications which require a high power output at high temperatures might then consider adding forced-air cooling in the enclosure – but the use of a fan is undesirable for all the reasons outlined above.

The problem, then, is how to deliver a low-cost, ruggedized power supply rated for up to 250W which can operate at full power at high temperature, but without requiring a fan.

A new category of power supplies that use conduction- and convection-cooling techniques, which are commonly found in DC-DC power converters, but which have not generally been used in AC-DC power supplies could be the answer. Unlike the conventional approach of enclosing a standard open-frame unit, the development of this new category of AC-DC power supplies is intended from the beginning to enable the power supply to be both enclosed and fanless, and they are optimized for this mode of operation.

In this type of power supply, the metal baseplate serves as both the mounting point to the host device and as a thermal interface to ambient air. The power supply therefore needs to be mounted to a thermally conductive surface – normally this will be the metal case or cabinet of the host device. This metal surface then serves as a large heat sink draining thermal energy from the power supply.

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Figure 2. Conduction-cooled versions of Emerson’s LCC250 offer higher usable power at high temperature than is common in fanless AC-DC power supplies

Convection-cooled variants are for systems in which heat cannot be transferred to ambient air via a surface of the host device. In these variants, the top surface of the power supply (which is not thermally conductive) provides the mounting points, and a large integrated heat sink attached to the base plate allows for heat to be dissipated. Figures 1 and 2 show that conduction-cooled power supplies offer a higher power output at high temperature than the convection-cooled variants.

But the provision of a thermal interface to ambient air is not the only reason for the high performance of these power supplies. As stated above, these new units are being designed from the outset for enclosed operation without a fan. This required optimization of the thermal path from components to the thermal interface (the base plate). So the layout of the unit’s circuit board is designed to provide direct thermal coupling between the system’s hottest components – such as mosfets and rectifiers – and the base plate. In other words, the board layout is not optimized for cost or ease of assembly, as it would be in a standard open-frame power supply unit, but for thermal performance.

In addition, an effective thermal path from other components which are not directly coupled to the base plate is provided by potting the enclosure with thermally conductive compound. As a result, the thermal resistance from components inside units to ambient air is far lower than in a ruggedized (enclosed) standard open frame unit. This approach offers full power operation supporting a 250W load right up to 85°C baseplate temperature, and down to -40°C, and de-rated operation between 85°C and 100°C baseplate temperature.

By comparison, standard mid- to high-power supply units typically offer full power operation up to just 50°C, and de-rated operation up to just 80°C; at the other extreme, standard units only operate normally down to 0°C.

As an alternative to today’s normal procedure of enclosing a standard open-frame unit, the new category of power supplies offers markedly superior power output and operating temperature range, while offering a moderate degree of ruggedization.

Conclusion
Through careful design of the thermal pathways inside the power supply unit, and from the unit to ambient, it is possible to produce a 250W AC-DC power supply which can operate at full power at very high and very low temperatures, and with no requirement for forced-air cooling at high temperature.

As an alternative to the common approach of enclosing a standard open-frame unit, this new category of power supplies offers far superior performance and reliability in applications which require moderate ruggedization, at comparable cost.

 


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Joel Zaens is a product manager with the Embedded Power business of Emerson Network Power, with responsibility for rapid modification and value-added services, fanless and conduction cooled products and board-mounted DC-DC modules. His experience in the power industry spans over 17 years in various manufacturing, project management and marketing roles. He holds a B.S. in electrical & communications engineering from De La Salle University in the Philippines.