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Alternatives to fan-cooled power supplies

Thu, 06/07/2012 - 10:57am
David Norton, TDK-Lambda Americas

The majority of power supplies in the mid to high power range require forced air-cooling when operated within a defined ambient air temperature range. Forced air-cooling is achieved either by fans that come as part of the power supply or by fans provided within the system or end-product. Since fans are electromechanical devices, they are subject to bearing fatigue and are therefore one of the least reliable components in an electronic product. To avoid fan failure, regular cleaning of inlet air filters is necessary. For fan-cooled supplies, periodically the power supplies must be cleaned by qualified technicians and the fans replaced prior to their end of life.

To the right is a photo of a fan-cooled power supply that operated for many years at a postal depot where mail is handled and sorted automatically. As can be seen with the fan removed, paper fragments and airborne dust contaminants were pulled into the supply by the fan and eventually caused a blown fuse. 

The alternative to forced air cooling with fans is the use of convection or conduction cooled power devices. Since power conversion efficiencies have greatly improved over the years, the amount of wasted heat generated within power supplies has been substantially reduced. These efficiency improvements have made it easier to produce power supplies that can be convection cooled by means of natural air flow, without the need for fans. Likewise, higher efficiencies have yielded a new breed of conduction cooled power supplies, where the user can sink the heat away from the power supply via contact with the system’s metal enclosure wall, external heatsinks or cold plates.

The obvious advantages of convection or conduction cooled fans are their extended field life, since the fan has been eliminated and the inflow of dust and other contaminants is substantially reduced. In addition, these types of supplies can be fitted in systems with less space required since a directed airflow entry and exit path is not a major concern. Moreover, these are ideal for some sealed or water proof enclosure applications.

A new breed of conduction-cooled power supplies has been developed that do not depend on fans for cooling. Instead, the required cooling is accomplished by conducting the internal heat loads to an external metal structure or enclosure, which act as a large heat sink surface.

In some applications, these conduction-cooled devices are mounted to liquid cooled cold plates that are made of metal with internal serpentine channels through which a liquid circulates while removing the unwanted heat. The net result is that the system operates with a substantial reduction in audible noise, reduced maintenance costs (no dust build-up and fan wear-out), and an enhanced MTBF.

Recently, I visited a television broadcasting station that consumes about 100 kilowatts of power. At this location, in separate areas, was a traditional fan-cooled system as well as the latest generation system, which uses conduction-cooled power supplies and RF amplifiers that are cooled via liquid flow cold plates. During the operation of the traditional system with fan cooling, the audible noise was so loud that personnel within 100 feet of the system had to wear hearing protection devices. By comparison, in the other area where the new system with liquid cooling was operating, the noise level was so low (similar to an office environment) that no hearing protection was required.

Another alternative to fan-cooled power supplies involves the application of convection-cooled units. These units can operate over a wide range of ambient temperatures while using only the natural air around the power supply to provide the required cooling.

In summary, since fans are electromechanical devices they reduce the system’s MTBF, exhibit unwanted audible noise and add to maintenance expenses. Therefore, there is a growing trend towards the application of conduction and convection-cooled power supplies that eliminate fans.

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