Using a heat pipe to conduct heat away involves the use of a pipe with a circulating liquid. One end of the pipe is in contact with the hot surface, which vaporizes the liquid. When the vaporized liquid encounters the cooler surface in the pipe, it condenses back into a liquid state and recirculates through the cooling system. Heat pipes can be effective in carrying heat away from the heat source to a heat transfer mechanism, yet designers need to cope with the additional space requirements and the potential for leaks damaging the electronics.
A third method, a cold plate, relies on a fluid—either water or some other refrigerant—to remove heat. Typically, the cold plate has small pipes in metal casings embedded beneath the surface of the plate. When the refrigerant moves through these pipes, it removes heat from the surface of the plate. Again, the extra space required for this method of cooling can be a consideration, along with compensating for potential water condensation.
An additional method of conductive cooling, called “potting,” involves filling the power supply enclosure with heat-transferring materials, such as epoxies, silicone elastomers, and urethane/polyurethane. This thermally conductive material carries heat to the top of the sealed enclosure where external airflow or other cooling mediums conduct the heat away.
While dissipating heat is a primary design challenge for engineers, extreme cold in outdoor settings also presents performance and reliability issues for power supplies. Extreme cold, though not as damaging as extreme heat, can also result in abnormal operation of the components, causing slow start up time, high ripple, instability, etc. In most cases, as a device starts up, the component temperature rises, bringing the operation within specification after a few minutes. In some cases, the device can be warmed by an external heating coil to ensure the components perform within specification at all times.
A conversation about air cooling also needs to include a discussion of elevation and atmosphere. Simply put, at higher elevations, the lower air pressure or density can result in less efficient convection cooling. For example, a 100 W power unit with an operating temperature rating of 50 C at sea level will have to be derated to less than 100 W power at 5,000 meters above sea level.
Ruggedized Modules for Board-Mounted Power
The equipment enclosure design and cooling techniques described above for PSUs can also greatly mitigate the challenges presented by temperature and mechanical stresses for board-mounted power. Additionally, for isolated and non-isolated DC-DC converters, designers can employ ruggedized versions of DC-DC converters that are available from many suppliers. These ruggedized versions are usually suitable for operating temperatures of up to 105 C and can withstand higher mechanical stresses than standard modules.
Harsh Environment Conditions
Environmental challenges—both outdoors and inside buildings—can also greatly degrade the performance, power rating, and longevity of power conversion modules. Outside, both water and humidity can erode performance and reliability, while inside manufacturing facilities, humidity, air-born particulates, and corrosive fumes create similar challenges. In addition to affecting performance and ratings, dirt, dust, and humidity can form arcing between high-voltage component leads, damaging components.
Traditionally, placing a cover over the system and power components mitigates some problems, but humidity, water, and dirt can still make their way onto the printed circuit board. A cover can work in limited indoor settings, but obviously is not a solution for outdoor applications.
To better protect against water and other contaminates, power designers can specify a conformal coating for the power component or for the entire board surface. Typically, a silicone or urethane coating covers the component, sealing it from water and humidity, dirt, and potentially corrosive fumes. This method can be a very cost-effective way to protect the components.
It does, however, have a drawback; conformal coatings restrict cooling airflow passing over the device, degrading either the power or temperature rating. For example, a 100 W power component rated for 65 C performs well within a performance specification operating at 50 C. With the air cooling limited by the coating, however, device temperatures can rise and result in a derated operating condition.
Excessive heat also can degrade the capacity of the power conversion device, such as an electrolytic capacitor, affecting its life cycle rating over time. For example, a device designed to operate at peak performance for 10 years might see the performance drop at 8 years if heat is not well dissipated.
Another extreme approach to protecting power conversion modules—whether used inside or outside—is to place the PSU in a sealed enclosure. While there are cost and performance trade-offs, the approach fully protects the device from air contaminants, humidity, water, and even a range of physical tampering—all defined as “Ingress Protection (IP).”
The definition of “harsh” can also apply to environments or processes where excess vibration can impact the performance, reliability or operating longevity of a PSU. For example, military vehicles, TV broadcast vans, and motor boats are subjected to shock and vibration that is transferred to the PSU mounted inside. To prevent failures under those conditions, PSUs in these settings generally use anti-vibration compounds, such as room temperature vulcanization, adhesive sealants or conformal coatings, to keep the components, screws, and boards in place.
Power designers face a number of issues, including thermal management, mechanical reliability, and harsh environmental conditions that all impact output capacity, performance, and longevity. An expanding set of PSUs offers a combination of tools and approaches to meet growing power conversion demands across industrial and commercial applications.