In an ideal world all the energy input to a power supply would be converted to useable electrical output power. Unfortunately real life systems incur losses and like all other types of power supplies, open frame power supplies generate heat which has to be dissipated during system operation. Key components that contribute to such losses are transformer coils, capacitors, power switches, resistors and the like.
Typically, open frame power supplies are either convection cooled or forced-air cooled. The main difference between convection and force-air cooled power supply is in the power density offered. For a given efficiency, convection cooled power supplies offer a lower power density. For instance, an open frame power supply on a 5” x 3” industry standard footprint may have a convection rating of 150W while the force-air cooled version may have a rating as high as 400W.
Convection Cooling
Convection is defined as the transfer of heat via air. The convection cooled open frame power supply is intended to be used in an environment where there is free air circulation. The system designer must ensure that there is sufficient space around and above the power supply unit for free air convection currents to cool the unit and must also ensure that the operating temperature is controlled to a level within its maximum ratings.
Forced Air Cooling
Forced-air cooled open frame power supply may incorporate a cooling fan, or may specify the cooling required to operate the unit at maximum load and operating temperature. The system designer must ensure that the maximum specified operating temperature is not exceeded for a given load rating and that the intake and exhaust areas are not blocked.
Typically, open frame power supplies that require the user to provide forced-air cooling will specify a minimum required airflow. In general, forced-air cooling requirements will be expressed in cubic feet per minute (CFM) which is also the common cooling rating for cooling fans.
When designing forced-air cooling, the fan should be sized to the cross-sectional area of the power supply within the enclosure to ensure the necessary velocity of air passes over the surface of the components. In general, air should be directed along the long axis of the power supply.
The system designer has a number of options available when considering the thermal management of the open frame power supply. While effective, forced-air cooling is not always the ideal choice for system designers if factors such as noise sensitivity are considered. By trading off power supply capacity against load or by selecting higher-efficiency, it is possible to achieve high performance with little to no acoustic noise through the use of convection air cooling.