Convection cooling: This involves the transfer of heat from a power device by the action of the natural air flow (air is actually a low-density fluid) surrounding and contacting the device. Many power devices are rated for natural convection cooling as long as the air surrounding the unit remains within a limited temperature range that is cooler than the device. The advantage of convection cooling is that no fans are required (Figure 1). Fans, due to their mechanical components, tend to reduce the mean-time-between-failures (MTBF) ratings of power supplies. Convection cooling works best when the products have a natural circulating source of air. For enclosed applications, to insure a natural exchange of air, a number of vents should be provided at various locations in the case or enclosure.

Figure 2. Diagram of typical Power Module being cooled by an attached heatsink. As shown, there are many thermal resistances which impede the free flow of the heat from the semiconductors and other devices that are internal to the Power Module. Techniques for reducing these thermal resistances include the use of thermal compounds (grease or pads), maximizing the surface area of the heatsink to suit the application, and, if necessary, adding forced air cooling.
Many heat- generating electronic devices, including DC-DC converters, microprocessors, hybrid circuits, etc., require the use of heatsinks to facilitate convection cooling and to assist in transferring the heat away from the devices to the cooler air. Heatsinks utilize both conduction cooling (i.e., the device must make good thermal contact with the heatsink) and convection cooling. Heatsinks are designed to transfer the heat from the device to the ambient air, primarily by substantially increasing the surface area that comes in contact with the air. Because the surfaces of electronic devices and heatsinks are not perfect, some type of thermally conductive interface material is necessary to fill the tiny voids. Although this material must be thermally conductive, in some instances it also needs to be an electrical insulator; for example, a thin silicon pad. In most other applications, a thin layer of thermal grease can be used as the interface material.