There are three main ways in which an electronic unit such as a power supply can lose heat; radiation, convection and conduction. Radiation through electromagnetic emission provides one source of heat loss but this is rarely the primary means of dissipation.
Convection provides one of the main pathways for heat to be transferred away from the power supply as energy is transferred from the solid components of the system to air as it moves past. The rate of heat loss is proportional to the rate at which the air flows over the system and away into the wider atmosphere. As a result, forced-air cooling -- usually driven by fans -- will provide a greater degree of cooling than the natural movement that results from hot components transferring energy to air molecules. With natural convection, expansion in air caused by its warming as it passes over hot components provides a degree of movement that allows the heat energy to be distributed through air vents to the outside world. Forced-air cooling provides a steady flow of cooler air to accept heat generated by the power supply's component but will add acoustic noise to the environment.
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Figure 1: The VHK dc-dc converter series employs an integrated heat sink design to help optimize thermal performance. (Source: CUI)
Conduction through a PCB substrate or system chassis provides a further avenue for removing heat from a power supply although, traditionally, it has been considered as less important than convection. In general, metals provide efficient conduction of heat. When excited by heat, the electrons in a piece of metal can leave their atoms and move within the lattice as free electrons. Kinetic energy from vibrating metals is transferred from hot parts of the metal to cooler parts by the free electrons, which will collide with ions as they move and can be recaptured if they lose enough energy. The high copper content of a PCB as well as the metal within an enclosure helps provide good paths for heat flow out of the power supply through conduction.
A heat sink uses conduction to increase the efficiency of cooling by convection. The heat sink is designed to increase the surface area of a device that is in contact with the surrounding air, helping to increase cooling efficiency.
To maximize heat conduction from the device to the heat sink, the use of thermal adhesive is recommended to fill any void between the device to be cooled, which may be a complete power converter, and the heat-sink surface. Bolts or clamps increase contact pressure, which also improves thermal transfer into the heat sink.