Convection is defined as the transfer of heat via a fluid, which in the case of power supplies, typically occurs via air rather than a liquid. If the fluid flow results from the gravitational forces on the fluid as its density varies (i.e. warm air rises), it is referred to as natural or free convection. However, if the fluid flow is created by external means such as fans or blowers, it is referred to as forced convection. Consequently, 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 the air as it moves past. As the rate of heat dissipation is proportional to the airflow rate, forced-air cooling will provide a greater degree of cooling than free convection. However, while forced-air cooling provides a steady flow of cooler air to transfer heat away from a power supply, it does incur the penalty of contributing acoustic noise to the system and its immediate environment. In addition, fans will increase power consumption within a system and can reduce overall lifetime of the application with the introduction of additional mechanical parts that may fail. The efficiency of cooling by convection can be improved with a heat sink that uses conduction to increase the surface area of a device that is in contact with the surrounding air. To maximize conduction from the heat generating components to the heat sink, the use of a thermal compound is recommended to fill any void between the devices to be cooled, which may be a complete power converter, and the heat-sink surface. Bolts or clamps that increase contact pressure also help to improve thermal transfer into the heat sink. To size a heatsink for a given application, it is necessary to know how much heat needs to be dissipated (the heat flow Q as defined earlier), the maximum ambient temperature (TA) and the maximum allowable case or baseplate temperature (TC) of the power supply. Hence the required thermal resistance for the heatsink must be no greater than: θCA = (TC – TA)/Q.