All electronic devices generate heat due to their unavoidable internal losses and inefficiencies. The higher the efficiency rating of the device, the less internal heat is generated within it. If we could achieve 100% efficiency, and technology is getting ever closer to that elusive goal, no heat would be generated within the device and, therefore, no cooling would be required. Until then, the generated heat must be dissipated to maximize the end product's reliability and prevent its premature failure.

Figure 1: Photo of a convection-cooled (no fan) AC-DC switchmode power supply. Notice the black heat sinks, some of which have fins, while others are vertically mounted black sheet metal, which also serve as heat sinks to the high power devices used in the power supply circuits. This model (ZWD-PAF) is capable of providing up to 225-watts of output power with an ambient air temperature ranging from -10°C to +50°C.
There are three primary techniques of transferring or dissipating heat from power and other electronic devices: These are conduction, convection, and radiant. In all cases, the heat is being transferred from the hot electronic device to another medium that is at a lower temperature. All cooling techniques include some components of conduction, convection, and radiant cooling in their process. Heat is constantly seeking to move from its source to an object, or through a medium that is cooler.

Conduction cooling: This is defined as the transfer of heat from one hot part to another cooler part by direct contact. For example, many DC-DC converters have a flat surface (baseplate) that is designed to mount directly to an external heatsink or cold plate that will conduct the heat away from the power device by direct contact, thereby cooling it. Most power supplies use internal heatsinks that are in contact with the power devices, via a thermal conductive paste or pad, to conduct away the heat. The heatsinks, in turn, depend upon convection cooling to transfer their conducted heat to the cooler surrounding air.