Computer designers keep products cool by following some fundamental equations for designing heat sinks that use forced air or natural convection cooling. High current diodes, numerous power-supply regulator assemblies, and power modules and transistors generate more heat than the mass can dissipate safely. To effectively increase the surface area and mass of heat-dissipating junctions, suitable heatsinks must be securely fastened to power devices.
Heat sinks are available in a variety of surface finishes, colors, sizes, and shapes for general and specific semiconductor applications. The amount of heat generated by a power module or semiconductor depends on the applied signal shape, signal amplitude, bias level, and operation mode. The devices need heatsinks to dissipate the generated internal power.
High current diodes and silicon power transistors need heat sinks that allow maximum junction temperature. Heat-generating devices that are not connected to heatsinks that provide greater surface area and mass will barely approach the voltage and junction current specifications, before exceeding the maximum temperature of the collector junction. That is why device specification sheets list characteristics with devices mounted on heat sinks of considerable size, usually referred to as ‘infinite’ heatsinks.
The surrounding air temperature of the heat-generating devices must be considerably less than the allowable temperature of the junction for devices to cool. Heat sinks attempt to reduce the temperature of the junction to that of surrounding mediums. To be thermally perfect the heatsinks would allow the transistor junction temperature to reach the temperature of the lower ambient.
Reaching that temperature is impossible in practice. The connections are thermally imperfect and consequently, produce temperature differentials. The variable between ambient and collector junctions are kept to a minimum with heat sinks.
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