When components, such as power transistors, are subjected to voltage and current during operation, they absorb power which is released in the form of calculated energy. The aim is to find a way of calculating how much surface area must be used to ensure that the component can function and at the same time be able to dissipate the heat energy through the fin. The thermal dissipation of a heat sink is measured by its Thermal resistance (Rt).
In order to calculate or select a heat sink, it is necessary to know the basic formulae of heat transfer or power dissipation, which can be expressed as follows:
PD = power dissipated by the semiconductor device in watts.
∆T = temperature difference or thermal jump causing the heat flow.
∑Rt = sum of thermal resistances to the heat flux where ∆T occurs.
∑Rt = Rt JC + Rt CS + Rt SA
Rt JC = thermal resistance between junction and semiconductor case in °C per watt (specified by semiconductor manufacturer).
Rt CS = interface thermal resistance between the semiconductor and the surface on which it is mounted, °C per watt.
Rt SA = thermal resistance between mounting surface and environment (heat sink thermal resistance) in °C per watt.
TJ - TA
Rt JC + Rt CS + Rt SA
TC - TA
Rt CS + Rt SA
TS - TA
TJ = junction temperature in °C (the maximum Tj is given by the semiconductor manufacturer).
TC = temperature in °C of the semiconductor case.
TS = temperature in °C of the surface of the heat sink in thermal contact with the semiconductor.
TA = ambient temperature in °C.