The basic equation for heat transfer or power dissipation may be stated as follows: PD equals the temperature change divided by the sum of the thermal resistances of the heat flow path across which the temperature change exists. This relationship may also be stated in the following forms, where: TJ equals the junction temperature in °C, TC equals the case temperature of the semiconductor device in °C, TS equals the temperature of the heat sink mounting surface in thermal contact with the semiconductor device in °C, and TA equals the ambient air temperature in °C. The terms for resistance in the equation are: RθJC equals the thermal resistance from junction to case of the semiconductor device in in °C/W, RθCS equals the thermal resistance through the interface between the semiconductor device and the surface on which it is mounted in in °C/W, and RθSA equals the thermal resistance from the mounting surface to ambient or thermal resistance in in °C/W. The equations shown on this slide are generally used to determine the required thermal resistance of the heat sink (RθSA), since the heat dissipation, maximum junction and/or case temperature, and ambient temperature are known or set.