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WSW Components Line of Board-Level Transistor Heat Sink Overview Slide 9

Power dissipated inside a semiconductor device is similar to a “constant current” heat generator, because the heat has to flow through the total thermal resistance between the junction and the ambient – Rth(ja) shown here on the diagram, raising the critical inside junction temperature T(j) (the hottest point on the component) above the surrounding ambient temperature T(amb). To keep the device at or below its proper operating temperature the designer would reduce the value of Rth(ja) by installing a heat sink. In the formula T(ja) is the temperature increase of the component’s heat producing junction above ambient (measured in degrees Celsius). Pd is the power being dissipated (measured in watts); and Rth(ja) is the total thermal resistance between the junction and ambient (in °C/W or °K/W). Similarly, to find the junction temperature – add T(ja) to the max ambient temperature. Once the maximum safe junction temperature (Tjmax) for the component is known as well as the power being dissipated (Pd) and the maximum ambient temperature (Tamb), one can then find the maximum total thermal resistance between the junction and surrounding ambient.

PTM Published on: 2013-05-06