When estimating the junction temperature of a power device used with a heatsink, which combination should be used as the junction-to-ambient thermal resistance?

Answer: B. The sum of θJC, the case-to-heatsink contact thermal resistance and the heatsink-to-ambient thermal resistance

θJC is the thermal resistance from the junction to the package's heat-dissipating surface (the case), measured with the case held at a constant temperature. With a heatsink attached, most heat flows from the case into the heatsink, so θJC, the contact thermal resistance θCS of thermal grease or similar, and the heatsink thermal resistance θSA are added in series. For example, 1.0 + 0.5 + 3.0 = 4.5 °C/W, so a 10 W loss gives a temperature rise of 45 °C. θJA is measured without a heatsink, with the device mounted on a test board, so it cannot be used in this case.