Consider a power transistor encapsulated in an aluminum

Chapter , Problem 3.33

(choose chapter or problem)

Consider a power transistor encapsulated in an aluminum case that is attached at its base to a square aluminum plate of thermal conductivity k 240 W/mK, thickness L 6 mm, and width W 20 mm. The case is joined to the plate by screws that maintain a contact pressure of 1 bar, and the back surface of the plate transfers heat by natural convection and radiation to ambient air and large surroundings at T Tsur 25 C. The surface has an emissivity of 0.9, and the convection coefficient is h 4 W/m2 K. The case is completely enclosed such that heat transfer may be assumed to occur exclusively through the base plate (a) If the air-filled aluminum-to-aluminum interface is characterized by an area of Ac 2 104 m2 and a roughness of 10 m, what is the maximum allowable power dissipation if the surface temperature of the case, Ts,c, is not to exceed 85 C? (b) The convection coefficient may be increased by subjecting the plate surface to a forced flow of air. Explore the effect of increasing the coefficient over the range 4 h 200 W/m2 K.