Four identical power transistors with aluminum casing are attached on one
side of a 1-cm-thick 20-cm 3 20-cm square copper plate (k 5 386 W/m·K) by
screws that exert an average pressure of 6 MPa (Fig. 3–18). The base area of
each transistor is 8 cm2, and each transistor is placed at the center of a 10-cm
3 10-cm quarter section of the plate. The interface roughness is estimated to
be about 1.5 mm. All transistors are covered by a thick Plexiglas layer, which is
a poor conductor of heat, and thus all the heat generated at the junction of the
transistor must be dissipated to the ambient at 20°C through the back surface
of the copper plate. The combined convection/radiation heat transfer coefficient
at the back surface can be taken to be 25 W/m2·K. If the case temperature of the transistor is not to exceed 70°C, determine the maximum power each
transistor can dissipate safely, and the temperature jump at the case-plate
interface.