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4. Six identical power transistors with aluminum casing are attached on one side of a 1.2-cm-thick, 20-cm x 30-cm copper plate (k = 386 W/m °C) by screws that exert an average pressure of 10 MPa. The base area of each transistor is 9 cm², and each transistor is placed at the center of a 10-cm x 10-cm section of the plate. The interface roughness is estimated to be about 1.4 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 23°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 30 W/m². °C. If the case temperature of the transistor is not to exceed 75°C, determine the maximum power each transistor can dissipate safely, and the temperature jump at the case-plate interface.
23°C 1.2 cm Copper Transistor plate 75°C Plexiglas cover