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Question: 153 consider a surfacemount type transistor on a circuit board...

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1.53 Consider a surface-mount type transistor on a circuit board whose temperature is maintained at 35°C, Air at 20°0C flows over the upper surface of dimensions 4 mm × 8 mm with a convection coefficient of 50 W/m. K. Three wire leads, each of cross section 1 mm x 0.25 mm and length 4 mm, conduct heat from the case to the circuit board. The gap between the case and the board is 0.2 mm Air Transistor case Wire lead Circuit board Gap(b) Using the conductive paste to fill the gap, we wish to determine the extent to which increased heat dis- sipation may be accommodated, subject to the con- straint that the case temperature not exceed 40°C. Options include increasing the air speed to achieve a larger convection coefficient h and/or changing the lead wire material to one of larger thermal con- ductivity. Independently considering leads fabri- cated from materials with thermal conductivities of 200 and 400 W/m K, compute and plot the maxi mum allowable heat dissipation for variations in Ih over the range 50 s h s 250 W/m2 K.

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