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Lecture-6 Thermal Design-2 Dr. Tahir Izhar 1

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Example-1: A certain power transistor dissipate 2 watts. The thermal resistance from junction to case is 8 o C/W and case to ambient is 20 o C/W. The free air temperature is 25 o C. Calculate the junction temperature and case temperature. 2

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Solution-1 3

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Example-2: The maximum permissible junction temperature of a certain power device is 150 o C. It is desired to operate the device at 15W in an ambient temperature of 40 o C. R jc = 0.5 o C/W and R ca = 10 o C/W. Determine whether a heat sink is required for this application. If sink is required, determine the maximum thermal resistance it can have assuming R cs nearly zero. 4

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Solution will exceed the maximum limit of 150 o C, therefore, heat sink is required.

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Solution-2 6 Set T j to maximum value of 150 o C.

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The maximum permissible power dissipation of a semiconductor device is specified by the manufacturer at a certain temperature (either junction or case). The maximum device power decrease as a function of temperature at elevated temperature. The decrease in device power dissipation at elevated temperature is called derating. Manufacturer also specify a derating factor in W/ o C. 7

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Example-3: Suppose a device has maximum power of 20 W at 25 o C and a derating factor of 100mW/ o C at temperature above 25 o C. The maximum permissible dissipation at 100 o C is P d =20W-(100 o C-25 o C)X100mW/ o C P d =20-7.5=12.5W 8

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Example-4: The temperature of a certain semiconductor device is 100 o C when it is dissipating 1.2W. The device temperature can not exceed 100 o C. If the total thermal resistance from device to ambient is 50 o C/W, above what ambient temperature should the dissipation be derated. 9

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Solution-4 10

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