Thermodynamics HW Solutions 227

Thermodynamics HW Solutions 227 - Chapter 15 Steady Heat...

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Chapter 15 Steady Heat Conduction 3-107 The maximum power rating of a transistor whose case temperature is not to exceed 80 ° is to be determined. C Assumptions 1 Steady operating conditions exist. 2 The transistor case is isothermal at 80 ° . C Properties The case-to-ambient thermal resistance is given to be 20 ° . C/W T R T s Analysis The maximum power at which this transistor can be operated safely is W 1.6 = ° ° = = Δ = C/W 25 C ) 40 80 ( ambient case case ambient case R T T R T Q & 3-108 A commercially available heat sink is to be selected to keep the case temperature of a transistor below 90 in an environment at 20 ° . ° C C Assumptions 1 Steady operating conditions exist. 2 The transistor case is isothermal at 90 ° . 3 The contact resistance between the transistor and the heat sink is negligible. C Analysis The thermal resistance between the transistor attached to the sink and the ambient air is determined to be & & () Q T R R TT Q =⎯ ⎯= = −° Δ case ambient case ambient transistor C W 90 20 40 1.75 C / W T R T s The thermal resistance of the heat sink must be below 17 .
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