Chapter 15 Steady Heat Conduction3-107 The maximum power rating of a transistor whose case temperature is not to exceed 80°is to be determined. CAssumptions1Steady operating conditions exist.2 The transistor case is isothermal at 80°. CPropertiesThe case-to-ambient thermal resistance is given to be 20°. C/WT∞RTsAnalysis The maximum power at which this transistor can be operated safely is W1.6=°°−=−=Δ=−∞−C/W25C)4080(ambientcasecaseambientcaseRTTRTQ&3-108 A commercially available heat sink is to be selected to keep the case temperature of a transistor below 90in an environment at 20°. °CCAssumptions1Steady operating conditions exist.2 The transistor case is isothermal at 90°. 3 The contact resistance between the transistor and the heat sink is negligible.CAnalysis The thermal resistance between the transistor attached to the sink and the ambient air is determined to be &&()QTRRTTQ=⎯→⎯=−=−°=°−−∞Δcase ambientcase ambienttransistorCW90 20401.75 C / W T∞RTsThe thermal resistance of the heat sink must be below 17.
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