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Question 19.4: Determine the minimum acceptable thermal resistance rating f...

Determine the minimum acceptable thermal resistance rating for a heatsink for use with a transistor under the following conditions:

Maximum junction temperature: 175°C
Thermal resistance of case to surface: 1.25°C/W
Thermal resistance of washer and mounting kit: 1.75°C/W
Maximum total power dissipation: 75W
Maximum ambient temperature: 45°C
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Applying equation (7) gives:

\theta _{SA}=\frac{T_{Jmax}-T_{Amax}}{P_{Tmax}} -(\theta _{JC}+\theta _{CS})

from which:

\theta _{SA}=\frac{175-45}{13} -(1.25+1.75)=10-3=7^\circ C/W

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