Discussion
A device dissipates \( 5 \ \mathrm{W} \) with resistances of \( 3 \), \( 0.5 \) and \( 2 \ \mathrm{^\circ C/W} \) in a \( 40 \ \mathrm{^\circ C} \) ambient. Its junction reaches
Answer: A
The three resistances add and the rise is added to ambient:
\[ \begin{aligned} R_{th} &= 3 + 0.5 + 2 = 5.5 \\ T_{j} &= 40 + 5(5.5) = 67.5 \ \mathrm{^\circ C} \end{aligned} \]
giving 67.5 degrees, with useful margin below a 150 degree limit. Omitting the interface gives 65, and omitting the heatsink gives 57.5 - both optimistic. Adding every term in the chain before multiplying is what makes the answer trustworthy.
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