Heat Transfer Application examples of using fins

[Hannibal247]

Hello,
Im struggling a bit with this problem. (uploaded)

I don't know how to sketch the equivalent thermal circuit for any of them. Does it have to look like anything like that?

the thermal circuits are for b) parallel connectd right? and for aswell?
And I am getting wrong answers for a). Shouldnt it be like this?
Q= h_e*A_b²*55 + 1/((1/h_i)+(L_b/k_b))*A_b²*55
or are they all parallel? like this Q=A_b²*1/((1/h_i)+(L_b/k_b)+(1/h_e)) .
My first thought was that the heat rate dissipating on the chip side is the same as the heat rate on the board side. why is this thought wrong? I am confused :D
thank you for your answers
Best regards

 

[Valkarie]

Yes, the thermal circuits for b) and c) are parallel connected. For a), the heat rate dissipating on the chip side is not the same as the heat rate on the board side. This is because the resistance of the board (Lb/kb) is different than the resistance of the chip (1/he). Thus, the heat rate will be different on each side. The correct equation for the heat rate is: Q = Ab2 * 1/((1/hi)+(Lb/kb)+(1/he))