Heat transfer from a transistor may be enhanced by inserting it in an aluminium sleeve (k = 200 W/m −
K) having integrally machined longitudinal fins on its
outer surface. Transistor radius and height are r1 =
2 mm & H = 6 mm respectively, while the fins are of
length L = r3 − r2 = 10 mm of uniform thickness t =
0.7 mm. The thickness of the sleeve base is r2 − r1 =
1 mm. Contact resistance of the sleeve transistor
interface is Rt, c

′′ = 10−3 m2

.K/W. Air at T∞ = 20°C
flows over the fin surface providing uniform
convection with h = 25 W
m2−K
. Assume 1-D radial heat

transfer,
a) Sketch the equivalent thermal circuit for heat
transfer from transistor case (r = r1) to the air.
Label each resistance. [1]
b) Evaluate each resistance. [2]
c) If the temperature of the transistor case is T1 = 80 °C, what is the rate of heat transfer from
the sleeve?

Heat transfer from a transistor may be enhanced by

inserting it in an aluminium sleeve (k = 200 W/m −

K) having integrally machined longitudinal fins on its

outer surface. Transistor radius and height are r1 =

2 mm & H = 6 mm respectively, while the fins are of

length L = r3 − r2 = 10 mm of uniform thickness t =

0.7 mm. The thickness of the sleeve base is r2 − r1 =

1 mm. Contact resistance of the sleeve transistor

interface is Rt,c

′′ = 10−3 m2

.K/W. Air at T∞ = 20°C

flows over the fin surface providing uniform

convection with h = 25 W

m2−K

. Assume 1-D radial heat

transfer,

Explanation:

the equation above shows how temperature f, measured in degrees fahrenheit, relates to a temperature c, measured in degrees celsius. based on the equation, which of the following must be true?

a temperature increase of 1 degree fahrenheit is equivalent to a temperature increase of

5

9

degree celsius.

a temperature increase of 1 degree celsius is equivalent to a temperature increase of 1.8 degrees fahrenheit.

a temperature increase of

5

9

degree fahrenheit is equivalent to a temperature increase of 1 degree celsius.

a) i only

b) ii only

c) iii only

d) i and ii only

answer:

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