Thermal Equivalent Circuit for a Transistor
ECE 442
1
Case Temperature
Heat Sink Temperature
Junction Temperature
Ambient Temperature
ECE 442
2
Thermal Resistance
from Junction to Case
Total Average Power Loss
Thermal Resistance
from Case to Heat Sink
Thermal Resistance from
Heat Sink to Ambient
ECE 442
3
T T P R
JC
T T P R
CS
T T P R
SA
C
S
A
J
C
S
T
T
T
T  T  P (R  R  R )
J
A
T
JC
ECE 442
CS
SA
4
Motorola MS-10 Heat Sink
ECE 442
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Performance Under Forced Air Flow
ECE 442
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Thermal Characteristic in Free Air
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Top View of MS-10 Heat Sink
ECE 442
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Hole Pattern for a TO-3 Case
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Mica Insulator
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Thermal Resistances for Mica Insulators
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ECE 442
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Clip-On Heat Sinks
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Thermal Performance
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Thermal Resistance for Clip-On
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Bond-On Extrusions for DIP Packages
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Thermal Performance
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Thermal Resistances for Bond-On
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Example
•
•
•
•
•
TO-220 case style dissipating 5 watts
RJC = 3.0 C / Watt (from device spec)
TJmax = 150 C
TAmax = 50 C
Find the proper heat sink to keep the
junction temperature from exceeding
150C in natural convection.
ECE 442
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T T
P 
R R R
T T
R 
 (R  R )
P
J
A
D
JC
CS
J
SA
A
SA
JC
CS
D
Need to determine a value for RCS – depends
on the mounting technique.
If the device is mounted without an insulator,
but only with Thermalcote, (silicone grease),
check page 57, Figure 9 for TO -220 case style.
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Use Thermalcote
RCS = 1 C/W
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No Insulator
T T
R 
 (R  R )
P
(150  50)C
C
R 
 (3  1)
5W
W
C
R  16
W
J
A
SA
JC
CS
D
SA
SA
ECE 442
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Thermalloy P/N 6030
Temperature rise above ambient@ 5 Watts Power Dissipation = 66 C
ECE 442
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Thermal Resistance RSA
66C
C
R 
 13.2
5W
W
SA
As long as RSA is less than 16 C/W, the
requirement is met.
ECE 442
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Thermalloy P/N 6030
ECE 442
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