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Chapter 12
12.2 Power Packages on a Heatsink (to-3, to-220, to-218, etc.)
This physical situation can be modeled as shown in Figure 12.2 . The thermal equation
would look like:
TPRRRT
jDJCCSSAA(max)
()

(12-1)
Since the heatsink performs the vast majority of the heat radiation, it is assumed that all
the power fl ows through all the other thermal elements.
Temperature tests can be conducted at ambient room temperature, but the designer must
remember that the typical product is enclosed in a case and its internal temperature rise
must be added to the readings. Another consideration is the highest external ambient
temperature the product may experience. In the desert, where this book was written, daytime
temperatures may reach 43 C in the shade and exceed 55 C inside an automobile.
Some typical thermal resistances associated with the different power packages are given
in Table 12.2 .
Table 12.2 : Thermal resistances of common thru-hole
power packages
Package Minimum Maximum Minimum Maximum
TO-3 * 30.0 0.7 1.56
TO-3P * 30.0 0.67 1.00
T0–218 * 30.0 0.7 1.00
TO-218FP * 30.0 2.0 3.20
TO-220 * 62.5 1.25 4.10
TO-225 * 62.5 3.12 10.0
TO-247 * 30.0 0.67 1.00
DPACK 71.0 100.0 6.25 8.33
P
T
j
T
C
T
A
T
HS
θ
jc
θ
CS
θ
SA
Figure 12.2 : The thermal model for a transistor on a heatsink

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