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Chapter 12
leaded packages are very compact and economical. For currents of 100 mA through
amperes, the package must ha
ve a tab or multiple leads connected directly to the die.
This is typically the drain, collector, or cathode. The common packages are the SOT223,
DPAK, SMB, and SMC. These tabs offer a very low resistance channel to remove the
heat from the die and get it onto the PC board for dissipation.
In surface mount printed circuit board applications, more than one issue usually must be
considered. Heatsinking must be considered, along with signal and EMI/RFI considerations.
The trace that must dissipate the greatest heat within a switching power supply is also the
node that has the largest dv/dt s which couple very easily to the surrounding traces.
Laying out heatsinking systems for surface mount packaging technology systems is
still an uncertain process. Semiconductor manufacturers still do not offer adequate
information for each power package to feel confi dent about the adequacy of the
heatsinking design. The graph in Figure 12.7 is a normalized plot based upon a SOT223
package. The curve is 2-oz. copper on the top of the PCB only. Curves such as that shown
in Figure 12.7 are needed to properly size the PC board heatsink island.
12.6 Examples of Some Thermal Applications
These examples will show the reader a typical application of thermal analysis but
with common application variations. These variations are useful in defi ning thermal
boundaries within a design.
1.0
0.9
0.8
0.7
0.6
0.1
0.2
0.3 0.4
Specified theta J-A
Heatsink pad area (sq-in)
Normalized thermal resistance J-A
Figure 12.7 : Example of the effect of increasing pad area versus JA
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373Thermal Analysis and Design
12.6.1 Determine the Smallest Heatsink (or Maximum Allowed Thermal
Resistance) for the Application
This approach is useful for determining the smallest possible heatsink that an application
can use before the thermal limit of a power device is exceeded. This is an example of a
consumer market approach to designing a heatsink system.
12.6.1.1 Specifi cation
The device is an FDP6670 (Fairchild MOSFET) in a switching power supply. Convection
cooling.
PD

10
50
20
053
watts
TC
C/W
C/W (Thermallo
A
JC
SA
(max)
.
.
°
°
°
yy P/N 53-77-5)
TC
J(max)
175°
The thermal model is shown in Figure 12.8 .
Rearranging Equation 12-1 and solving for the thermal resistance of the heatsink,

SA J A D JC CS(max)
()/ TTP
(12-5)
CS
is assumed at 1.0 ° C/W. Being conservative by not requiring the junction at its
maximum temperature makes the maximum allowable junction temperature 150 ° C. The
result is
SA
C/W
(max)
.70°
The PC board-mounted heatsink choices are: Thermalloy part numbers 7021B through
7025B for low-cost sheet-metal type heatsinks.
10 W
T
j
150°C
T
C
don’t care
T
A
50°C
T
HS
don’t care
θ
jC
1.53°C/W
θ
CS
0.52°C/W
θ
SA
?
Figure 12.8 : Thermal model for design example 12.6.1
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Chapter 12
12.6.2 Determine the Maximum Power That Can Be Dissipated by a Three-
Terminal Regulator at the Maximum Specifi ed Ambient Temperature without
a Heatsink
A three-terminal regulator s overcurrent protection is totally dependent upon
the heatsinking system. When the die reaches approximately 165 ° C, the regulator
shuts down. This example demonstrates the nonheatsink capabilities of
a A7805.
12.6.2.1 Specifi cation
The desired three-terminal regulator is a mA7805KC (TO220) (Texas Instruments).
T
J(max)
150 ° C
T
A(
max)
50 ° C
V
in(max)
10.0 VDC
I
out(max)
200 mA
JA
C/W22°
The power dissipated by the regulator is
PV VI
D in out out(max)
()
(max)
(12-6)
or
P
D
1.0W
The thermal model is that of Figure 12.4 , and the thermal equation is Equation 12-2
rearranged to
TTP
T
T
AJDJA
A
A
C W deg C/W
(max) (max)
(max)
(m
(. )( )


150 1 0 22°
aax)
128°C
(12-7)
So the A7805KC will operate within its maximum junction temperature ratings for this
application.
12.6.3 Determine the Junction Temperature of a Rectifi er with a Known Lead
Temperature
This is useful in verifying whether a diode s junction temperature is within its safe
operating temperature.

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