NCV8508
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19
Table 3. SOIC 8−Lead EP Thermal RC Network Models
54 mm
2
717 mm
2
54 mm
2
717 mm
2
Cu Area
Cauer Network Foster Network
C’s C’s Units Tau Tau units
1 2.28E−06 2.28E−06 W−s/C 2.99E−07 2.99E−07 sec
2 1.08E−05 1.08E−05 W−s/C 4.40E−06 4.40E−06 sec
3 3.24E−05 3.24E−05 W−s/C 4.36E−05 4.36E−05 sec
4 9.71E−05 9.71E−05 W−s/C 3.59E−04 3.74E−04 sec
5 6.28E−04 7.55E−04 W−s/C 3.17E−03 4.59E−03 sec
6 7.13E−03 1.49E−02 W−s/C 0.030 0.162 sec
7 1.54E−02 9.28E−02 W−s/C 0.341 0.473 sec
8 6.16E−02 1.72E−01 W−s/C 2.909 1.653 sec
9 1.94E−01 3.83E−01 W−s/C 16.126 8.488 sec
10 1.52E+00 2.41E+00 W−s/C 54.334 71.562 sec
R’s R’s R’s R’s
1 0.161 0.161 C/W 0.11 0.11 C/W
2 0.482 0.482 C/W 0.26 0.26 C/W
3 1.445 1.445 C/W 0.73 0.73 C/W
4 3.00 3.00 C/W 2.60 2.83 C/W
5 4.47 5.34 C/W 4.80 5.82 C/W
6 5.92 12.21 C/W 2.98 8.95 C/W
7 20.11 16.03 C/W 12.20 0.61 C/W
8 51.85 4.89 C/W 26.10 12.91 C/W
9 68.87 15.34 C/W 62.22 16.96 C/W
10 27.52 22.36 C/W 71.83 32.09 C/W
NOTE: Bold face items in the Cauer network above, represent the package without the external thermal system. The Bold face items in
the Foster network are computed by the square root of time constant R(t) = 225 * sqrt(time(sec)). The constant is derived based
on the active area of the device with silicon and epoxy at the interface of the heat generation.
The Cauer networks generally have physical significance and may be divided between nodes to separate thermal behavior
due to one portion of the network from another. The Foster networks, though when sorted by time constant (as above) bear
a rough correlation with the Cauer networks, are really only convenient mathematical models. Cauer networks can be easily
implemented using circuit simulating tools, whereas Foster networks may be more easily implemented using mathematical
tools (for instance, in a spreadsheet program), according to the following formula:
R(t) +
n
S
i + 1
R
i
ǒ
1−e
−tń tau
i
Ǔ
