NCV7356
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18
Table 1. SOIC−8 Thermal RC Network Models*
53 mm
2
719 mm
2
Copper Area 53 mm2 719 mm
2
Copper Area
Cauer Network Foster Network
C’s C’s Units Tau Tau Units
5.86E−06 5.86E−06 W−s/C 1.00E−06 1.00E−06 sec
2.29E−05 2.29E−05 W−s/C 1.00E−05 1.00E−05 sec
6.98E−05 6.97E−05 W−s/C 1.00E−04 1.00E−04 sec
3.68E−04 3.68E−04 W−s/C 1.99E−04 1.99E−04 sec
3.75E−04 3.74E−04 W−s/C 1.00E−03 1.00E−03 sec
1.57E−03 1.56E−03 W−s/C 1.64E−02 1.64E−02 sec
2.05E−02 2.24E−02 W−s/C 5.60E−01 5.60E−01 sec
9.13E−02 7.35E−02 W−s/C 4.50E+00 4.50E+00 sec
2.64E−01 1.22E+00 W−s/C 7.61E+01 7.61E+01 sec
1.66E+01 9.74E+00 W−s/C 3.00E+01 3.00E+01 sec
R’s R’s R’s R’s
0.22 0.22 C/W 1.30E−01 1.30E−01 C/W
0.50 0.50 C/W 2.82E−01 2.82E−01 C/W
1.30 1.30 C/W 8.91E−01 8.91E−01 C/W
1.80 1.79 C/W 0.17 0.18 C/W
0.95 0.96 C/W 1.88 1.88 C/W
7.43 7.37 C/W 7.15 7.24 C/W
31.19 31.59 C/W 19.80 16.27 C/W
59.97 47.70 C/W 30.1 54.7 C/W
75.79 28.63 C/W 14.1 23.3 C/W
4.41 6.15 C/W 109.0 21.3 C/W
*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) = 130 * 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.
Both Foster and 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
−tńtau
i
Ǔ
