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11-18 M68040 USER’S MANUAL MOTOROLA
11.9.2 Forced Air
In this study, a small sample of MC68040 packages was tested in forced-air cooling in a
wind tunnel with no heat sink. The test was performed with approximately 6 W of power
being dissipated from within the package. As previously mentioned, since the variance in
θJA within the possible power range is negligible, it can be assumed for calculation
purposes that θJA is constant at all power levels. Using the previous equations, Table 11-3
lists the results of the maximum power dissipation at maximum θJC with airflow and no
heat sink for the MC68040, and Table 11-4 lists the results for the MC68LC040 and
MC68EC040. Refer to Table 11-1 for calculating other power dissipation values.
Table 11-3. Thermal Parameters with Forced Airflow
and No Heat Sink for the MC68040
Thermal Mgmt.
Technique Defined Parameters Measured Calculated
MHz Airflow Velocity PDTJθJC θJA θCA TCTA
25
25
25
33
33
33
100 LFM
6.3 W
6.6 W
8.6 W
7.7 W
8.0 W
10.0 W
110 °C3 °C/W 12.7 °C/W 9.7 °C/W
91.1 °C
90.2 °C
84.9 °C
86.9 °C
86.0 °C
80.0 °C
29.90 °C
26.18 °C
00.76 °C
12.21 °C
08.40 °C
00.00°C
25
25
25
33
33
33
250 LFM
6.3 W
6.6 W
8.6 W
7.7 W
8.0 W
10.0 W
110 °C3 °C/W 11.0 °C/W 8.0 °C/W
91.1 °C
90.2 °C
84.2 °C
86.9 °C
86.0 °C
80.0 °C
40.70 °C
37.40 °C
15.40 °C
25.30 °C
22.00 °C
00.00 °C
25
25
25
33
33
33
500 LFM
6.3 W
6.6 W
8.6 W
7.7 W
8.0 W
10.0 W
110 °C3 °C/W 9.9 °C/W 6.9 °C/W
91.1 °C
90.2 °C
84.2 °C
86.9 °C
86.0 °C
80.0 °C
47.63 °C
44.66 °C
24.86 °C
33.77 °C
30.80 °C
11.00 °C
25
25
25
33
33
33
750 LFM
6.3 W
6.6 W
8.6 W
7.7 W
8.0 W
10.0 W
110 °C3 °C/W 9.5 °C/W 6.5 °C/W
91.1 °C
90.2 °C
84.2 °C
86.9 °C
86.0 °C
80.0 °C
50.15 °C
47.30 °C
28.30 °C
36.85 °C
34.00 °C
15.00 °C
25
25
25
33
33
33
1000 LFM
6.3 W
6.6 W
8.6 W
7.7 W
8.0 W
10.0 W
110 °C3 °C/W 9.3 °C/W 6.3 °C/W
91.1 °C
90.2 °C
84.2 °C
86.9 °C
80.0 °C
81.8 °C
51.41 °C
48.62 °C
30.02 °C
38.39 °C
17.00 °C
22.58 °C
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