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Figure 2 shows STD cards as heat sources
in an enclosed .digital system.
The
heat generated by a card is the sum of numerous heat sources:
the
heat generated by ICs plus any singular components.
Most heat is generated in
the different junctions within the IC.
An average thermal resistance between
any junction and the case is assumed.
Heat flow from an IC with a junction
te~perature
T J encounters the junction-to-case thermal resistance ReJC and
raises the. case temperature to TC'
From the IC case, the heat flows through
ReCAI to the ambient air in the enclosure, and finally through ReA to give TA
the ambient temperature·outside the enclosure
-
-
JUNCTION
enclosure
HEAT
SOURCES
Tc..
ReJC
IC CASE
ReCAI
I
l
r
I
I
I
(
AMB I ENT IN ENCLOSURE
l
f
ReA
.J
-
-
-
--
TA
AMBIENT OUTSIDE ENCLOSURE
T
J
- IC junction temperature
TC - IC case temperature
TAl';"
Ambient temperature in enclosure
TA - Ambient temperature outside enclosure
ReJC - Thermal resistance, junction-to-case
R
eCAI
- Thermal resistance, junction-to-ambient in
enclosu~e
ReA -
Thermal resistance, ambient in enclosure to ambient outside
enclosure
FIGURE 2 - HEAT FLOW IN ENCLOS£D DIGITAL SYSTEM
When many heat sources are involved, the thermal circuit becomes quite complex.
The use of analysis with assumptions, approximations, and experimental
techniques is necessary to understand the problems and find practical
solutions.
17
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