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How To Select a Heat Sink
How to select a heat sink
The basic equation for heat transfer or power dissipation may be stated as follows:
PD =
Δ T
Σ R θ
Where:
PD = the power dissipated by the semiconductor device in watts.
Δ T = the temperature difference of driving potential which causes the flow of heat.
Σ R θ = the sum of the thermal resistances of the heat flow path across which Δ T exists.
The above relationship may be stated in the following forms:
PD =
TJ–TA
R θ JC + R θ CS + R θ SA
PD =
TC–TA
R θ CS + R θ SA
PD =
TS–TA
R θ SA
Where:
TJ = the junction temperature in ° C (maximum is usually stated by the manufacturer of the semiconductor device).
TC = case temperature of the semiconductor device in ° C.
TS = temperature of the heat sink mounting surface in thermal contact with the semiconductor device in ° C.
TA = ambient air temperature in ° C.
R θ JC = thermal resistance from junction to case of the semiconductor device in ° C per watt
(usually stated by manufacturer of semiconductor device).
R θ CS = thermal resistance through the interface between the semiconductor device
and the surface on which it is mounted in ° C per watt.
R θ SA = thermal resistance from mounting surface to ambient or thermal resistance of heat sink in ° C per watt.
The above equations are generally used to determine the required thermal resistance of the heat sink (R θ SA),
since the heat dissipation, maximum junction and/or case temperature, and ambient temperature are known or set.
Figure 1 indicates the location of the various heat
flow paths, temperatures and thermal resistances.
FIGURE 1
Mounting surface
(cooler/dissipator)
Interface
Atmosphere
or ambient
The common practice is to represent the system with
a network of resistances in series as shown in Figure 2.
FIGURE 2
Junction
(heat source)
T
A
TJ
TC
TS
TA
P
D
P
D
Semiconductor case
R θ JC
R θ CS
R θ SA
R
θ CS
R
θ SA
Heat flow path mounting
surface to ambient, equation (3)
R
θ JC
T
J
T
T
C
S
Heat flow path case
to ambient equation (2)
Heat flow path junction
to ambient, equation (1)
T
A
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