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:
ΣRθ
Where:
PD = the power dissipated by the semiconductor device in watts.
ΔT
= the temperature difference of driving potential which causes the flow of heat.
ΣRθ
= the sum of the thermal resistances of the heat flow path across which
ΔT
exists.
The above relationship may be stated in the following forms:
TC–TA
TS–TA
TJ–TA
PD =
PD =
PD =
R
θCS
+ R
θSA
R
θSA
R
θJC
+ R
θCS
+ R
θSA
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
θJC
= thermal resistance from junction to case of the semiconductor device in
°C
per watt
(usually stated by manufacturer of semiconductor device).
R
θCS
= thermal resistance through the interface between the semiconductor device
and the surface on which it is mounted in
°C
per watt.
R
θSA
= 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
θSA
),
since the heat dissipation, maximum junction and/or case temperature, and ambient temperature are known or set.
The common practice is to represent the system with
a network of resistances in series as shown in Figure 2.
Figure 1 indicates the location of the various heat
flow paths, temperatures and thermal resistances.
FIGURE 1
Mounting surface
(cooler/dissipator)
Interface
Junction
(heat source)
T
A
Atmosphere
or ambient
FIGURE 2
TJ
P
Semiconductor case
D
P
D
TC
TS
TA
R
θJC
R
θCS
R
θSA
R
θCS
R
θSA
Heat flow path mounting
surface to ambient, equation (3)
R
T
θJC
T
T
C
S
Heat flow path case
to ambient equation (2)
Heat flow path junction
to ambient, equation (1)
T
A
J
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9
HOW TO SELECT A HEAT SINK
PD =
Δ
T
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