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