9
HOW TO SELECT A HEAT SINK
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The basic equation for heat transfer or power dissipation may be stated as follows:
Where:
P
D =
the power dissipated by the semiconductor device in watts.
ΔT = the temperature difference of driving potential which causes the flow of heat.
ΣR
θ
= 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:
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.
How To Select a Heat Sink
Where:
T
J
= the junction temperature in °C (maximum is usually stated by the manufacturer of the semiconductor device).
T
C
= case temperature of the semiconductor device in °C.
T
S
= temperature of the heat sink mounting surface in thermal contact with the semiconductor device in °C.
T
A
= 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.
Figure 1 indicates the location of the various heat
flow paths, temperatures and thermal resistances.
The common practice is to represent the system with
a network of resistances in series as shown in Figure 2.