Application Note
44 of 53
V 1.0
2019-04-01
IM393 Application note
IM393 IPM Technical Description
Thermal design
increasing the number of fins does not necessarily reduce the thermal resistance as discussed in section
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.
•
Heat transfer coefficient (aerodynamics):
This coefficient is strongly depending on the air flow velocity as
shown in Figure 36. If there is no externally induced flow, one speaks of natural convection, otherwise it is
called forced convection. Heat sinks with very small fin spacing are not good for air flow. If a fan is used, the
fin gap may be smaller than for natural convection, as the fan forces the air through the space between the
fins.
Figure 36
Thermal resistance as a function of the air flow velocity
Furthermore, in the case of natural convection, the heat sink efficiency depends on the temperature difference
of heat sink and ambient (i.e. on the dissipated power). Some manufacturers, like Aavid thermalloy, provide a
correction table which allows users to calculate thermal resistance depending on the temperature difference.
Figure 37 shows the heat sink efficiency degradation for natural convection, as provided in the previous
equation [6]. Please note that the thermal resistance is 25 % higher at 30 W than at 75 W.
Figure 37
Correction factors for temperature
0.5
1
1.5
2
0
1
2
3
4
5
Air flow velocity [m/s]
T
h
e
rm
a
l
re
s
is
ta
n
c
e
0.95
1.00
1.05
1.10
1.15
1.20
1.25
1.30
20
30
40
50
60
70
80
Temperature difference heat sink to ambient [K]
C
o
rr
e
c
ti
o
n
f
a
c
to
r
