Application Note
45 of 53
V 1.0
2019-04-01
IM393 Application note
IM393 IPM Technical Description
Thermal design
The positioning of the heat sink also plays an important role for aerodynamics. In the case of natural
convection, the best mounting position is with vertical fins, as the heated air tends to move upwards due to
buoyancy. Furthermore, one should make sure that there are no significant obstructions impeding the air flow.
Radiation occurs as well supporting the heat transfer from heat sink to ambient. In order to increase radiation
one can use anodized heat sinks with a black surface. However, this decreases the thermal resistance of the
heat sink only by a few percent in the case of natural convection. Radiated heat is negligible in the case of
forced convection. Hence, black heat sinks can be used if no fan is used with the heat sink.
The discussion in this section clearly show there cannot be a single thermal resistance value assigned to a
certain heat sink.
7.5.3
Selecting a heat sink
Unfortunately there are no straightforward recipes for selecting heat sinks. Finding a sufficient heat sink will
include an iterative process of choosing and testing heat sinks. In order to get a first rough estimation of the
required volume of the heat sink, one can start with estimated volumetric thermal resistances, as given in Table
17 (Taken from [7]). This table provides only first evidence, as the actual resistance may vary depending on
many parameters such as actual dimensions, type and orientation.
Table 17
Electrical characteristics of internal bootstrap parameters
Flow conditions [m/s]
Volumetric resistance [cm³ °C/W]
Natural convection
500 ~ 800
1.0
150 ~ 250
2.5
80 ~ 150
5.0
50 ~ 80
One can roughly assume that the volume of a heat sink needs to be quadrupled in order to half its thermal
resistance. This gives an idea whether natural convection is sufficient for the available space, or if forced
convection is required.
In order to find an optimized heat sink for a given application, one needs to contact heat sink manufacturers or
consultants. Further tips and references can be found in [7].
When contacting heat sink manufacturers to find a suitable heat sink, please ensure that the conditions for the
specified thermal resistance values apply. These might be given either for a point source or for a heat source
which is evenly distributed over the entire base area of the heat sink. Also ensure that the fin spacing is
optimized for the corresponding flow conditions.
7.6
Online simulation tool
Infineon has developed an online simulation tool based on PLECS
™
to help designers select the proper module
that fits their system. The online tool can be found at:
https://plex.infineon.com/plexim/ipmmotor.html