Page 75 - Power Electronics Handbook

68 Thermal design
dissipation will vary due to differences in clamping pressure between
the device and its heatsink, and these are difficult to predict.
(iii) Many of the constants used in the thermal equations are actually
low-order variables and choosing the right value often needs
judgement and experience.
Analysis of forced air-cooled systems gives less accurate results than
analysis of natural air-cooled equipment because:

(i) There are differences in flow over interior and exterior surfaces.
(ii) It is not possible to calculate the air velocity at each point in the flow
path.
(iii) Thermal analysis usually assumes symmetrical shapes, e.g. cylinders
and spheres, and in practice these shapes rarely occur.
Performance can be improved by mounting heatsinks vertically within an
enclosure with openings at the top and bottom, to create a chimney effect.
Several devices can also be mounted on a common heatsink, since this
would result in a higher temperature differential between heatsink and
ambient, and so improve its efficiency, although the devices now run hotter
and upstream components will be working at a higher case temperature.
The advantage of mounting devices on the same heatsink is that there is
good thermal coupling, as is needed when they are being operated in
parallel.

2.4.2 Construction of heatdnks
Heatsinks are available in a variety of shapes and sizes to accommodate the
many different package types used for power semiconductors. For
example, Figure 2.6(a) shows a heatsink for a hockey puck construction

f nductor
Heatsink

semiconductor

Figure 2.6 Examples of heatsinks: (a) air cooled; (b) liquid cooled