IGBT Modules - Technologies, Driver and Application (Second Edition) - page 376

Fig. 10.9
Open heatsink to be fittedwith a directly cooledmodule
Thus, the associated application rules of the manufacturer need to be observed. In
many cases a seal constructionwith anO-ring is to be used in preference over onewith
a flat gasket. The reason is that the seal is compressedwhen themodule ismounted to
the heatsink. For an O-ring it suffices to achieve a certain pre-tension, which depends
on the type of O-ring, on the groove in which the O-ring is placed as well as the torque
of the mounting screws. The final sealing relevant in operation is achieved by the
pressure exerted onto the O-ring by the fluid in the cooling circuit. With flat gaskets, on
the other hand, the sealing is achieved by the tightening torque and does not increase
with the existing pressure of the cooling fluid. The result is an irregular distribution of the
pre-tension (in this case it is to be treated as the final seal tension) with its maximum
values present directly at the mounting points. At these spots the flat gasket is
compressed themost, whereas there is less force in themiddle between the bolts. This
exerts a pressure onto the module baseplate which may possibly lead to a minor
warping. The worst case could be a warping so severe that, in operation, cooling fluid
will leak ou
. As a countermeasure spacers should be used at themounting points as a
support, whichwill avoid undue compression of the gasket at these spots andwill hence
lead to an even height of the seal over the entire length. The result is thenminimum or
no deformation of thebaseplate.
With very severebending the risk is that theDCB inside themodulewill break which results in an insulation fault
and/or electrical disconnection (open circuit).
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