) e1( r
The calculation is the same for amodulewithout a baseplate.
Where the losses are known, which can be determined, for example, using the formulae
andmeasured temperature T
, the junction temperature for each particular
application can bedetermined together with the specifications for the datasheet.
)t(T )t( Z)t(P)t(T
as the temperature-dependent parameter, as it can be assumed
that the temperature T
will not be constant in the actual application. T
influenced by the connection between the heatsink (TIM) and the cooling system itself.
Accordingly, metrological dimensioning that takes the entire system into account and is
performed under conditions that are as realistic as possible is the method that delivers
themost precise results. To prevent orminimise the amount of measuring needing to be
carried out, there is an option to combine the separate model of the IGBT module
(based on the datasheet values) with the separate model of the cooling system (based
on datasheet values or separate measurements). However, the result depends very
much on the type of cooling system and can only ever represent an approximation. The
greater the time constant of the cooling system, the less influence it has on the results
for the IGBT model. Likewise, a cooling system with a small time constant, such as a
water cooler, will exert a significant influence on the IGBTmodel. By contrast, air-cooled
systemswithout forced coolinghave long time constants.
The resistance R
(baseplate to heatsink) is calculated by the manufacturers
so that it can be included in the datasheets. The layer thickness d,
of the thermal compound or foil applied between the baseplate and the heatsink is set at
levels of 50µm to 100µm. The value
is often used for the specific conductivity of
the thermal compound or foil. The value of R
determined in this way and stated in
the datasheet is therefore merely a reference value because users almost always use
other parameter valueswhen they actually use the application.