temperature variations can be calculated directly when this information is fed into the
thermal layermodel and comparedwith existing static temperature values. The basis for
calculating the temperature variations is, however, knowledge of themechanical design
of the power electronic switch and selection of a suitable thermal model to describe this
design. The latter can bedepictedas a continued fraction or partial fractionmodel:
Continued fraction model or Cauer mode
is a direct representation of the
actual physical layers and materials in an IGBT module. Use of this model
requires the precise material parameters, especially the parameter for thermal
lateral spread in the relevant layers. The number of RC combinations used
depends on the degreeof resolution that themodel is expected toprovide.
Partial fractionmodel or Fostermodel
has no relation to the actual physical
layers and materials but is well suited to metrological determination of the
thermal resistances and impedances. It is not necessary to know the precise
material parameters in order to use the partial fraction model. The number of
RC combinations depends on the number of measuring points and usually falls
between 3 and 6.
To determine the datasheet valuesR
and other thermal resistancesand
impedances by measuring, the actual junction temperature of the semiconductor must
be known. This is determined using a calibration curve and measuring the U
voltage. The calibration curve is preparedaccording to the following pattern.
Named after Germanmathematicianand physicistWilhelmCauer (1900 - 1945).
Named after UnitedStatesmathematicianRonaldMartinFoster (1896 - 1998).