IGBT Modules - Technologies, Driver and Application (Second Edition) - page 193

181
Then, the current flow is reduced to the value of the measuring current I
ref
and the
saturation or forward voltage is measured. The current junction temperature can be
deduced using the previously prepared calibration curve. The temperature T
C
of the
baseplate under the power semiconductor being examined
) is determined
usingSensor 2. In thisway, the thermal impedance of thedesign can beobtained:
P
)t(T)t(T
)t( Z
C
j
jc,th
=
Eq. 4.18
Sensor 1 is used for amodulewithout a baseplate. It is located 1 to 2mm below the TIM
layer within the heatsink, and displays the temperature T
h
of the heatsink. In this case, it
is not the impedanceZ
th,jc
, but Z
th,jh
that ismeasured:
P
)t(T)t(T
)t( Z
h
j
jh,th
=
Eq. 4.19
The reason for the different referencing of the two impedances – for a module with a
baseplate and one without – is that Sensor 2 influences the transfer of heat from the
module to the heatsink. In amodule with a baseplate, an induced error of <5% can be
assumed to result from thermal lateral spread in the baseplate. In contrast, this error
increases significantly in a module without a baseplate, in which there is no thermal
lateral spread from the baseplate and a smaller active junction area available to
transport the heat. Therefore, for modules of this type, the impedances always relate to
the reference temperature T
h
rather than T
C
. The R
th
value is obtained from the above
formulae, by setting the time t higher 5
τ
and then deducting the value of the TIM, theR
th
of which is assumed tobe already known or has already been set.
Fig. 4.21
Examples of datasheet specifications for Z
th
for modules with and without a
baseplate
As shown in
datasheets usually use the Foster model, for which the
parameters r and
τ
are stated for each RC combination. The total Z
thj,jc
curve can thus
be calculated analytically by putting nRC combinations together:
1...,183,184,185,186,187,188,189,190,191,192 194,195,196,197,198,199,200,201,202,203,...548