© by SEMIKRON / 2017-09-07 / Technical Explanation / SKiiP
4
Page 51/73
The data sheet values for the thermal resistance are based on measured values. The point of the
temperature measurement has a major influence on the thermal resistance because of a temperature
profile between the different chip positions and across the heatsink surface.
The reference points for SKiiP
®
4 systems are: virtual junction temperature of the hottest chip (T
j
); heat
sink temperature underneath the hottest chip (T
sink
) and DCB-sensor temperature (T
sensor
). A principle
sketch with the positions is shown in Figure 7.1. The values for R
th(j-s)
, R
th(s-a)
and R
th(j-r)
are calculated from
these temperatures. SKiiP modules have no base plate, therefore the case temperature T
C
can not be
measured without disturbance of the thermal system and the thermal resistance R
th(j-c)
cannot be given.
T
Sink
is measured in a drill hole 2mm underneath the heatsink surface. The 2 mm distance guaranties a low
disturbance of the thermal path and a minimum effect of heat sink parameters like size, thermal
conductivity, cooling medium etc.
The temperature sensor is located between IGBT and diode chips on the same DBC copper layer at high
voltage potential. This ensures an excellent thermal coupling between both power semiconductors and
sensor and furthermore a short reaction time on changes in power dissipation.
Only one of the temperature sensors is monitored by the Gate driver. The monitored sensor is in the
middle of the SKiiP (refer to Figure 7.3). The protection level is matched to the maximum operation
temperature of the power semiconductors. Since the temperature sensor is close to the IGBT and mounted
on the substrate the actual and precise IGBT temperature cannot be acquired. Consequently the protection
function derived from the substrate temperature sensor’s temperature needs evaluation under the real
application conditions such as peak current, load profile etc
During operation there will be a temperature profile along the heatsink from cool at the inlet of the coolant
to warm at the outlet.
Figure 7.3: Sensor position in SKiiP
4 GB (6fold, 4fold, 3fold) with proposed cooling (water
inlet) direction
SKiiP
®
4 are equipped with high performance heat sinks. The data sheets contain transient thermal data
referenced to the built-in temperature sensor. This allows the calculation of junction temperature T
j
, if the
generated losses are known. The thermal resistances given in the data sheets represent worst case values.
Evaluation of thermal impedance:
• Junction to sensor
Z
th(j-r)
=Ʃ R
th(j-r)n
* (1-e
-t/
Ƭ
n
), n=1,2,3,….
• Sink to ambient
Z
th(s-a)
=Ʃ R
th(s-a)n
* (1-e
-t/
Ƭ
n
), n=1,2,3,….
Please note: The values for transient thermal impedance given in the data sheets (Z
th(j-r)
) are only
valid together with the SEMIKRON standard heat sinks and under conditions given in the data sheets.
The usage of these values for other heat sinks/conditions might cause deviations in calculation of
thermal resistance!
7.5 Cooling and coolant circuit
7.5.1 Materials used and fluid composition
The cooling circuit (heat sink) of the water-cooled SKiiP
®
4 IPM consists of two side parts (with inlet and
outlet for coolant marked on the corresponding data sheet) and the cooling channels inside the SKiiP
AC
DC
SKiiP
AC
DC
AC
DC
AC
DC
AC
DC
AC
DC
1
2
3
4
5
6
SKiiP
AC
DC
AC
DC
AC
DC
1
2
3
AC
DC
4
SKiiP
AC
DC
AC
DC
AC
DC
1
2
3
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