# IGBT Modules - Technologies, Driver and Application (Second Edition) - page 456

444
It should be ensured that, later, during operation, it will be possible to measure the
internal temperaturewithout any external measuring equipment. Oneway to do this is to
use an NTC resistor, a component that changes its resistance value depending on the
temperature. When using an NTC, the resistance value decreases as the temperature
rises, i.e. it shows a negatively temperature coefficient. Hence the name NTC.
Alternatively, a component that shows positive behaviour at rising temperature levels
(PTC resistor) can be chosen.
Many IGBT modules have an NTC resistor made of ceramic semiconductor material
that can be used as a reference for determining the temperature of the baseplate. Of
course, such NTC resistors can be attached to the heatsink at one or several points.
shows internal utilisation of an NTC resistor (the circuit shown here is just
), which functions as follows:
The timer component NE555
is configured as an astable multivibrator. The output
signal is a function of the resistors R
NTC
, R
1
and the capacitor C
1
. The output of the
component is "high" for a time t
on
defined according to:
(
)
1CR R 693 .0 t
1
NTC
on
+
=
Eq. 12.20
Accordingly, for the time t
off
, the output frequency that applies is f
out
:
1CR 693 .0 t
1
off
=
Eq. 12.21
The output frequency f
out
itself is a function of the NTC resistor which, in turn, is a
functionof the temperature:
(
)
1 1
NTC
off
on
out
CR2 R 693 .0
1
t
t
1
f
⋅ +
=
+
=
Eq. 12.22
For the sample values i
therefore, the result is the plotted function curve f
out
= (T
NTC
. Galvanic isolation can be achieved via a subsequent optocoupler and the
signal thenutilised further.
24
The following formulae were taken from the datasheet for the NE555, SA555, SE555 published by
STMicroelectronics. Different formulaemayapply for components from othermanufacturers.
25
The values of theNTC resistor are taken from the datasheet for ShibauraElectronics' KGThermistor series.
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