Adjustment of internal gate resistors if applicable. The trend is that an increase
of the internal gate resistor will reduce oscillations if not completely eliminate
Adjust the gate-emitter threshold voltage U
. An increase of the threshold
voltage counteracts oscillations.
Optimise the internal layout of the module in order to minimise the feedback
loops or to bring the effect of such feedback outside of the relevant field of
application for this IGBTmodule.
shows the comparison of amodulewith optimised layout and onewhich has not
Impact the module layout has on the oscillation characteristics of an IGBT
module during a short circuit SC1
9.3 Oscillationsduring IGBT turn-off
During the turn-off of an IGBT, feedback occurs from the change of the collector-emitter
via the Miller capacitance C
onto the gate. A displacement current i
is formed according to
During the slow rise of U
in the time when the gate
is on theMiller plateau this displacement current stabilises initially and aids
to keep the plateau voltage constant. The magnitude of the displacement current is
directly dependent on the feedback capacitance C
. This in turn is dependent on the
internal oxide capacitance of the IGBT and the current-dependent junction capacitance.
The higher the current I
to be switched off by the IGBT, the greater will be C
reason for this is the constricted space-charge region internally in the IGBT which
makes up the junction capacitance. It can be reasoned, thus, that a greater collector
to be switched off will lead to a greater displacement current i
. This positive
feedbackmay lead to oscillations in semiconductors connected in parallel.
the influencing factorswhich aid theoccurrenceof oscillations.
Here too the manufacturers of IGBTs and IGBT modules usually conduct tests
regarding the oscillation characteristics during the development phase and implement
Adjustment of internal gate resistors if applicable.