IGBT Modules - Technologies, Driver and Application (Second Edition) - page 361

9 RFoscillations
When switching IGBT modules at certain operating points it can come to a high
frequency oscillation in the gate voltage, the collector-emitter voltage and/or the
collector current. The reasons and possible countermeasures regarding these
oscillations are consideredbelow.
9.1 Introduction
Basically an IGBT can be considered as an inverting amplifier
). while
operating in its active region. If feedback from the output signal returns to the input
without a phase shift the amplification of the system is reduced (negative feedback). If
the feedback is phase shifted, however, the amplification increases (positive feedback)
and thismay finally lead to oscillations. Feedback paths develop inside the IGBT due to
parasitic capacitances such as the Miller capacitance
, as well as the stray
inductances caused by the construction of the module. This creates an LC-oscillator
comprised of a three-way capacitive coupling which is also called a Colpitts
. AColpitts oscillator based on a grounded emitter circuit is also referred to as
Fig. 9.1
IGBTas anoscillatory system
As a consequence of these oscillations, disturbances in other electronic circuits may
arise. This includes its own gate drive andmay possibly lead to incorrect control of the
IGBT. Further, these oscillations could cause unbalanced current distribution of
semiconductors connected in parallel, which in turn may lead to the overload of one or
more semiconductors and thus to the failure of the entire module. In high power
modules, for example, many individual semiconductors are connected in parallel in
order to achieve the required current rating. The higher the nominal current of the IGBT
module the more chips will work in parallel connection. In a 3.6kA module up to 24
IGBTs and diodes areoperated inparallel.
Named after theCanadian physicist EdwinHenryColpitts (1872 –1949).
Named after theAmerican physicist GeorgeWashingtonPierce (1872 – 1956).
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