IGBT Modules - Technologies, Driver and Application (Second Edition) - page 275

Neither Active Clamping nor Conditional Active Clamping is suitable for limiting
over-voltages in ordinary switching operation. Measures such as adjusting the stray
inductance in the commutation path or reducing the current slope during turn-off are
required to keep over-voltages in normal switching operations under control. Constant,
long-term use of Active Clamping would overload the components, which are designed
only for transient, i.e. short-term, non-repetitive processes. It would also significantly
increase the switching losses of the IGBT. Dynamic voltage rise control (DVRC)
IGBTs with a fieldstop layer (chapter
tend not to form a tail current when turning
off at high DC-bus voltages or general high switching over-voltages. The effect is
particularly pronounced at low temperatures. This is because of the structure of the
IGBTs. At the time it is turned off, the electrical field inside the IGBT expands and all the
charge carriers within the area are extracted. The remaining charge carriers in the area
not affected by the electrical field contribute to the tail current. If the electrical field is
sufficient to reach the fieldstop layer of the IGBT at a high collector-emitter voltage, no
charge carriers are left and the current decreases quickly, without a tail, so that
oscillations occur. This behaviour is particularly strongly developed in first-generation
high current Trench-FS IGBTs
, as the stray inductance in high current applications
cannot usually be reduced to the extent as the rated currents and
increase. This
leads to high switching over-voltages, which eventually cause these oscillations at turn-
off, as shown i
Fig. 6.57
Oscillations at turn-off of a first-generation2.4kA/1.7kVTrench-FS IGBT
One way to suppress the oscillations in this operating scenario is to use a large gate
resistor to reduce the resulting
andwith it the switching over-voltages. However, this
Later Trench-FS IGBTs are optimised according to their intended application, e.g. low, medium or high power.
For example, the high current Trench-FS IGBTs of the second generation are designed to bemuch softer in their
turn-off behaviour, preventing oscillations under the same conditions as for first-generation IGBTs.
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