IGBT Modules - Technologies, Driver and Application (Second Edition) - page 218

6 I
the internal circuitry without latching-up. When these voltages are exceeded, internal
pn-junctions will become forward biased andmay cause unpredictable switching states
or may destroy the device through uncontrolled internal currents. To circumvent this
disadvantage, level shifter in SOI (Silicon on Insulator) technology were developed.
Such SOI devices provide isolation of the individual internal circuitry by an insulating
layer (usually silicon dioxideSiO
) and not by pn-junctions operated in reverse bias. The
ruggedness against negative voltages can thus be increased to values of up to -100V.
Fig. 6.6
Decouplingof the bottom driver by addinga second level shifter
As shown i
the ground reference GND of the input circuit is identical with the
reference point GNDL of the lower IGBT driver. Accordingly, both reference points have
to be connectedwith each other. However, if GNDLwould be instead directly connected
to the emitter of the bottom IGBT, a voltage difference can occur between GND and
GNDL due to parasitic inductances and resistances always present. As long as thesedo
not lead to a latch-up, this separation is not critical. Alternatively, however, a driver can
be used, which not only has a level shifter for the upper channel, but also for the bottom
channel. This decouples GND and GNDL and especially makes sense if, as happens
often for instance in consumer applications, a shunt resistor R
is located in the
negative path of theDC-bus
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