collector-emitter voltage U
very soon arrives at the saturation value so that the
resulting switching losses areminimal. If the gate resistor R
is increased the current
oscillations diminish, however, the switching losses rise sharply due to the reduced
When the IGBT is turned off the switching over-voltage U
across the IGBT
diminishes with increasing IGBT gate resistor R
. In this case the turn-off losses
increase relativelymoderate comparedwith the turn-on losses.
When using IGBT modules with SiC Schottky diodes (in particular with modules in the
medium power range) the gate resistor has to be tuned exactly in order to minimise
existing oscillations and to avoid an unnecessary increase of switching losses. It is also
true that the less SiC chips are connected in parallel inside the module the better the
switching behaviour will be. This makes the implementation of SiC freewheeling diodes
attractive, in particular in the lowpower range up tonominal 100 to150A.
7.11 Load reducedswitching and (quasi-) resonant switching
The variants of switching with active switches considered so far are called hard
switching topologies. As depicted i
there are significant losses P
to a high collector-emitter voltage U
and a high collector current I
concurrently when switching. Temporarily the lossesmay peak in a range of kilowatt or
megawatt, depending on the specific application.
IGBT turn-off behaviour (hard switching)
The higher the switching frequency, the higher the resulting losses. These increase
proportionatelywith the switching frequency f
E E f~ P
Therefore, hard switching topologies in applications with a high switching frequency will
have their limits, because the losses cannot be justified and the heat dissipation of the
semiconductor cannot be guaranteed. Also there might be EMI/EMC problems due to
during turn-onor turn-off time.