IGBT Modules - Technologies, Driver and Application (Second Edition) - page 358

The challenge when connecting IGBTmodules in series is to ensure that the voltage is
properly distributed, both when the device is being turned on and off and when it is
operating in blockingmode. This is the opposite goal to that in parallel connection, when
balancing the current distribution is thepriority.
To illustrate this, the imbalance in dynamic and static operation of three IGBTs
connected in series is presented in
Due to differences in turn-off switching
speeds (arising from the driver stage and/or variations in the component characteristics)
the fastest IGBT takes the highest voltage, while the slowest IGBT takes it at the time
the device turns on. Themain reasons for the step up in voltage at the time the device
turns off are the voltage distribution of the series connection and the stray inductance of
the commutation path. During turn-on, the voltage overshoot also occurs because of the
voltage distribution across the IGBTs. The variation in voltage mismatching for static
operation results from variations in the leakage currents I
as a function of the applied
blocking voltageand the temperature.
The various approaches to improving the switching behaviour and the dynamic and
static voltagedistribution aredescribedbelow.
Balancing resistors
). Balanced voltage matching in static
operation can be achieved by using resistors parallel to the IGBTs, as long as
the shunt current in the resistors is greater than the leakage current of the
IGBTs. A ratio of 3:1 to10:1of I
to I
can be taken as aguideline.
. The dynamic voltage distribution can be improved by
using snubbers parallel to each IGBT. The snubber can comprise only one
capacitor or a network of capacitor, resistor and/or diode. The disadvantage of
this solution is that it requires the use of expensive, and sometimes large,
snubber capacitors and retardationof the switching.
Active Clamping
. Dynamic turn-off overvoltage can be kept to
a certain level by inserting Z-diodes between the collector and the gate of the
IGBTs as soon as the break-over voltage of the Z-diodes is reached. Usually,
several Z-diodes are connected in series in order to achieve an avalanche
voltage of the necessary magnitude. An additional diode in series separates
the gate voltage and the collector voltage. The disadvantage is that the
avalanche voltages of the Z-diodes have a high level of tolerance and
temperature dependence. Furthermore, permanent use of Active Clamping
during all switching causes increased losses in both the clamping diodes and
in the IGBTs, which have to be rated specifically for this scenario.
Correcting the switching times
). During starting up (increasing of
the DC-bus voltage), the status of the balancing is determined by sensors and
control circuits installed for this purpose and a transition time correction is
added to the control of each IGBT. This makes it possible to switch on and off
synchronously. The disadvantages of this implementation are the expense and
the need for sensor and control technology.
). This involves determining the collector-emitter
voltage gradient
when it turns on and off and comparing it to a reference
value. If there are deviations from the desired value, modifications aremade to
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