Driverswith integratedUVLOdetection andan associated fault signal that unequivocally
notifies the operating state of the output stage of the driver includeAvago Technologies’
HCPL-316J and InfineonTechnologies’ 1ED020I12-F.
6.10 Parallel and series connection
The motivation to connect IGBTs in parallel or in series and the application
requirements and driver solutions are discussed in chapter
The focus of this chapter
is topresent fundamental considerations only.
In parallel connection of IGBTs, the focus is on balancing the current distribution in the
static and dynamic states. The selection of suitable IGBTs, mechanical interconnection
and the driver stage are all significant to achieve evendistributionof the current.
Aspects relevant to the designof thedriver stage are:
Use of a joint driver coreand connection of several IGBTs to this driver stage.
Use of a separate driver core for each IGBT.
Decoupling of signals of each IGBT.
Balancing thegate impedances.
The deciding factor for the first two points, i.e. whether to have a joint driver for all the
IGBTs or individual drivers (one for each IGBT), is the transient response of the driver
stage or stages. The key parameter of the driver in this connection is the difference or
the tolerance in the transition time of the control signal from the input to the output of the
driver. This is known as propagation delay mismatch. The greater the deviation of the
transition time of the driver, themore difficult it will be to use individual drivers for each
IGBT. In this case, one driver stage must be used for all the IGBTs as this is the only
way to achieve an identical transition time delay for all gate signals. This is equally
necessary if the deviation of the gate switching voltages between the relevant driver
stages is too high. This is usually the casewhen the tolerances of the power supplies of
the stages are too high. Different gate voltages create different delay times in the control
of the IGBTs, which negatively affect the dynamic current distribution. When there are
differences in the positive gate voltage, there will also be repercussions in the static
current distribution of the IGBTs connected inparallel.
Viewed purely objectively, there are obvious cost benefits, and hence the best solution,
to using one shared driver. But this is not necessarily the case. Viewed from the point of
the system as a whole, sharing a driver can be detrimental to the current sharing of the
IGBTs and make the systemmore expensive overall. Therefore, in many cases, it can
be more advantageous to use separate drivers, as this makes each potential
combination of driver and IGBT an identical unit. In terms of design, development and
maintenance, thismaymean savings because of the unified design. The inductances of
the AC outputs do not play a role, so that themechanical design has some advantages.
In addition, there are no oscillations between the gates, to mention just a few aspects.
On the other hand, with only one driver stage for several IGBTs, specific adjustments
may be required depending on howmany IGBTs there are. These adjustments entail,
for example, the need to decouple the signals and balance the gate impedances. The
latter are determined essentially by differences in cable lengths and connection paths
(and therefore different lead inductances) from the driver to the IGBT. It is essential to