This dissertation studies an AC-link-based drivetrain concept for battery-electric
vehicles. At the heart of the drivetrain lies a Three-phase modular multilevel in-
verter topology that employs a novel submodule voltage balancing scheme. Pre-
vious multilevel inverter topologies required the usage of redundant switches and
large capacitors to balance their submodule voltages, resulting in lower power den-
sity and complicated topologies and control. In the proposed topology, this problem
is solved with a balancing magnetic field, interfaced through a multi-active bridge.
This topology allows to remove all redundant switching states and thus simplifies
the modulation while increasing the power density.
A special focus was laid on developing methods that are easy to upscale to larger
numbers of modules. To investigate the detailed behaviour of the drivetrain, a sys-
tem model of the topology was created and simplifications were subsequently de-
rived, including linearization, averaging and model order reduction.
Several methods to reduce the losses of the device were presented throughout the
project. The switching losses of the multi-active bridge were minimized by the de-
sign of a specialized gate driving circuit, employing a combination of soft-switching
and adiabatic switching. Guidelines for the design and operation of the gate driver
circuit were subsequently discussed.
To verify the findings, a Three-phase prototype was constructed within the project
based on a planar transformer with custom-made ferrites. To reduce the power
density and increase the efficiency of the prototype different techniques including
interleaving techniques, generalized from two-winding transformers, input signal
coding, and a 3D assembly tailored to the prototype were used.