Abstract:
Inductive power transfer (IPT) is gaining popularity in Electric Vehicle (EV) charging
applications. It allows EVs to be wirelessly charged by parking over a designated
ground-based charging pad. In recent years there has been an increasing demand
for developing higher power IPT systems to reduce the refuelling time of EVs.
However, traditional full-bridge based IPT systems have significant drawbacks at
high-power levels. Therefore, as an alternative solution, this thesis investigates and
proposes new multilevel converters for IPT systems.
In comparison to two-level converters, multilevel converters have the advantages
of lower device stresses, additional degrees of control, modular realisation,
and higher quality output waveforms generation. However, the use of multilevel
converters in IPT applications is relatively new, and most of the existing designs
utilise Cascaded H-Bridge Converter (CHBC) topologies, which require many independent
DC sources and are not desirable due to high cost and system complexity.
Therefore, this thesis investigates the use of different multilevel topologies for the
primary converter of an IPT system and then proposes new multilevel converter
topologies with improvements. All of the experimental prototypes in this thesis
comply with the stringent requirements specified in the SAE J2954 WPT2/Z2 wireless
EV charging standard and the results can provide a benchmark for future work
in this area.
The first main contribution of this thesis is the introduction of a new multilevel
converter topology termed Integrated Boost Multilevel Converter (IBMC). An
IBMC consists of series-connected half-bridge submodules (SMs) in each arm, and
a single DC supply is connected to each arm through a DC inductor. This circuit
arrangement allows an IBMC to generate a boosted multilevel output voltage and
to regulate the power flow efficiently over a wide load/coupling range, making it a
suitable converter topology for high-power IPT systems.
This thesis also proposes a new digitised modulation scheme to further improve
the IBMC, whereby it can generate a boosted square-wave shaped AC voltage with
a controllable amplitude, enabling it to transfer power efficiently over a wide load
and coupling range while achieving Zero Voltage Switching (ZVS) for all switches.
Because of the ZVS operation, low-cost Si (Silicon) switches can be used for high-
power IPT systems, providing a wide range of options for selecting the semiconductor
switches. An SAE J2954 WPT2/Z2 compliant wireless EV charger prototype
employing 200V Si MOSFETs has been built and tested. This system maintains an
efficiency between 90% to 93% when transferring 7.7 kW under the loading and the
coupling range specified in WPT2/Z2 standard.
The final contribution of this thesis is a proposal for a new high-power IPT system,
based on a Modular Multilayer Converter (MMC), which takes advantage of
the digitised modulation scheme to realise ZVS operation, and integrates the arm
inductors with the compensation network inductors to reduce the component count.
The high-frequency operations of the proposed MMC differ significantly from traditional
MMCs which are designed to operate at low-frequency. To validate the
proposed MMC based IPT system, an SAE J2954 WPT2/Z2 compliant prototype
has been built and tested, and it achieves up to 93% efficiency over the wide range
of test conditions specified in WPT2/Z2 standard.
To conclude, the benefits of the proposed multilevel converter based IPT systems
are highlighted by comparing them to the state-of-the-art designs, and further
improvements are proposed.