Ph.D. Thesis Modular Multiport Power Converter Topologies for
Electric Vehicle Charging Stations
Graduate School of Yeungnam University
Department of Electrical Engineering
Major in Control and Electric Machinery Power Conversion
by Ngoc Dat Dao
Advisor: Professor Dong-Choon Lee
August 2021
Abstract The main objective of this thesis is to develop converter topologies with high efficiency
and low cost for fast charging stations. Based on the investigation of different power
conversion structures for fast charging stations, new topologies have been derived, which
can offer additional benefits to EV fast charging stations.
Firstly, a novel isolated three-port DC/DC converter is proposed, which is based on a
series resonant converter (SRC) and a dual active bridge (DAB) converter for electricvehicle
(EV) charging stations with fast and slow charging functions. With this three-port
structure, the proposed converter has fewer components, which results in lower system cost
and volume compared with separate charger systems. A simple control method using phase
shift and frequency modulations was developed to control the output power of the fast and
slow charging ports simultaneously. An optimal phase shift angle was also derived to
minimize the transformer current for when only the DAB converter is operated for slow
charging. To verify the converter operation, a 5-kW SiC-based prototype with a power
density of 2.74-kW/dm3 was built and tested with an input voltage of 600-V. A high
efficiency performance over a wide output voltage range has been achieved, and the peak
efficiency is 98.2% at the rated conditions
Secondly, a half-bridge bidirectional isolated matrix-based AC/DC converter is
proposed for compact AC/DC power stages in fast charging stations. The converter can
control not only the DC voltage or current, but also the power factor of the AC current with
a single conversion stage, which helps to achieve a higher power density with a lower
complexity. The converter operates with zero-voltage switching (ZVS) or zero-current
switching (ZCS) in all switches. Hence, the switching frequency of the converter can be
increased higher, leading to smaller passive components. Besides the simple circuit, the
modulation scheme derived from time domain analyses is also easy to implement. The
proposed topology has been verified by experimental results for a 2-kW SiC-based
prototype. A high efficiency of 96.8 % was achieved at a full load condition. The current
THD is lower than 4 % and the power density is 1.8-kW/dm3. Furthermore, an experiment
for two modules has been carried out to demonstrate the feasible of the proposed converter
for modular SST-based charging stations.
Finally, a novel semi-modular three-phase AC/DC structure is proposed for SST-based
fast charging stations. The proposed system employs single-stage indirect matrix-based
AC/DC converters. A single full-bridge rectifier is used for each phase of the medium
voltage (MV) grid so that each AC/DC module does not need a voltage rectifier. As a result,
the number of semiconductor devices in the proposed system is reduced significantly
compared with other existing SST-based systems. A decentralized control scheme without
high-speed communication has been developed to regulate the input and output currents
while keeping the voltages of each module balance. A design of a 360-kW system has been
carried out to evaluate the efficiency of the proposed system, which is 97.7 % at rated
power. With delta connection in three-phase system, a third harmonic current can be
injected to the phase currents to increase the output current and power by 15 %. Simulation
results for a 360-kW 3.3-kV system are provided to verify the performance of the proposed
system. Finally, a 3-kW hardware prototype has been built and tested to demonstrate the
feasibility of the proposed system.
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