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School of Electrical, Computer and Energy Engineering
PhD Final Oral Defense
Soft-switching techniques for power conversion system in automotive chargers
by
Siddharth Kulasekaran
4/13/2017
11:00 am
ERC 111
Committee:
Dr. Raja Ayyanar (chair)
Dr. George Karady
Dr. Jiangchao Qin
Dr. Qin Li
Abstract
This thesis investigates different unidirectional topologies for the on-board charger in an
electric vehicle and proposes soft-switching solutions in both the AC/DC and DC/DC
stage of converter with a power rating of 3.3 kW. With an overview on different charger
topologies and their applicability with respect to the target specification a soft-switching
technique to reduce the switching losses of a single-phase boost-type PFC is proposed.
This work is followed by a modification to the popular soft-switching topology, the dual
active bridge (DAB) converter for application requiring unidirectional power flow. The
topology named as the semi-dual active bridge (S-DAB) is obtained by replacing the
fully active (four switches) bridge on the load side of a DAB by a semi-active (two
switches and two diodes) bridge. The operating principles, waveforms in different
intervals and expression for power transfer, which differ significantly from the basic
DAB topology, are presented in detail. The zero-voltage switching (ZVS) characteristics
and requirements are analyzed in detail and compared to those of DAB. A small-signal
model of the new configuration is also derived. The analysis and performance of S-DAB
are validated through extensive simulation and experimental results from a 1 kW
hardware prototype.
Secondly a low-loss auxiliary circuit for a power factor correction (PFC) circuit to
achieve zero voltage transition is also proposed to improve the efficiency and operating
frequency of the converter. The high dynamic energy generated in the switching node
during turn-on is diverted by providing a parallel path through an auxiliary inductor and a
transistor placed across the main inductor. The paper discusses the operating principles,
design and merits of the proposed scheme with hardware validation on a 3.3 kW/ 500
kHz PFC prototype. Modifications to the proposed zero voltage transition (ZVT) circuit
is also investigated by implementing two topological variations. Firstly, an integrated
magnetic structure is built combining the main inductor and auxiliary inductor in a single
core reducing the total footprint of the circuit board. This improvement also reduces the
size of the auxiliary capacitor required in the ZVT operation. The second modification
redirects the ZVT energy from the input-end to the DC link through additional halfbridge circuit and inductor. The half-bridge operating at constant 50% duty cycle
simulates a switching leg of the following DC/DC stage of the converter. A hardware
prototype of the above-mentioned PFC and DC/DC stage was developed and the
operating principles were verified using the same.