Author: Yaqoob, Muhammad
Title: Advanced topologies and modulation schemes for high-efficiency operation of dual-active-bridge series-resonant DC-DC converter
Degree: Ph.D.
Year: 2018
Subject: Hong Kong Polytechnic University -- Dissertations
DC-to-DC converters
Bridge circuits
Department: Department of Electronic and Information Engineering
Pages: xxix, 139 pages : color illustrations
Language: English
Abstract: Dual-active-bridge (DAB) dc-dc converter has gained considerable attention of researchers in recent years due to its applications in integration of various energy storages in micro-grid systems and electric vehicles. It provides single-stage bidirectional power fow with galvanic isolation, high-power density, low semi­conductor device stress and high efficiency. The conventional topology of DAB converter incorporates an inductive tank as the main power transfer element but the presence of higher order harmonics in the inductor/transformer current due to its piecewise linear profile leads to complex design and analysis. DAB series-resonant converter (DABSRC) was proposed to overcome the aforementioned constraint by introducing a series capacitor in addition to an inductor. The modified tank behaves as a low-pass filter with inherent dc-blocking capability which produces a near sinusoidal tank current without any dc o set which could arise due to the application of asymmetric voltages across the tank. The near sinusoidal profile of the tank current has a lower root-mean-square (rms) value as compared to that in the conventional single inductor based DAB converter. However, at high-frequency operation and under wide-range variations in input­to-output voltage ratio and power control, both DAB converter and DABSRC su er from conduction and switching loss. Conduction loss arises from circulating current and reactive power while hard switching operation of active switches leads to switching loss. The operating region over which hard-switching occurs in DABSRC is a function of the input-to-output voltage ratio (defned as the voltage gain hereafter). In general, the primary-side (i.e., input side) switches suffer from hard switching when the input voltage of the tank is less than its output voltage (voltage gain > 1), and the secondary-side (i.e., output side) switches are hard switched when the output voltage of the tank is less than its input voltage (voltage gain < 1).
In this thesis, a comprehensive literature review is frst presented discussing the prior works on the various variants of DAB converter and DABSRC. It is followed by the proposed modifed DABSRC topology aimed at extending the soft-switching operating region of the secondary-side switches (voltage gain ≤ 1) with the aid of switched-controlled-inductor (SCI). In order to achieve reduced conduction loss, a nominal operating point is chosen such that the root-mean­square (rms) resonant tank current is the minimum over a specifed range voltage gain. However, in case of wide-range variations in the input voltage leading to a voltage gain > 1 (i.e., the input voltage is less than the output voltage of the tank), the proposed SCI based DABSRC topology leads to hard-switching operation of the primary-side switches. To achieve soft switching and minimum-tank-current operations for all voltage gains, a DABSRC with switched-impedance-based is proposed. The minimum-tank-current operation aims to reduce conduction loss arising from circulating current. Full-range soft switching is achieved in all switches, thus switching loss is signifcantly reduced. With this new topology, power control is achieved by controlling a switch-controlled capacitor (SCC) in the series-resonant tank while ensuring minimum-tank-current operation and soft switching in all switches. However, due to the adoption of the conventional single-phase-shift (SPS) modulation scheme, both the proposed SCI and SCC based modified DABSRC topologies fail to eliminate reactive power completely. In order to achieve total power loss minimization in DABSRC, a four-degrees­of-freedom (4-DOF) modulation scheme is proposed. This modulation utilizes internal, external phase shifts, and switching frequency as modulation parameters to achieve zero reactive power, minimum-tank-current and complete soft-switching operations. Analysis of the proposed modulation scheme is given for both buck and boost-mode operations. The effectiveness of the proposed topologies and modulation schemes are validated using a hardware prototype designed for charging/discharging a super-capacitor at various output power and voltage gain levels. Simulations and experimental results obtained from the proposed topologies and modulation schemes are compared to those utilizing conventional SPS and modulation schemes proposed in prior research works. A maximum power-conversion efficiency of 94.6 % and 97.4 % is achieved, respectively, by the proposed SCI and SCC based DABSRC topologies whereas the proposed 4-DOF modulation scheme attained the highest efficiency of 97.6 % among the three proposed DABSRC topologies/modulation schemes

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