open access

https://doi.org/10.14943/doctoral.k15082

https://hdl.handle.net/2115/85461

A Study on Isolated AC-DC Converters Using High-Frequency Output Matrix Converters

日本語

Power Decoulping

2022-03-24

Hokkaido University

ⅹⅱ, 138p

Isolated AC-DC converters are widely used in various electrical applications, such as switching-mode power supplies, battery chargers, and solar photovoltaic systems (PVs) to isolate an AC grid and a DC load. A conventional isolated AC-DC converter consists of a power factor correction(PFC) circuit and an isolated DC-DC converter. The PFC controls the grid current to maintain unity power factor, while the isolated DC-DC converter adjusts the output DC power with isolation between the AC grid and the DC load. This converter has three-stage power conversion (AC–DC-high-frequency (HF) AC–DC), and a large-capacity electrolytic capacitor, which is large and has a short lifespan, is used in the DC link between the PFC and DC-DC converter. Therefore, the conventional isolated AC-DC converter has low efficiency, large size, and a short lifespan.Isolated AC-DC converters using matrix converters (MCs) are more compact, have a longer lifespan, and are more efficient than the conventional isolated AC-DC converters. The MC directly converts a grid voltage into an AC voltage in the primary of an isolation transformer and simulta-neously controls the AC grid current with unity power factor and the output DC power. Therefore this converter requires no electrolytic capacitors for the DC link and has two-stage power conversion (AC–HFAC–DC), resulting in the compact size, long lifespan, and high efficiency. However,most of the MC control methods converts a grid voltage into a low MC output frequency. The volume of isolation transformer in the isolated AC-DC converter is inversely proportional to the MC output frequency. Thus, the isolated AC-DC converters with these MC methods require large isolation transformers and generate large switching losses.This study proposes a novel MC control for an isolated AC-DC converter fed by a single- or three- phases supply. The MC modulates a grid voltage into an AC voltage with high-frequency equal to a switching frequency of the MC to minimize the isolation transformer and generates a sinusoidal grid current with unity power factor. A variable MC output frequency control with an LC resonant circuit realizes soft switching, which is loss-free switching, and maximizes the MC output power factor. Additionally, the proposed converter can be fed by not only a three-phase supply but also a single-phase supply without any additional components. The proposed method has been evaluated through simulations and experiments with laboratory prototypes.Chapter 1 presents the backgrounds and aims of this study, and then, chapter 2 introduces the MC principles and a survey of the most popular control, modulation strategies, and circuit configurations studied for MCs.Chapter 3 presents six MC switching patterns which achieves a high MC output frequency equivalent to the switching frequency to reduce the volume of the isolation transformer in the isolated AC-DC converter, which is inversely proportional to the MC output frequency. A simple linear mathematical model (control equations) of the proposed three-phase input/HF single-phase output MC is also developed to obtain the ON time ratios of the MC power semiconductor switches which realize the sinusoidal MC input current with unity power factor and arbitrary output DC voltage. The simulation results shows the effectiveness and validity of the MC control method based on the proposed switching pattern and the control equations.Chapter 4 introduces a fixed MC output frequency control using an LC resonant circuit between the MC and isolation transformer. The MC output current is lagging to the MC output voltage to achieve soft switching when the MC output frequency is higher than the LC resonant frequency, like a conventional series resonant DC-DC converter. This chapter clarify the soft switching conditions for the switching pattern proposed in chapter 3. First Harmonic Approximation (FHA) analysis, which approximates the secondary circuit of the isolation transformer as a linear element and assumes sinusoidal MC output current, simplifies the MC control for the isolated AC-DC converter with the LC resonant circuit. It is noted that the control equations derived by the FHA analysis are non-linear. The effectiveness and validity of the proposed fixed MC output frequency control is confirmed by simulation.Chapter 5 proposes a variable MC output frequency control using the LC resonant circuit to achieve soft switching with the maximum MC output power factor. The MC adjusts the phase of the MC output current to maximize the MC output power factor by the variable MC output frequency considering the characteristics of the LC resonant circuit. The output DC voltage feedback controller and the analysis procedure using a circuit simulator to calculate the ON time ratios of the MC switches and MC output frequency considering non-linearity of the diode rectifier are also discussed. The proposed method is evaluated experimentally with the laboratory prototypes. Chapter 6 presents an isolated AC-DC converter using the MC fed by the single-phase. A single-phase supply inherently has a power ripple at twice the grid frequency, resulting in low-frequency output DC voltage ripples. The proposed MC control performs the active power decoupling using input filters, which diverts the power ripple at twice the grid frequency to the MC input filter capacitors, to suppress this power ripple. Therefore, this isolated AC-DC converter can use only film capacitors. The proposed isolated AC-DC converter can also be fed by a three-phase supply without any additional components. This converter has been evaluated through simulations and experiments with laboratory prototypes.Finally, chapter 7 concludes this study and discusses future works.

2022-03-24

甲第15082号

博士(工学)

(主査) 教授 小笠原 悟司, 教授 五十嵐 一, 教授 北 裕幸

情報科学院(情報科学専攻)

博士論文

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