Publishing type：Research paper (international conference proceedings)  

In recent years, using SiC devices for fast switching have led to the miniaturization of power modules. However, due to the high di/dt of fast switching, false triggering and parallel oscillation caused by high di/dt have become an issue. A method for extracting equivalent circuits, including parasitic inductance, is essential to solve these issues. In power modules, multiple devices are often connected in parallel to meet the demand for high power density. Therefore, complex mutual coupling between wirings makes it challenging to determine the equivalent circuit, including parasitic inductance. In this paper, we propose an equivalent circuit extraction method for power modules that focuses on current loops instead of focusing on the wiring section as in previous studies. The proposed method can extract the effective inductance considering the mutual coupling between wires with fewer measurements than the conventional method. We first summarize the concept of inductance and explain the advantages of the extraction method, focusi ng on current loops. Next, the equivalent circuit, including the parasitic inductance of a double-pulse test circuit board with SiC-MOSFETs connected in parallel, is extracted using the proposed method. To confirm the validity of the proposed method, a double resonance system was constructed using the test circuit board wiring, and the frequency characteristics of the actual measurement and the simulation based on the extracted parasitic inductance were compared.

DOI： 10.1109/APEC48139.2024.10509099

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Publishing type：Research paper (international conference proceedings)  

Power electronics is an electric circuit technology for electric power conversion and is taught in many universities due to its importance in modern society. This technology is traditionally taught in lecture courses, which are based on the operation analysis of the power conversion circuits based on the circuit theory. However, unlike other fields regarding the electric circuit, the quality of the power conversion circuits is fundamentally dependent on the non-ideal features of the circuit elements as well as the size including the cooling system. Therefore, the practical design of the power conversion circuit profoundly relies not only on understanding the basic circuit operation but also on the knowledge of the actual circuit elements, including their non-ideal features. The traditional class based on theoretical circuit operation analysis was difficult to cover the latter knowledge, which may have resulted in insufficient educational achievement in power electronics technology. This paper proposes a teaching approach that embeds mini-experiments in the lecture to overcome this difficulty. These mini-experiments are carefully designed to be seamlessly combined with the lecture and shown to the students during the lecture by utilizing IT-based measurement instruments. According to the students' survey, the students felt the improvement of the lecture quality by the embedded experiment, although the difficulty in the time scheduling for the embedded experiment is a remaining problem to be solved in future studies.

DOI： 10.1109/ICELIE58531.2023.10313105

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DOI： 10.1109/ISIE51358.2023.10228017

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Other Link： https://dblp.uni-trier.de/db/conf/isie/isie2023.html#UmetaniTIH23

Publishing type：Research paper (international conference proceedings)  

In many practical LLC DC-DC converters, the isolation transformer is commonly designed to have a secondary-side center-tapped configuration with one-turn secondary windings. Conventionally, the rectification circuit is installed outside the transformer. However, the recent LLC converters are designed to operate at increasingly high switching frequencies, which indispensably causes the proximity effect in the rectification circuit as well as the wire that connects this circuit to the transformer secondary windings, resulting in large copper loss generation outside the transformer. The previous paper addressed this problem by proposing a secondary winding structure integrating the rectification circuit, although this structure caused the parasitic resonance inside the rectification circuit and reduced the rectification efficiency. To prevent the reduction in the rectification efficiency, this paper proposes an improved winding structure based on the printed-circuit-board technology for suppressing parasitic resonance. The experiment revealed further improvement in the rectification efficiency compared to the previously-proposed structure, suggesting the effectiveness of the proposed winding structure.

DOI： 10.23919/ICPE2023-ECCEAsia54778.2023.10213516

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Publishing type：Research paper (international conference proceedings)  

In recent years, the engineering field is becoming more and more complicated, and collaboration with other disciplines is becoming more critical. As a result, interdisciplinary programs are rising at the graduate level and higher education stages, centered around universities. One drawback of this trend is that students who lack foundational knowledge related to certain lectures may take specialized courses. This can lead to significant differences in the knowledge acquired by the students, depending on how the lecture content is tailored to accommodate their respective levels. To address this issue, various efforts have been made to solve this problem. This paper presents a case study of introducing a circuit simulator to conduct a semiconductor power conversion engineering, in a class with not a few students who do not have circuit theoretical backgrounds.

DOI： 10.1109/ICELIE58531.2023.10313100

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Publishing type：Research paper (scientific journal)  

Recently, the resonant inductive coupling wireless power transfer (RIC-WPT) technique is being widely used to charge small IoT devices dispersed in a wide area, e.g., an office desk and warehouse rack. In this application, the repeater is utilized to expand the chargeable area without an additional AC power source. The repeater comprises a LC resonator commonly with a high-quality factor to induce large current in the repeater coil. However, this causes the problem that the repeater performance is highly susceptible to the variation in its resonant frequency due to the manufacturing tolerance of coil inductance and resonant capacitance as well as the variation of the magnetic coupling between the transmitter and repeater coils. To solve this problem, a previous study has proposed a concept of the autonomous resonant frequency tuning circuit to stably maximize repeater coil current, although this study implemented a part of this control system as a practical circuit. This paper aims to develop a complete autonomous resonant frequency tuning circuit that implements the aforementioned concept. The proposed circuit adjusts the resonant frequencies of the transmitter and repeater by utilizing the Automatic Tuning Assist Circuits. These frequencies are optimized using the infrared wireless signal communication from the transmitter to the repeater. Along with the operating principle of the proposed circuit, this study presents detailed circuit design as well as the experimental results, which verified the stable autonomous maximization of the repeater coil current against the variation in the resonant frequency of the repeater resonator.

DOI： 10.1541/ieejjia.22010814

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Publishing type：Research paper (international conference proceedings)  

The gallium nitride high electron mobility transistors (GaN-HEMTs) are increasingly applied for high-power switching converters. However, their fast switching in high-power applications tends to generate enormous switching surges at the turn-off, which may destroy the switching device. This problem can be mitigated by optimizing the wire structure to minimize the power loop inductance, although satisfactory reduction of this inductance is difficult in many practical designs. This paper address this issue by optimizing the power loop inductance rather than seeking the entire elimination of this inductance. This paper analytically elucidates the existence of the optimal power loop inductance that ideally generates no turn-off surge with fast-switching GaN-HEMTs. An experiment supported the analytical result, suggesting the optimal design of power loop inductance is promising for the power module design of GaN-HEMTs.

DOI： 10.23919/EPE23ECCEEurope58414.2023.10264302

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Publishing type：Research paper (international conference proceedings)  

Gallium nitride high electron mobility transistor (GaN-HEMT) is attractive for fast switching capability and small on-resistance, which motivates researchers to adopt GaN-HEMT for high-power applications. High-power application of GaN-HEMT requires small power loop inductance to suppress switching surge under fast switching but also requires a large power module size for sufficient cooling performance by heat spreading, which results in large wire length and increases the power loop inductance. This paper addresses this problem by proposing a module structure with small power loop inductance and high cooling capability. The proposed module structure incorporates double-sided and single-sided aluminum-clad printed-circuit boards (PCBs) that sandwich the switching devices. These PCBs offer double-sided cooling and small power loop inductance owing to the eddy current generation inside the aluminum clad. A 3kw half-bridge prototype module was constructed to test the proposed structure in comparison with the preceding module structure, which incorporates the double-sided FR4 PCB for the power circuit and the single-sided aluminum-clad PCB for heat spreading. The result successfully revealed that the proposed structure exhibited a small power loop inductance of 1.4nH and a small thermal resistance of 0.72/W, which indicates the reduction in power loop inductance by 53% and in thermal resistance by 20%.

DOI： 10.1109/ECCE53617.2023.10362213

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Publishing type：Research paper (international conference proceedings)  

The kelvin-source packages are becoming popular for high-power ultrafast switching devices to reduce the common source inductance (CSI), which induces the noise voltage in the gating circuit at the transitional change of the drain current and causes false triggering. Despite its importance, however, the actual measurement of CSI has not been reported for kelvin-source packages due to their small inductance values. This paper measured the CSI of typical kelvin-source package TO-247-4L. The result revealed that the CSI of this kelvin-source package ranged 0.2nH-0.7nH, which cannot be neglected at a switching speed greater than 1A/ns. Furthermore, this variation in CSI was found to arise from the magnetic coupling between the bonding wires of power source and gating source, implying that separating the bonding wires can further reduce the CSI.

DOI： 10.1109/ECCE53617.2023.10362400

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Publishing type：Research paper (international conference proceedings)  

Recently, GaN-HEMTs are expected to be applied to high-power applications because of their high-speed switching and low on-resistance, which enables high-efficiency. In high-power applications, the GaN-HEMT is commonly connected in parallel to reduce the current stress. However, the paralleled GaN-HEMTs often suffer from oscillatory false triggering due to fast switching capability. If oscillatory false triggering is occurred, may damage the circuit so that we must be prevented in industrial products. To solve this problem, the purpose of this study is to elucidate design strategy for the parasitic inductance to prevent the oscillatory false triggering in the paralleled GaN-HEMTs. To achieve the purpose, this paper analyzes an equivalent circuit of the single-pulse test circuit with two paralleled GaN-HEMTs after the turn-off transition. As a result, we derived design strategy for parasitic inductances to prevent the oscillatory false triggering and verified the validity of the design strategy by simulations and experiments.

DOI： 10.1109/ECCE53617.2023.10362878

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Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

DOI： 10.1109/icelie55228.2022.9969416

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DOI： 10.1109/icelie55228.2022.9969423

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DOI： 10.1109/ecce50734.2022.9947525

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Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

DOI： 10.1109/ecce50734.2022.9948072

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Publishing type：Research paper (international conference proceedings)  

The data centers of the telecommunication infrastructure are recently consuming more electric power than before. This increasing electrical burden is requiring a small but high-power isolated power converter with a high step-down ratio. Conventionally, the LLC converter is regarded to be a promising candidate, although this converter needs vacant space for galvanic isolation due to extremely high voltage caused by the resonance in the primary side, i.e. high voltage side, of the isolation transformer, which hinders the compact implementation inside the server computers in high power design. The purpose of this paper is to propose a novel LLC converter topology to reduce this extremely high voltage by locating the LC resonator on the secondary side. The performance analysis revealed that the proposed converter exhibits a similar frequency-voltage characteristic enabling the boost and buck operation modes as well as the small rectifier conduction loss similar to the full-wave rectifier with a center-tapped transformer. The simulation confirmed the operating principles and reduction of the resonance voltage amplitude of the proposed LLC converter in comparison with the conventional LLC converter. Furthermore, the experiment also supported the operating principles, which suggests the usefulness of the proposed converter for a high voltage dc power supply system.

DOI： 10.23919/IPEC-Himeji2022-ECCE53331.2022.9806826

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Publishing type：Research paper (international conference proceedings)  

The solenoid coil is a typical heating coil structure for induction heating of cylindrical heating objects. Conventionally, a bare solenoid coil without any soft-magnetic core has been commonly utilized. However, this conventional structure suffers from worse heating efficiency caused by the large coil copper loss and the low magnetic field induction at the heating object. This paper addresses these difficulties by proposing a novel heating coil structure. In addition to the solenoid coil, the proposed heating coils structure further incorporates the soft-magnetic cores that cover the outer coil periphery and the coil opening edges. These additional cores can reduce the local intense magnetic field around the coil, thus mitigating the eddy current induction inside the wire to reduce the copper loss, and also can increase the magnetic field inside the heating object, thus increasing the heat generation. The simulation was carried out to test the effectiveness of the proposed heating coil structure. The results inferred the reduction of the coil copper loss by 25% and the increase in the heat generation in the heating object by 53%, both of which suggest the promising features of the proposed heating coil structure for heating the cylindrical magnetic material. The heating operation of the proposed heating coil structure was tested by the experiment, although the current prototype was found to have larger power loss than expected from the simulation. The reason may lie in unintended eddy current generation of the metal case of the heating coil, which will be addressed in future work.

DOI： 10.1109/ISIE51582.2022.9831459

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Publishing type：Research paper (international conference proceedings)  

The LLC converter is a very effective topology for miniaturization of power supplies. However, its application is currently limited. That is because of the problem of degraded system stability when the load condition changes. This paper proposed a practical control circuit that controls the power factor cos\theta by adjusting the phase difference \theta between the inverter voltage and primary current of the LLC converter to control the output voltage to address this problem. As a result, experimental result confirm that the proposed control method has robust control characteristics against wide load impedance and shows low output impedance regardless of the output voltage.

DOI： 10.1109/SPEC55080.2022.10058371

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Publishing type：Research paper (international conference proceedings)  

Isolated DC-DC converter in data centers are now required to have a high step-down ratio and be suitable for high-power applications. As such DC-DC converters, LLC resonant converters have been attracting attention. However, the LLC resonant converter generates a large resonance voltage at the resonator, so the resonance inductor and resonance capacitor must be large in size. Recently, a previous study proposed a secondary-side resonance LLC converter to reduce the resonant voltages of the passive components. Nevertheless, this LLC converter requires a resonant capacitor in parallel with the rectifier to achieve boost mode operation, which complicates the circuit operation and design. Therefore, we have considered the use of the magnetizing inductance of the coupled transformer of the current doubler rectifier as a method to realize the boost mode operation. In this paper, we propose a new method of boost mode operation for LLC resonant converter using secondary-side resonance and the current doubler rectifier.

DOI： 10.1109/SPEC55080.2022.10058227

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Publishing type：Research paper (international conference proceedings)  

Reluctance machines such as the switched reluctance machines (SRMs) and the synchronous reluctance machines (SynRMs) are recently investigated for propulsion motors of the electrified vehicles owing to their robust mechanical construction, high thermal tolerance, and cost-competitiveness. However, practical application of the reluctance machines to the vehicle propulsion has been hindered by the fact that the conventional SRMs and SynRMs are difficult to meet all the three preferable features for the vehicular application: 1. High power density, 2. Low torque and input-current ripples, 3. Being drivable by the normal three-phase inverter. This paper addresses this issue by proposing a novel reluctance machine. The prominent features of the proposed reluctance machine are the sinusoidal reluctance profile and the delta-connected phase windings, both of which differ from conventional reluctance machines. Along with the theoretical discussion of the basic operating principles of the proposed reluctance machines, this paper presents a simple performance estimation of the proposed machine, the SRM, and the SynRM, which supported that the proposed machine can meet the three preferable features. Furthermore, the experiment successfully supported the operating principles of the proposed machine, suggesting the feasibility of the proposed machine for vehicle applications.

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Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers ({IEEE})  

Multiple-receiver resonant inductive coupling wireless power transfer (RIC-WPT) technique is emerging as a promising charging method for multiple household appliances, mobile devices, and wearable devices. However, this technique often suffers from output power fluctuation owing to the cross-interference among receivers as well as the manufacturing and aging tolerance of the resonant frequency (i.e., resonant frequency tolerance). This article proposes an autonomous RIC-WPT system concept to solve this difficulty. The proposed concept addresses the cross-interference and resonant frequency tolerance by incorporating the following two functions. First, the amplitude of the transmitter current is controlled to have a constant amplitude. Second, the phase of each receiver current is controlled to be orthogonal to that of the transmitter current by using an active reactance compensator installed in each receiver. The experiment of a two-receiver RIC-WPT system successfully verified that the two functions of the proposed system concept can stabilize the output voltage against the cross-interference and resonant frequency tolerance.

DOI： 10.1109/TIA.2021.3081071

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Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

GaN-FETs are recently spreading in high-power switching converters, where GaN-FETs are commonly parallel connected to switch the large current. However, the parallel-connected GaN-FETs often suffer from false triggering because the parallel connection incorporates multiple LC resonators of the parasitic capacitance of GaN-FETs and the parasitic inductance of the printed circuit board, which can be easily excited by the switching noise and fluctuate the gate voltage. Particularly, GaN-FETs are susceptible to the self-sustaining repetition of the false triggering, i.e. the oscillatory false triggering, which must be prevented in industrial products. For prevention of this phenomenon in the case of a single GaN-FET, the preceding studies have proposed the design instruction of the parasitic inductance. However, few insights are available for the parallel-connected GaN-FETs. The purpose of this paper is to elucidate the design instruction to prevent the oscillatory false triggering for parallel-connected GaN-FETs through analyzing the equivalent circuit model of this phenomenon. The result revealed that parallel-connected GaN-FETs need the design instruction slightly modified from that for a single GaN-FET. The appropriateness of this modified instruction was verified by the simulation, suggesting the feasibility of this instruction for applying the parallel-connected GaN-FETs in high-power switching converters.

DOI： 10.1109/IECON48115.2021.9589541

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Other Link： https://dblp.uni-trier.de/db/conf/iecon/iecon2021.html#HatakenakaUIHT21

Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

Due to COVID-19, Okayama University, one of the national universities in Japan, has been unable to enter the campus since March 2020. Therefore, all classes, meetings, and even graduation ceremonies were held online. Even now that large-scale vaccination is in progress, most of the lectures are stil online except for some small-scale classes that can secure the social distance. This paper introduces small attempts to effectively conduct the interactive online lectures of undergraduate and graduate schools.

DOI： 10.1109/IECON48115.2021.9589676

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Other Link： https://dblp.uni-trier.de/db/conf/iecon/iecon2021.html#HirakiIU21

Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

Gallium-nitride-field-effect transistors (GaN-FETs) are expected as a key component to the power density improvement of switching power converter for electric vehicles (EVs) because of their low on-resistance and fast switching capability. It is well known that the switching loss is influenced by the drain inductance, which is the parasitic inductance of the power loop, and can be minimized in principle by an appropriate design of the drain inductance. However, in switching power converters using Si-based power devices such as the Si-MOSFET and the Si-IGBT, it is usually difficult to design the drain inductance so that the switching loss minimizes because an appropriate drain inductance becomes too large, thus resulting in large surge voltages of the switching device. On the other hand, this may not be the case when using the GaN-FETs because the inductance that can minimize the switching loss may become small due to the high-di/dt switching. Therefore, the purpose of this study is to show the feasibility of the parasitic drain inductance design that the switching loss of the GaN-FET in the bridge circuit can be minimized while keeping the surge voltage of the GaN-FET within acceptable limits. The appropriateness of this insight is verified by simulation.

DOI： 10.1109/ISIE45552.2021.9576373

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Other Link： https://dblp.uni-trier.de/db/conf/isie/isie2021.html#AbeIHUH21

Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

The resonant inductive coupling wireless power transfer technique (RIC-WPT) is emerging as a promising wireless power transfer method for small mobile devices with comparatively high-power transfer capability and good efficiency. However, the performance of the RIC-WPT system is severely deteriorated by the resonant frequency detuning of the transmitter, repeater, and receiver resonators, which can be naturally caused by the manufacturing tolerance and the aging effect of these resonators. Particularly, the detuning of the repeater resonator tends to severely affect the performance due to its high-quality factor, which should be solved for practical application of RIC-WPT technology. This paper addresses this difficulty by proposing an autonomous resonant frequency tuning controller circuit for the repeater resonator. The controller tunes the resonant frequency of the repeater resonator to the operating frequency of the inverter, driving the transmitter, without an external power supply and wireless communication with the inverter controller, which restricts the freedom of the installation location and therefore preferably be avoided in the wireless power transfer system. The operating principles of the proposed controller were confirmed by the circuit simulator, supporting the feasibility of the proposed controller.

DOI： 10.1109/ISIE45552.2021.9576268

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Other Link： https://dblp.uni-trier.de/db/conf/isie/isie2021.html#KonishiUIH21

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

The LLC converter is attractive for its small volume and comparatively high efficiency. However, the application of this converter is limited to the power supply to a static load with almost constant output voltage and current because the LLC design covering a wide range in the output voltage and the load variation can deteriorate the output voltage stability and efficiency. This paper addresses this issue by proposing a novel control method. Unlike the conventional control, which adjusts the operating frequency to stabilize the output voltage, the proposed control adjusts the power factor of the transformer primary-side to achieve the same excellent dynamic performance as the buck chopper. Simulation supported the effectiveness of the proposed control at a wide load variation range.

DOI： 10.1109/ECCE47101.2021.9595420

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

Resonant inductive coupling wireless power transfer (RIC-WPT) systems with multiple transmitters and multiple receivers often suffer from cross-interference among transmitters as well as among receivers. The cross-interference causes the fluctuation of the output power of the receiver and hard switching of the inverter on the transmitter. To avoid the cross-interference, controlling the amplitude and the phase of each transmitter current is an effective method. Therefore, this paper proposes a multiple-transmitter that can compensate for the cross-interference influence by implementing two controls. First, adopting the phase-shift control on the full-bridge inverter, the current amplitude is controlled to be constant. Second, applying the reactance control, each phase of the current is controlled to be in phase. In addition to the above operations, especially under a high-frequency operation, zero-voltage switching (ZVS) at a fixed operating frequency is essential for improving the power efficiency while not exceeding an allowable narrow bandwidth. Thus, the reactance control also works to ensure ZVS in all load conditions without adjusting the frequency. The RIC-WPT prototype with two transmitters and a single receiver is built to verify that the proposed multiple-transmitter can compensate for the cross-interference influence while achieving ZVS in different load conditions at a fixed operating frequency.

DOI： 10.1109/ECCE47101.2021.9595950

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers ({IEEE})  

Series-series (SS) and series-parallel (SP) topologies are widely used in resonant inductive coupling wireless power transfer systems for various applications. However, the selection of an appropriate topology to achieve higher output power or higher efficiency is typically difficult because design optimization of the circuit parameters (e.g., characteristic impedance, load resistance, and mutual inductance) for each topology is generally separately discussed using different equivalent circuits with multiple resonance modes. Therefore, the purpose of this study involves proposing a simple strategy to select an appropriate topology. The proposed strategy is based on quasi-duality between the SS and SP topologies that are elucidated from the novel equivalent circuits derived using Lagrangian dynamics. Based on the quasi-duality, the output power and efficiency of the SP topology are calculated via the equivalent circuit of SS topology. Thus, the quasi-duality offers a simple comparison between the SS and SP topologies. The proposed strategy selects an appropriate topology by comparing only the equivalent ac load resistance, which is the ac resistance including the rectifying circuit and the load resistance, the characteristic impedance, and the ac load resistance that achieves the maximum efficiency ormaximum output power of the SS topology. Experiments verify the appropriateness and effectiveness of the proposed strategy.

DOI： 10.1109/TPEL.2019.2956732

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

Next-generation switching devices as GaN-FETs are recently emerging as promising switching devices capable of extremely high-speed switching. High-speed switching enables the high-frequency operation of the power converters, which can reduce the size of the passive components. However, highspeed switching can induce the resonance between the parasitic capacitance of the switching device and the parasitic inductance of the circuit board wiring, which appears as the gate voltage fluctuation at the switching. Particularly, GaN-FETs tend to have comparatively low gate threshold voltage and therefore are susceptible to the false turn-on, which is caused by the gate voltage fluctuation in the switching device just after the turn-off transition. For preventing this phenomenon, this paper analytically investigates the design requirement of these parasitic parameters to reduce the gate voltage fluctuation after the turn-off transition. As a result, the optimal ratio of the gate-drain capacitance and the common source inductance is elucidated to be the key to minimize the gate voltage fluctuation. The simulation and the experiment supported that the optimal design of this ratio can reduce the gate voltage fluctuation, supporting the usefulness of this novel insight for preventing the false turn-on.

DOI： 10.1109/ECCE44975.2020.9236428

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

Recently, a resonant inductive coupling wireless power transfer (RIC-WPT) system with multiple transmitters is emerging as a promising power supply method for household appliances, mobile devices, and wearable devices dispersedly placed in a wide area. However, the multiple-transmitter often suffers from an unstable operation of an inverter, feeding AC current to the transmitter coil, due to the cross-interference (i.e., cross-coupling) among the transmitters. When the cross-interference occurs, the inverter may not achieve high power factor and soft switching, which damages the power density and reliability of the multiple-transmitter. Therefore, this paper proposes a multiple-transmitter, including its controller, that can compensate for the effect of the cross-interference. In the proposed multiple-transmitter, each transmitter has a simple switching circuit that can automatically cancel the induced voltage due to the cross-interference with only simple control. Furthermore, the proposed multiple-transmitter also achieves a load-independent transmitter current by the control of the input voltage of the inverter, which results in a stable magnetic field regardless of load variation. Experiments verify the effectiveness and appropriateness of the proposed multiple-transmitter.

DOI： 10.1109/ECCE44975.2020.9235430

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：INST ENGINEERING TECHNOLOGY-IET  

Resonant inductive coupling wireless power transfer is widely known to be a possible convenient power supply method for the small mobile apparatus. However, the limited receiving-coil size tends to lower the efficiency and limit the output power owing to the small mutual inductance and comparatively large stray alternating current (AC) resistance of the receiving-coil. This study mitigates this issue by proposing a novel receiving-coil structure. This proposed structure comprises a coil and a drum core with a thin axis. The coil is wound on the axis to form a single winding layer. The proposed structure can reduce stray AC resistance by suppressing the proximity effect and reducing the wire length without deteriorating the mutual inductance significantly. Therefore, better efficiency and larger output power can be achieved. Simulations and experiments were performed to verify the proposed structure. Consequently, both simulations and experiments supported the reduction in AC resistance compared to the conventional structure. Furthermore, the experiment revealed improvements by the proposed structure in both efficiency and output power. These results support the effectiveness of the proposed structure for wireless power transfer to small mobile apparatus.

DOI： 10.1049/iet-pel.2018.5358

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

The ability to charge to multiple receivers simultaneously with a single transmitter is one of the advantages of resonant inductive coupling wireless power transfer (RICWPT) technologies. However, in multiple-receiver RIC-WPT systems, each receiver often suffers from the magnetic coupling among receivers, i.e., cross-coupling. The cross-coupling not only complicates the control of the receivers but also significantly decreases the output power of each receiver. Hence, the purpose of this paper is to propose a compensation method of the influence due to the cross-coupling. To achieve this purpose, we first analyze the requirement to compensate for the influence of the cross-coupling. As a result, we reveal that it is necessary to control the phase of the current in all the receiver to be orthogonal to the phase of the transmitter current. Then, we propose the method to adjust the current phase of each receiver automatically to the desired phase by using only the phase information of the transmitter current, which results in the full compensation of the influence due to the cross-coupling. Furthermore, the proposed method also compensates for the influence of a detuning of the resonant frequency of each receiver due to the natural tolerance. Experiments of a two-receiver RIC-WPT system successfully verify the effectiveness of the proposed method.

DOI： 10.1109/ECCE.2019.8913279

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

Placing a repeater, which relays the magnetic field from the transmitter to the receiver, is promising as a method to increase the spatial freedom of resonant inductive coupling wireless power transfer systems (RTC-WPT) working at 6.78MHz. However, the capability of the repeater is often affected by a frequency splitting phenomenon. When this phenomenon occurs, the resonance in the repeater becomes sufficiently small at a fixed operating frequency and make it difficult to improve the spatial freedom. To solve this problem, we apply an impedance matching method using simple switching circuits to the 6.78 RTC-WPT system with the repeater. Then we carry out experiments to verify the effectiveness of the impedance matching method. The experimental results show that the repeater improves the spatial freedom of the 6.78 MHz RTC-WPT regardless of the frequency splitting phenomenon.

DOI： 10.23919/EPE.2019.8915171

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

Intermediate resonators (repeaters) of resonant inductive coupling wireless power transfer systems can improve the transmission distance as well as the output power. However, frequency bandwidths in which the repeater can operate effectively are very narrow because the repeater usually has a high quality-factor. Furthermore, these frequency bandwidths shift for the following two factors. The first factor is the intensity of the magnetic coupling between the repeater and the other resonator. The second factor is the variation in the natural resonance frequency of the resonators due to a production error, temperature characteristic, and aging degradation. Therefore, the repeater is not practical because the repeater requires accurate adjusting of the circuit parameters every time according to the various conditions. To address this problem, we propose an impedance matching method for the repeater. The proposed method can maximize the induced current in the repeater in wide frequency bandwidth regardless of the variations in the intensity of the magnetic coupling and the natural resonance frequency. Therefore, the proposed method can realize the repeater which can stably improve the performance of the wireless power transfer. Experiment and simulation successfully verified the effectiveness of the proposed method as well as the appropriateness of the theoretical analysis.

DOI： 10.1109/ECCE.2018.8557827

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

Intermediate resonator (repeater) in resonant inductive coupling wireless power transfer (RIC-WPT) systems can increase the transmission distance between the transmitting and receiving coils. The induced current in the repeater is easily affected by the frequency splitting phenomenon because the quality factor (Q-factor) of the repeater is generally as high as several hundreds. If this phenomenon occurs, induction of large current is often difficult in the repeater because the one peak characteristic of the induced current is no longer expected and the single peak of resonance splits into multiple peaks which shift the resonant frequency corresponding to the circuit parameters. In this paper, we approach this difficulty by applying an auxiliary circuit to the RIC-WPT system with the repeater. As a result, under the fixed operating frequency, the induced current in the repeater can be kept large even if the frequency splitting phenomenon occurs. The effectiveness of proposed system was supported by the simulation and experiment.

DOI： 10.23919/IPEC.2018.8507768

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

Intermediate resonators (repeaters) for resonant inductive coupling wireless power transfer have been widely studied as a method of improving not only the transmission distance but also the output power. For the repeater to operate effectively, it is needed to induce a large current in the repeater to enhance the magnetic field far from a transmitting resonator. However, it is often difficult to induce a large current in the repeater due to frequency splitting phenomenon. This phenomenon easily occurs when the resonator having high quality factor such as the repeater is used. The frequency characteristic of the induced current in the repeater has multiple peaks when the frequency splitting phenomenon occurs. In addition, these multiple peaks shift according to slight variation in the parameters of the coil and the capacitor that constitute the resonator. This slight variation is easily caused by production error, temperature characteristic, and aging degradation of the coil and the capacitor. The induced current in the repeater is significantly decreased by the slight variation in the parameters, namely, the slight variation in the resonance frequency. Therefore, the repeater has low robustness against variation in the resonance frequency. To address these difficulties, we apply an auxiliary circuit to the repeater. The auxiliary circuit can dynamically adjust a phase of the induced current in the repeater, namely, the resonance frequency without complicated control. As a result, a large induced current can be maintained even if the frequencies corresponding to the peaks shift. Consequently, we can provide the repeater having a stable characteristic against the variation in the resonance frequency. The effectiveness of the repeater applied the auxiliary circuit and the appropriateness of analysis results are supported with simulation and experimental results.

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Interleaved DC-DC converter employing close-coupled inductors is a popular topology among other power converters topologies. Close-coupled inductors allow the power converter to achieve high power density and high efficiency. This paper proposes a novel magnetic structure of close-coupled inductors suitable for increasing the thermal handling capability. The proposed magnetic structure is combined of different magnetic materials, namely, ferrite and powder cores. The design method of the integrated close-coupled inductors are presented. Furthermore, this design method is considering the DC bias superposition characteristics, and the iron and copper losses as well. A 300W prototype is built to validate the proposed analysis. Finally, excellent heat dissipation of the proposed magnetic structure of the integrated close-coupled inductors is also reported.

DOI： 10.1109/ECCE.2017.8095782

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Series-Series (SS) and Series-Parallel (SP) topologies are widely utilized in practical resonant inductive coupling wireless power transfer (RIC-WPT) systems owing to their simple circuit configurations. Conventionally, design optimization of the circuit parameters of these topologies were investigated separately, because these topologies are expressed by different equivalent circuits. However, analysis of these equivalent circuits is generally complicated due to the multiple resonant modes contained in the operation, which may cause difficulty in comparing the performance between the SS and SP topologies after design optimization of these topologies. This difficulty may prevent elucidating the methodology to select an appropriate topology that offers better output power or efficiency after design optimization. The purpose of this paper is to suggest a selection methodology of the SS and SP topologies by elucidating a novel insight that these topologies have the quasi-duality relation, in which the SP topology works approximately as the dual of the SS topology. This insight enables to share the analysis results between the topologies. As a result, both of these topologies were found to be expressed by a same novel equivalent circuit. Furthermore, the only difference between the SS and SP topologies were found to be the equivalent load resistance of this equivalent circuit, thus reducing the topology selection into selection of preferable load resistance. The appropriateness of the quasi-duality relation and the resultant equivalent circuit was successfully confirmed by the simulation and the experiment.

DOI： 10.1109/PEDS.2017.8289248

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Interleaved techniques and magnetic integration in a boost converter have gained attention in electric powertrains system for electric, hybrid and fuel cell vehicles in order to achieve high power density or to improve power conversion efficiency. Furthermore, the proposed multi-phase boost converter is equipped with a phase drive control to improve the efficiency at all load ranges. Furthermore, a design method of a coupled-inductor for an interleaved boost converter with phase drive control is also proposed. However, the interleaved DC-DC converter using coupled method with phase drive control has many problems. In this paper, this problem of interleaved DCDC converter using coupled inductor with phase drive control (PDC) is analyzed. In addition, defensive method of this method.

DOI： 10.1109/INTLEC.2017.8214190

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Resonant inductive coupling wireless power transfer (RIC-WPT) is attracting attention as a convenient power supply method to small mobile apparatus. The efficiency and the power transfer capability of RIC-WPT has been known to be profoundly dependent on the quality factor of the receiving resonator. However, the quality factor of the receiving resonator tends to be deteriorated because of the conduction loss at the diode rectifier, particularly for low output voltage applications. In order to improve the efficiency and the power transfer capability, this paper propose a novel simple synchronous rectifier, which can reduce the conduction loss. The proposed rectifier has simple circuit configuration, which contributes to straightforward application to the wireless power transfer to small mobile apparatus with limited installation space. Experiment was carried out to verify the operation principle of the proposed rectifier. As a result, the proposed rectifier revealed successful suppression of the conduction loss. In addition, the experimental wireless power transfer system verified successful improvement in the rectification efficiency, supporting usefulness of the proposed rectifier for practical applications of the RIC-WPT for small mobile apparatus.

DOI： 10.1109/INTLEC.2017.8214163

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Series-series (SS) topology and series-parallel (SP) topology of the electric-field coupling wireless power transfer system are widely utilised for practical applications. However, selecting the appropriate circuit topology, which maximise the output power by design optimisation, is often difficult, because these circuit topologies are generally analysed using different circuit models. This difficulty by analysing novel equivalent circuits for SS and SP topologies is addressed. As a result, the output power of SP topology is found to be approximated by that in SS topology with transformed load impedance. This result can offer a simple strategy for topology selection as well as design optimisation.

DOI： 10.1049/el.2016.1253

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In this paper, a novel magnetic structure suitable for boost converters is proposed. Multi-phase interleaved method using coupled-inductor has gained attention in electric powertrains for electric, hybrid and fuel cell vehicles in order to achieve high power density. In fact, a four-phase boost converter using coupled inductor is used in the drive system of the Honda CLARITY. In particular, magnetic coupling method is used in coupled inductors, Loosely-Coupled Inductors (LCI) and Closed-Coupled Inductors (CCI). This study is focused on these methods, especially using the CCI. This paper presents a design method of a closed-coupled inductors using generic cores for a four-phase interleaved boost converter. In addition a comparison between the proposed topology with other conventional non-coupled methods is carried out. Furthermore, the evaluation of miniaturization is studied. As a result, the proposed method can achieve a huge reduction in the core volume and mass.

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This paper describes lateral misalignment characteristics of electrical resonance coupling wireless power transfer (ERC-WPT) from experimental points of view. A dead zone is an important factor for ERC-WPT because ERC-WPT cannot transfer electric power in the zone. Electrical coupling units that are composed of 2 electrodes lined up laterally have a dead zone. The dead zone exists near outer edge areas of the electrodes and certain distant areas.

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Flyback switch mode power supplies have been widely used in low-power applications, such as DC/DC converters, solar micro-inverters and LED drivers. However, flyback converters have several problems related to the volume of the output capacitor as well as high output voltage noise; this is produced by the discontinuous output current. Consequently, interleaved operation with parallel connection on the secondary side can reduce the output current ripple compared with the single-phase flyback converter. Nevertheless, besides the output capacitor, the interleaved operation is unsuitable for transformers if it is desired to obtain light weight and compact performance. To address this problem, the interleaved flyback converter with integrated transformer has been proposed for achieving miniaturization of the output capacitor and transformers. There are mainly two types of the proposed flyback converter with integrated transformer: Parallel and Series types. These two types are categorized based on the connection on the primary side. Nevertheless, quantitative comparison of the volume and power loss has not been analyzed yet. Moreover, the design method of the integrated transformer also has not been conducted with clarity. Hence, in this paper, in order to provide a guide for the design of an interleaved flyback converter for achieving high power density, we analyze the quantitative volume and power losses of the integrated transformer and the input capacitor. Finally, this paper shows some experimental results that validate the appropriateness of the design method for the integrated transformer.

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This paper proposes Series Input Parallel Output (SIPO) interleaved Active Clump Forward (ACF) converter with novel integrated magnetic components that may achieve high power density. The proposed converter consists of two ACF converters in SIPO connection to share the input voltage and output current. The natural magnetizing reset can be realized by the active clump circuit. The SIPO connection distributes the voltage stress of the primary side switches and the thermal stress of the output rectifiers and filters. The interleaved PWM operation also diminishes the current ripple in the output capacitor to reduce the size the output filter. In addition, the integrated magnetic components are used for inductors and transformers in the proposed converter to realize size-reduction of the magnetic components. Therefore, this converter is suitable for high input voltage and high output current applications such as dc-dc converters EVs and HEVs. In this paper, the operation principle and the theoretical analysis for the proposed integrated magnetic components are presented. Following we use core models for the integrated magnetic components, and the superiority of them in term of reducing the size and losses in the magnetic components is demonstrated clearly through comparison with a conventional single ACF converter and an conventional SIPO interleaved ACF converter. As a result, it is cleared that the proposed converter is effective for size-reduction and low-losses of the magnetic components under the same conditions. To verify the theoretical results, the experiments using a 500 W output prototype are conducted.

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This paper describes mid-range electrical resonance wireless power transfer in kHz range. Wireless power transfer using electrical field is minor since this method is understood only for close-range applications. However, our experimental units, the resonance frequency of which is 85 kHz, achieved 180 mm distance power transmission by vector network analyzer (VNA) measurement while keeping peak efficiency 68 %. In case of frequency response analyzer (FRA) measurement, the units can transfer power the most to the load at 300 mm but the efficiency is low. The closer the distance between the units, the better the efficiency is.

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New semiconductor devices such as Silicon Carbide (SiC) and Gallium Nitride (GaN) are very attractive for improving converter efficiency and power density in power electronics. Especially, GaN FETs are promising devices for the high speed switching and good cost performance due to being able to use silicon wafer, so that we focus on GaN FETs. There are two types of GaN FETs, and one, non-isolated GaN FETs, has a diode structure on the gate-to-source, and another, isolated GAN FETs, doesn't have this feature. Non-isolated GaN FETs show better performance than isolated GaN FETs in terms of withstand voltage, allowable current, and other parameters. However, simple gate circuits used for MOS FET cannot be applied. Hence, many gate drive circuits for non-isolated GaN FETs are proposed. In this paper, we proposed a novel gate drive circuit for a non-isolated GaN FET, which uses two parasitic capacitors of the switches in the gate drive circuit. The experiment for the proposed gate drive circuit is executed with a simple test circuit. Furthermore, drive loss is analyzed and compared with a previous proposed gate drive circuits. Furthermore, we develop the theoretical equations for the gate drive loss and verify them experimentally. The drive and reverse conduction losses of the proposed capacitor-less gate drive circuit are compared with dividing capacitor gate drive circuit and previous capacitor-less gate drive circuits.

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Other Link： http://id.ndl.go.jp/bib/031934588

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