Resonant converters are poor stability about load reguration. New control methods have been proposed to stabilize resonant converters versus load variations. This paper describes the LC series resonant converter using a power factor control with stable control characteristic for load regulation. The proposed control adjusts the power factor that determined by primary current and voltage. Previous research proposed phase shift control method for power factor. This method controls the power factor by using the voltage across the resonant capacitor. The problem with this method is that it assumes the resonant capacitor voltage is sinusoidal when controlling the power factor. Therefore, power factor control becomes more difficult when the waveform is distorted. This research proposed a new power factor control method to address this problem in simulations. Hence, this paper shows that the proposed method has stable control characteristic versus load regulation even in actual circuits.

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.

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.

Power conversion technology is characterized by the comparatively small variation of circuit topologies. Because product competitiveness mainly lies in the performance quality as efficiency and low noise emission, the professional skills of this technology commonly require the ability to improve the quality under the restriction of the space, the material resource, and the cost. The conventional teaching method of the power conversion technology is mainly based on the lecture classes and the hands-on classes, which teach the fixed knowledge of the circuit operation. Therefore, this conventional method tends to be insufficient to cover the various practical strategies to improve the quality. To overcome this difficulty, this paper proposes application of the competition-based learning (CBL), in which teams of the students design their power converters and compete on the efficiency. For many professional engineers, competition among companies is the basic motivation to think of strategies to improve product competitiveness. Therefore, the CBL may be effective for teaching design strategies. For promotion of sharing the know-how, the proposed teaching method introduces the events, in which each team share its design strategies with other teams and improve its design based on the know-hows of other teams These events emulate the economic activity, in which a company investigates its competitors' products to extract the know-how. Along with the basic methodology of the proposed teaching method, this paper also reports the experience of a teaching course.

Resonant inverters are widely utilized to generate high-frequency high voltage AC power for the power supply to the plasma generator. These inverters commonly require the load impedance to be inductive for achieving soft-switching operation. However, the impedance often fluctuates to be momentarily capacitive due to the unstable state of the plasma, thus causing enormous switching loss and possibly destroying the inverters. This paper addresses this difficulty by using a recently-proposed resonant inverter that can achieve soft-switching operation under the capacitive load impedance. For this purpose, this paper proposes a novel soft-switching controller for applying this inverter for plasma generator application. The proposed controller observes the load impedance and controls the resonant inverter according to the impedance value. Because of the high-frequency AC power supply to the plasma generator, the voltage and current measurement are difficult for estimation of the load impedance. Therefore, the proposed controller estimates the load impedance based on traveling and reflected wave measurement. The experiment revealed successful control of the inverter according to the load impedance, suggesting high feasibility for applying the inverter to the plasma generator.

Litz wire is widely utilized for heating coils of the induction cookers to suppress the proximity effect. However, low space factor of the Litz wire often significantly increases the heating coil height, which may be an obstacle of wide-spread of the induction cookers. In order to overcome this problem, a preceding study has proposed a heating coil structure of the copper foil, which suppresses the proximity effect without using the Litz wire. The preceding study has proven effective suppression of the proximity effect, when the material to be heated is placed to cover the heating coil. However, this heating coil structure still suffers from large AC resistance, when the material to be heated does not exist or partially covers the heating coil. Furthermore, the ferrite walls, installed to suppress the proximity effect, need larger height than the coil, hindering effective reduction of the total heating coil height. The purpose of this paper is to propose an improved heating coil structure. The proposed heating coil structure further incorporates a thin layer of low-permeability soft-magnetic material covering the top of the coil. This layer enables elimination of the proximity effect regardless of the existence or the disposition of the material to be heated. Furthermore, this layer enables the ferrite walls to have the same height as the coil, thus further reducing the total heating coil height. The FEM based simulation was carried out to verify the operating principle of this structure. The result supported reduction of the heating coil height and effective suppression of the AC resistance regardless of the existence and the disposition of the material to be heated.

Recently, resonant inverters have been employed to generate high voltage high frequency AC power for plasma generator of the semiconductor processing equipment. This inverter is beneficial in low switching loss owing to its natural soft-switching capability under inductive load impedance. Generally, the resonant inverter for plasma generator is designed to have inductive load using an impedance matching circuit that interfaces the inverter and the plasma reactor. However, the load impedance often fluctuates according to the state of the plasma, momentarily causing the capacitive load impedance. In this case, the inverter is generally controlled to restrict the output power for circuit protection from excessive switching loss, although this will result in unstable plasma, deteriorating the quality of the semiconductor processing. This paper addressed this difficulty by applying a soft-switching technique to the resonant inverter. This soft-switching technique can be utilized regardless to the capacitive or inductive load. Experiment was carried out to evaluate the effectiveness of the proposed technique. As a result, the effective reduction of the switching loss was successfully verified even under the capacitive load impedance.

Three-phase power factor correction (PFC) converters have been widely utilized in industrial power applications to improve the power factor and reduce current harmonics in three-phase power grid. The PFC converters commonly incorporate large-sized inductors as a noise filter to suppress the noise emission to the grid. However, as many other power converters, the PFC converters are intensely required to achieve high power density by reducing the size of the inductors. The purpose of this paper is to propose a magnetic structure to miniaturize the inductors. The proposed magnetic structure consists of an integrated magnetic component and a three-phase common-mode choke. By integrating the inductors, the proposed magnetic structure can miniaturize the magnetic core volume, while keeping the saturation current level and the noise filtering capability unchanged. Experiment was carried out to evaluate the appropriateness of the proposed magnetic structure as well as possible volume reduction. As a result, 32.2% volume reduction was found in the proposed magnetic structure without deterioration of the filtering capability.

Switched reluctance motors (SRMs) are expected to be applied to propulsion systems of electric vehicles for their robust mechanical construction and cost-effectiveness. On the other hand, their large source current ripple and large torque ripple are main obstacles in practical applications of SRMs to vehicle propulsion. Certainly, a number of studies have been dedicated to address the torque ripple. However, unlike other motors driven by sinusoidal phase current waveforms, the large source current ripple of SRMs generally remains, even if the torque ripple is removed. The purpose of this paper is to propose a simple control technique of SRMs for vehicular propulsion by eliminating the source current ripple as well as the torque ripple. The proposed control is a current tracking control based on a pre-computed current profile. Because vehicular propulsion requires to instantaneously output large torque in sudden acceleration, SRMs tend to be designed to be propelled below the magnetic saturation in normal vehicle travel. Therefore, this proposed current profile is derived using a simple SRM analytical model without the magnetic saturation. In addition, the proposed current profile is determined so that the peak magnetic flux is minimized to offer high-speed current response at a high rotating velocity. Along with theoretical derivation of the proposed control, this paper also presents an experiment to verify the principle of the proposed control technique, which successfully revealed reduction of both the source current ripple and the torque ripple.

The Dowell model is widely utilized for copper loss analysis of forward transformers. However, this model is not directly applicable to transformers with parallel-connected windings. The reason is that the Dowell model requires determining the AC current of all the windings in advance of analysis, although the AC current distribution in parallel-connected windings is severely affected by disposition of the windings. This issue is addressed in this paper by employing the Dowell model in combination with a novel insight that the AC current is distributed in parallel-connected windings to give an extremum of the magnetic co-energy of the transformer. Simulation and experiment were carried out; and, the results supported appropriateness of the proposed copper loss analysis method.

In this paper, a multiple-output high-frequency soft switching inverter is presented for home and business use “all metal” IH cooking appliances. The frequency and handling power of each output can be selected and regulated individually. The proposed inverter is composed of two single high-frequency circuit cells which are connected in parallel. Therefore, the number of switching devices in the proposed inverter can be significantly reduced. The simulation and experimental results indicate that the proposed circuit is suitable for the multiple output IH concepts such as free-zone induction heating, multi burner type cooking set-up and so on.

The anxiety of merging tiny materials and resin materials in the manufacturing process has become one of the serious issues because it may become cause of overheat such as lithium-ion batteries, flat panel displays, and so on. Therefore, a study on detecting method of the tiny metals is a significant issue. A combination of high-frequency induction heating and thermal detection method is one of the useful approaches. In this paper, MHz-frequency GaN inverter based IH system is built and tested to verify heating size limitation of the tiny metals. Furthermore, the validity of the proposed method to detect the tiny metals by proposed IH thermal technique is evaluated and discussed in detail.

This paper represents the practical evaluations of time-sharing high-frequency resonant soft-switching inverter composed of two multi-resonant sub inverter cells in parallel for common series resonant load for all metal induction heating (IH) cooking appliances. Together with a fixed switching frequency resonant current, IH power regulation is carried out by phasor angle control scheme with phase-shifted PWM scheme. In particular, this high-frequency multi-resonant inverter can efficiently operate under the conditions of load resonant frequency changing mode; frequency doubler mode (100 kHz) for low resistivity metal IH loads and fundamental switching frequency mode (50 kHz) for high resistivity metal IH loads, respectively. The feasible performance of the time-sharing high-frequencys inverter are demonstrated and discussed in detail.

In tandem with the penetration of PHV as well as EV, contact-less power supply system in parking towers have become important subjects. In parking towers, EV and PHV are charged on the traveling pallets, supply power is to be transmitted from the building to travelling pallets. In this paper, high-frequency link AC-AC converter for contact-less power supply system in parking tower is newly proposed and its basic operation and performance have been demonstrated from computer aided simulation and experiment.