Papers by Kazuhiro Umetani
IEEJ Journal of Industry Applications, 2018
2020 IEEE Energy Conversion Congress and Exposition (ECCE)
The Litz wire is widely utilized in recent high frequency switching regulators as a magnetic wire... more The Litz wire is widely utilized in recent high frequency switching regulators as a magnetic wire with small AC resistance. The AC resistance of the Litz wire is dependent on the twisting structure of the strands as well as the resultant cross-section geometry of the Litz wire. Therefore, optimization of the Litz wire is becoming important, although the numerical prediction of the AC resistance is difficult due to the complicated structure of the Litz wire. Certainly, a recent study has proposed an analytical AC resistance prediction method. However, this method cannot be applied to Litz wires with the rectangular cross-section, which is recently developed to improve the packing factor of the winding. This paper aims at extending this previous method to cover these Litz wires by partly utilizing a simple static magnetic field FEM analysis. The experiment revealed successful AC resistance prediction of the Litz wire with the rectangular cross-section.
IECON 2020 The 46th Annual Conference of the IEEE Industrial Electronics Society
Power conversion technology is characterized by the comparatively small variation of circuit topo... more 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.
2020 IEEE 29th International Symposium on Industrial Electronics (ISIE)
With the growth of GaN HEMTs applications in power electronic commercial markets, higher frequenc... more With the growth of GaN HEMTs applications in power electronic commercial markets, higher frequency operation became possible allowing different advantages with regards to downsizing of passive components in power converters. High switching and heat performance makes GaN HEMTs a better solution for high power applications. This technology is different from that of conventional Si devices, raising several new challenges to the power converter design procedures. For instance, the effect of the fast transient operation of GaN HEMTs on the operation magnetic components has not been completely understood yet. Consequently, it is important to start analyzing how GaN semiconductors are driven, despite the few gate drivers designed to drive GaN HEMTs. This paper analyzes and summarizes the driving techniques and solutions of E-mode GaN HEMTs and their gate drivers. The commercially available products are reviewed. Simulation of E-modes GaN HEMT switching performance is performed and discussed.
2017 19th European Conference on Power Electronics and Applications (EPE'17 ECCE Europe)
In the LLC resonant converter, the transformer core is usually gapped to realize the designed val... more In the LLC resonant converter, the transformer core is usually gapped to realize the designed value of magnetizing inductance that allows for soft switching. The air gap length which is in a scale of millimeters may be affected by the humidity and long working hours of the ferrite material, resulting into a variation of the core permeability. Moreover, a slight variation of the transformer core permeability may be encountered with any possible change in the core temperature. In a proper transformer design, the transformer shall operate with an optimum value of effective permeability where the sum of both copper and core losses is minimal. In this paper, the transformer optimal design is presented. Moreover, the influence of the permeability on the transformer's copper and core losses is revealed. Furthermore, with the help of newly derived equations, the effect of the transformer's effective permeability on the LLC converter conduction turn-on and turn-off losses is reported. Along with the theoretical discussion, the analysis is supported with simulation and experimental results utilizing a 250W–200kHz prototype.
2017 IEEE Energy Conversion Congress and Exposition (ECCE), 2017
SiC-MOSFETs have attracting increasing attention because of their outstanding characteristics tha... more SiC-MOSFETs have attracting increasing attention because of their outstanding characteristics that contributes to high efficiency and high power density of power converters. However, compared to conventional Si-IGBTs, SiC-MOSFETs are susceptible to false triggering, because they tend to generate large switching noise due to ultrafast switching capability and have a lower threshold voltage in high temperature operation. Particularly, disastrous oscillation of repetitive false triggering can occur after a fast turn-off, which is the severe issue for practical application of SiC-MOSFETs. The purpose of this paper is to give an instruction to avoid this phenomenon. This paper hypothesized that the repetitive false triggering is the parasitic oscillation caused by parasitic capacitance of SiC-MOSFET, and parasitic inductance of wiring. Based on this hypothesis, this paper analyzed the oscillatory condition of the parasitic oscillator to propose a design instruction to avoid the oscillatory false triggering. The result revealed that the parasitic inductance of the gate, drain, and source wiring should be designed so that the resonance frequency of the parasitic LC resonator in the gating circuit is far apart from that of the power circuit. This paper also presents experimental results that support appropriateness of the proposed design instruction.
IECON 2017 - 43rd Annual Conference of the IEEE Industrial Electronics Society, 2017
Three-phase power factor correction (PFC) converters have been widely utilized in industrial powe... more 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.
2018 International Power Electronics Conference (IPEC-Niigata 2018 -ECCE Asia), 2018
Intermediate resonator (repeater) in resonant inductive coupling wireless power transfer (RIC-WPT... more 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.
Journal of the Japan Institute of Power Electronics, 2020
2017 IEEE International Telecommunications Energy Conference (INTELEC), 2017
Resonant inductive coupling wireless power transfer (RIC-WPT) is attracting attention as a conven... more 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 suppre...
2018 20th European Conference on Power Electronics and Applications (EPE'18 ECCE Europe), 2018
Intermediate resonators (repeaters) for resonant inductive coupling wireless power transfer have ... more 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 ...
2019 21st European Conference on Power Electronics and Applications (EPE '19 ECCE Europe), 2019
Placing a repeater, which relays the magnetic field from the transmitter to the receiver, is prom... more 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 (RIC-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 RIC-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 RIC-WPT regardless of the frequency splitting phenomenon.
2018 IEEE Energy Conversion Congress and Exposition (ECCE), 2018
Intermediate resonators (repeaters) of resonant inductive coupling wireless power transfer system... more 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.
2018 IEEE Energy Conversion Congress and Exposition (ECCE), 2018
Switched reluctance motors (SRMs) are attractive motors for electric vehicle propulsion because o... more Switched reluctance motors (SRMs) are attractive motors for electric vehicle propulsion because of their cost-effectiveness or robust structures. However, SRMs suffer from input current ripple and torque ripple. The authors have proposed derivation method of phase current profile which suppresses input current ripple and torque ripple and experimentally verified the suppression of both ripples. However, the rms value of derived phase current was much greater than square wave current, which is implemented widely, and increased copper loss. For the EV application, the rms value should be reduced. To break through the issue, the author addressed the characteristic that phase current profiles are shaped based on each reluctance profile, which is shaped by motor configuration, and noticed that phase current profile with low rms value may be derived by a novel rotor configuration. Therefore, this paper proposes a novel rotor configuration which can reduce the copper loss. The proposed rotor configuration is designed to satisfy the reluctance profile which is derived by previously proposed derivation method of phase current. This paper also presents the experiment for the evaluation of the reduction of copper loss.
2017 IEEE 3rd International Future Energy Electronics Conference and ECCE Asia (IFEEC 2017 - ECCE Asia), 2017
The common source inductance is one of the major causes of deterioration of the switching speed a... more The common source inductance is one of the major causes of deterioration of the switching speed and susceptibility of the false triggering for semiconductor switching devices. Practical design of this inductance requires selection of an appropriate semiconductor package because this inductance is greatly dependent on the physical package structure. However, few studies have reported a list of the common source inductance of commercially available packages. In order to list and compare the common source inductance among typical semiconductor packages, this paper carried out measurement of this inductance of actual switching devices mounted on experimental PCBs. This paper employed a recently proposed straightforward measurement technique directly applicable to switching devices mounted on PCBs. As a result, a basic database of the common source inductance of typical packages was presented. The common source inductance was found to be determined mainly by the package type. However, in TO-247 and TO-220 packages, approximately one-third of the total common source inductance was found to possibly vary among the switching devices of the same package due to the dependence on the current rating. Investigation of the physical package structure implied that this dependence was caused by the stray inductance of the bonding wires connecting the semiconductor chip to the source terminal.
2018 IEEE 27th International Symposium on Industrial Electronics (ISIE), 2018
Switched reluctance motors (SRMs) are promising for propulsion motors of electric vehicles (EVs) ... more Switched reluctance motors (SRMs) are promising for propulsion motors of electric vehicles (EVs) because of their robust mechanical construction and cost effectiveness. However, practical application of SRMs to vehicular propulsion is hindered by comparatively large torque ripple and radial force ripple, which cause noise vibration and acoustic noise. Previous studies have proposed phase current profiles which suppress both ripples. However, in the derivation process of phase current, these previous studies need numerical calculation hindering the EV development speed. The purpose of this paper is to propose a novel phase current profile to suppress both the torque ripple and the radial force ripple of SRMs without numerical calculation. The profile can be straightforwardly calculated using a simple analytical model of the SRMs. In addition, the profile can be determined for any reluctance profiles of actual SRMs. Experiment was carried out to verify the proposed method. The result supported suppression of the torque ripple and the acoustic noise by the proposed method.
2019 IEEE International Conference on Industrial Technology (ICIT), 2019
To estimate the core loss in SRMs, core loss estimation by equivalent circuit is preferable espec... more To estimate the core loss in SRMs, core loss estimation by equivalent circuit is preferable especially in the EV development because of its simple calculation. However, previously proposed models may limit applicable operating condition. These models refer instantaneous value, whereas hysteresis loop is a key factor to estimate core loss and the hysteresis loop directly associates with the history of magnetic flux. This paper focuses on the relation between the width of hysteresis loop and the peak to peak value of magnetic flux because the relation directly expresses the core loss. Therefore, the purpose of this paper is to propose a novel core loss model for equivalent circuit which estimates core loss in SRMs. This paper refers the peak to peak value of magnetic flux linkage and electrical angle. Along with the theoretical formulation of the model, this paper presents the model construction method. As a result, the proposed model successfully estimated core loss.
IEEJ Journal of Industry Applications, 2018
Recently, the dual transmitting resonator wireless power transfer system (DTR-WPT) has been propo... more Recently, the dual transmitting resonator wireless power transfer system (DTR-WPT) has been proposed as a promising technique for the power supply of mobile apparatus. Although this technique has been reported to be effective for increasing the output power as well as for covering a wide area during wireless power transfer, the complicated magnetic coupling among two transmitting resonators and one receiving resonator makes it difficult to develop practical design optimization methods, thus hindering practical applications of this technique. The purpose of this paper is to propose a design optimization method for the load impedance of DTR-WPT. This method is derived based on a novel simple equivalent circuit model of the DTR-WPT. The optimum impedance derived using this method as well as the appropriateness of the equivalent circuit were verified experimentally, thus validating usefulness of the proposed method for the practical application of DTR-WPT.
IEEE Journal of Emerging and Selected Topics in Power Electronics, 2018
Integrated magnetics is applied to replace the threediscrete transformers by a single core transf... more Integrated magnetics is applied to replace the threediscrete transformers by a single core transformer in a threephase LLC resonant converter. The magnetic circuit of the integrated transformer is analyzed to derive coupling factors between the phases; these coupling factors are intentionally minimized to realize the magnetic behavior of the three discrete transformers, with the benefit of eliminating the dead space between them. However, in a practical design, the transformer parameters in a multi-phase LLC resonant converter are never exactly identical among the phases, leading to unbalanced current sharing between the paralleled modules. In this regard, a current balancing method is proposed in this paper. The proposed method can improve the current sharing between the paralleled phases relying on a single balancing transformer, and its theory is based on Ampere's law, by forcing the sum of the three resonant currents to zero. Theoretically, if an ideal balancing transformer has been utilized, it would impose the same effect of connecting the integrated transformer in a solid star connection. However, as the core permeability of the balancing transformer is finite, the unbalanced current can't be completely suppressed. Nonetheless, utilizing a single balancing transformer has an advantage over the star connection, as it keeps the interleaving structure simple which allows for traditional phase-shedding techniques, and it can be a solution for the other multi-phase topologies where realizing a star connection is not feasible. Along with the theoretical discussion, simulation and experimental results are also presented to evaluate the proposed method considering various sources of the unbalance such as a mismatch in: 1) Resonant and magnetizing inductances 2) Resonant capacitors. 3) Transistor on-resistances of the Mosfets. 4) Propagation Delay of the gate drivers.
IEEE Transactions on Power Electronics, 2019
Resonant converters rely on a precise knowledge of leakage inductance of the equipped transformer... more Resonant converters rely on a precise knowledge of leakage inductance of the equipped transformers. Resonant circuit topologies such as LLC usually utilize the transformer leakage as an inductive component in the resonant tank, allowing for a drastic reduction in the converter weight, size and volume. The existence of the secondary leakage inductance affects the whole operation of the LLC resonant converter. This paper reveals that placing the secondary winding near the air gap would increase the resonant tank input impedance, vertically shrink the voltage-gain curve of the converter, and consequently minimize the frequency range (i.e frequency bandwidth with respect to load variation). On contrary, placing the secondary winding in a close contact with the magnetic core would decrease the resonant tank input impedance, vertically stretches the voltage-gain curve of the converter, and widen the frequency variation range. It has been reported that the winding location with respect to the air gap has an impact on the leakage inductance value. In other words, placing the secondary winding in a close contact with the magnetic core (zero mmf position) would maximize the leakage energy storage originated from the secondary winding, and hence maximize the secondary leakage inductance and vice versa. The theoretical discussion is presented which is merely based on Ampere’s law and Dowell’s model. Furthermore, transformer prototypes had been constructed and tested in a 390V/12V-220W LLC converter prototype to evaluate the proposed analysis.
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Papers by Kazuhiro Umetani