Kimihiro Nanamori

Verification of novel recovery-less boost converter with saturable inductor

Higher efficiency performances, lower EMI/RFI noise, and low cost boost converter have been required in power conversion system for Electric Vehicle, Plug-in Hybrid Electric Vehicle. Recovery-less boost converter are satisfied all those demands, without active power switch. So far, recovery-less boost converter are proposed two method; with the auxiliary inductor type or the saturable inductor type. The purpose of this paper is to indicate the effectiveness of recovery-less boost converter with the saturable inductor. First, the problem of recovery-less boost converter using auxiliary inductor is taken up from trade-off between reduction of recovery loss and deterioration of circuit performance. Second, the principle of recovery-less boost converter using saturable inductor which can solve the trade-off problem, is described. Finally, the effectiveness of recovery-less boost converter with the saturable inductor is discussed from experimental point of view.

Three-phase LLC resonant converter with integrated magnetics

Recently, Electric Vehicles (EVs) have required high power density and high efficiency systems in order to save energy and costs. Specifically, in the DC-DC converter that feeds the non-propulsive loads in these vehicles, where the output voltage is much lower than the one of the energy storage unit. Therefore, the output current becomes quite high, and the efficiency and power density are reduced due to the high current ratings. Furthermore, magnetic components usually are the biggest contributors to the mass and volume in these converters. This paper proposes a Three-phase LLC resonant converter with one integrated transformer where all the windings of the three independent transformers are installed into only one core. Using this technique, a high reduction in the core size and thereby an increment in the power density and a reduction of the production cost are obtained. In addition, this integrated transformer is intended to be applied in the novel Three-phase LLC resonant converter with Star connection that is expected to offer reduction of the imbalanced output current, which is produced by tolerances between the phase components. Finally, the proposed converter with the novel integrated transformer is discussed and evaluated from the experimental point of view. As a result, a 70% reduction in the mass of the magnetic cores was achieved.

Oscillation Analysis and Current Peak Reduction in Paralleled SiC MOSFETs

Parallel connection of power metal oxide semiconductor field effect transistors (MOSFETs) is often used in the high current side of power conversion systems to obtain a thermal dispersion and low conduction losses. However, a parallel connection may lead to a current unbalance due to the difference of parasitic parameters and switching characteristics of the paralleled devices. The current unbalance generates current oscillations, and in the worst case, it may lead to complete destruction of the power devices. This study analyses an inherent oscillation of two paralleled SiC MOSFETs, under current unbalance conditions. Based on the proposed analysis, it is found that the parasitic inductance is the main cause of the coupled oscillation, which is composed of two different oscillation frequencies. In this study, the coupled oscillation leads to a difference of peak currents between paralleled devices. The circuit conditions, considering the parasitic inductances, are investigated to suppress the coupled oscillation. As a result, a reduction of the common parasitic inductance allows preventing the coupled oscillation and to suppress the peak combined current of paralleled devices. Moreover, a peak current reduction by 37.8% can be achieved, as a result of eliminating the coupled oscillation.

A Current Sharing Method Utilizing Single Balancing Transformer for a Multiphase LLC Resonant Converter With Integrated Magnetics

Integrated magnetics is applied to replace the three-discrete transformers by a single core transformer in a three-phase 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 multiphase 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 cannot 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 multiphase 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; and 4) propagation delay of the gate drivers.

Analytical investigation of interleaved DC-DC converter using closed-coupled inductor with phase drive control

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 DC-DC converter using coupled inductor with phase drive control (PDC) is analyzed. In addition, defensive method of this method.

Analytical investigation on design instruction to avoid oscillatory false triggering of fast switching SiC-MOSFETs

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.