Masatoshi Uno

Adequate preheating for electric vehicles' lithium-ion batteries (LIBs) in cold climates is indispensable to prevent a decrease in cruising distance. Internal ac heating techniques utilizing Joule heat originating from LIBs' internal resistances have been proposed as an efficient and rapid heating method. However, an additional inverter is necessary to generate ac currents, resulting in increased system complexity. Furthermore, conventional triangular and sinusoidal current waves are not ideal as their rms values at a given peak current are relatively low. This article proposes a dual active bridge (DAB) converter integrating an ac internal heating inverter. A full-bridge circuit is utilized not only for the DAB converter but also for the ac heating inverter, achieving a simplified system. A novel PWM technique is also proposed to generate a trapezoidal current wave that achieves higher rms currents at a given peak current. A 400-W prototype with 50-W internal heating capability was built for a 48-V 3400-mAh LIB. Experimental results demonstrated that, in addition to the bidirectional power conversion capability, the trapezoidal wave could warm up the LIB from −10°C to 0°C in 2.6 minutes.

Preheating for automotive lithium-ion batteries (LIBs) is indispensable to enhance cruising distance at subzero temperatures and to improve fast-charging performance. Internal ac heating techniques that utilize Joule heat originating from LIBs’ internal resistances are an efficient heating method. However, an additional inverter is necessary to generate ac currents for LIBs, increasing the system complexity and cost. This article proposes an integrated cell equalizer that operates either in a cell equalization mode or internal ac heating mode. The proposed equalizer is based on a selective module-to-cell equalizer using a dual active bridge (DAB) converter. The DAB converter in the equalization mode exchanges energies between the LIB module and cells. In the ac heating mode, a winding inductance of a transformer resonates to generate ac heating currents for the LIB module. A prototype for a 3400-mAh twelve-cell module was built and tested. Experimental results demonstrated the successful internal heating capability and cell equalization performance.

This paper proposes a fault tolerant modular differential power processing (DPP) converter for photovoltaic (PV) systems under partial shading conditions. The proposed DPP converter can be derived by integrating panel-level and module-level DPP converters, reducing the total switch count, and achieving a simplified circuit. The modular design of the proposed converter allows PV systems to be flexibly scaled up without redesigning converters. Furthermore, series-resonant tanks in the proposed DPP converter exhibit constant-current–constant-voltage characteristics that realize the inherent fault tolerance against short-circuit and open-circuit fault of PV panels. The prototype for a 1.5-kW PV system consisting of two modules, each comprising four panels connected in series, was built and tested. In addition to the fault tolerance, the energy yield enhancement of 19.6% was experimentally demonstrated.

The occurrence of partial shading is known to significantly reduce the energy yield of photovoltaic (PV) panels. Differential power processing (DPP) converters can prevent negative influences of partial shading at a substring level. For PV strings comprising numerous series-connected panels, however, conventional DPP converters face various issues, such as increased converter count, poor extendibility, and cumulative power conversion loss. This article proposes a modular DPP converter with an isolated dc bus for PV strings. The proposed DPP converter is the integration of panel- and module-level DPP converters. Four panels are grouped as a module having a DPP converter, and each DPP converter can handle four panels. PV systems can be scalable with no need to redesign the DPP converter as all modules are connected through the isolated dc bus. The isolated dc bus allows the direct power transfer from unshaded high-voltage modules to shaded low-voltage ones, reducing the cumulative power conversion loss. A field testing for a three-module PV string was performed using a prototype for a four-panel 1-kW module. The experimental results demonstrated 5.6% energy yield enhancement by the proposed DPP converter.

Ordinary modular energy storage systems require cell- and module-level equalizers, in addition to a main bidirectional converter, increasing the system complexity and cost. This article proposes a bidirectional buck-boost converter using cascaded energy storage modules. Each module contains a cell-level equalizer with a half-bridge cell. The half-bridge cell in each module is utilized not only for the cell-level equalizer but also for the cascaded buck-boost converter. Module voltages are equalized by adjusting duty cycles of the half-bridge cells, allowing the elimination of dedicated module-level equalizers. Furthermore, the cascaded structure equivalently increases the switching frequency and reduces the inductor current ripple. An experimental charge-discharge cycling test using the prototype was performed for three electric double-layer capacitor modules. Cell and module voltages were gradually equalized during cycling, demonstrating the bidirectional power conversion and cell- and module-voltage equalization capabilities of the proposed converter.

Voltages of series-connected energy storage cells, such as electric double-layer capacitors (EDLCs) and lithium-ion batteries, need to be equalized to ensure years of safe operation. However, to this end, a voltage equalizer is necessary in addition to a charger, increasing the system complexity and cost. This paper proposes a family of transformerless single-switch integrated chargers that merge a charger and equalizer into a single unit, achieving a simplified system and circuit. Proposed integrated chargers are derived by stacking multiple conventional pulse width modulation (PWM) converters, such as a superbuck converter, that contain two inductors and one energy transfer capacitor. Detailed operation analyses, including an investigation on the impact of component tolerance on voltage equalization performance, are also performed. Experimental charging tests using a 12-W prototype were performed for four EDLC cells. All cells were charged with eliminating voltage imbalance and demonstrating the charging and equalization performance of the proposed integrated charger.

Multiport converters (MPCs) that can regulate multiple input/output ports have been proposed to reduce the converter count in electric vehicle battery systems consisting of multiple batteries (12-V, 48-V, and high-voltage batteries). Conventional MPCs consist of an interleaved PWM converter and DAB converter, and an active current balancing control is indispensable for the interleaved converter. Furthermore, the transformer utilization unavoidably decreases because switches are driven with a nominal duty cycle of 0.75 for 48-/12-V batteries. This article proposes a novel MPC with an automatic current balancing capability and improved transformer utilization. A flying capacitor (FC) is added to the interleaved PWM converter to automatically balance inductor currents. The added FC also allows operations with a nominal duty cycle of 0.5 for 48-/12-V batteries, improving the transformer utilization. The experimental verification tests using a 1-kW prototype demonstrated the automatic current balancing capability, improved transformer utilization, and flexible power exchange among three batteries.

In photovoltaic (PV) panels comprising multiple substrings, a mismatch in substring characteristics is known to reduce energy yield significantly. Substring characteristics in solar roofs of plug-in hybrid vehicles (PHVs) are always mismatched to some extent due to the curved surface of the panel. This paper proposes a PWM differential power processing (DPP) converter based on a panel-to-substring power redistribution architecture and its dual maximum power point tracking (MPPT) control technique for PHEVs’ solar roofs. Depending on the degree of characteristic mismatch, the proposed DPP converter adjusts its output current to maximize the energy yield of the panel. With the proposed dual MPPT control, a duty cycle dDPP of the DPP converter is manipulated to optimize the panel characteristic while an external boost converter manipulates its duty cycle Dboost to track an MPP of the panel. A prototype for 180-W solar roofs comprising seven substrings was built, and laboratory testing was performed using solar array simulators to emulate characteristic mismatch conditions. Thanks to the dual MPPT control technique, the proposed DPP converter operated with the optimal dDPP while the external boost converter tracked the MPP by manipulating Dboost, realizing the enhanced energy yield.

Multiport converters (MPCs) that integrate multiple converters into a single unit have been proposed to reduce the converter count in electric vehicle power systems where multiple batteries are employed. However, conventional MPCs comprising a dual active bridge (DAB) converter and an interleaved PWM converter require feedback control loops and current sensors to balance the inductor currents. Furthermore, the transformer utilization decreases as the duty cycle of the interleaved PWM converter drifts away from 0.5. This paper proposes a novel MPC that integrates a DAB converter and an interleaved PWM converter with an automatic current balancing capability and improved transformer utilization. A 1-kW prototype with a half-bridge or full-bridge circuit on the high-voltage side was built for the experimental verification, and loss analysis was performed. The results demonstrated the automatic current balancing capability, improved transformer utilization, and flexible power exchange among three ports.

Voltages of electric double-layer capacitor (EDLC) modules vary rather wider than traditional secondary batteries. Although EDLCs should desirably be cycled in a voltage range as wide as possible to achieve a high energy utilization ratio, the wide voltage variation of EDLC modules impairs the performance of DC–DC converters. To address such issues, previous works reported series-parallel reconfiguration techniques, which are roughly divided into balance- and unbalance-shift circuits. However, conventional balance-shift circuits are not applicable to modules comprising odd number cells, impairing modularity. Unbalance-shift circuits, on the other hand, unavoidably cause cell voltage imbalance that reduces energy utilization ratio. This paper proposes a novel series-parallel reconfigurable EDLC module with cell voltage equalization capability. The proposed reconfigurable EDLC module is applicable to any number of cells, realizing good modularity. Furthermore, all cells in the proposed module can be charged and discharged uniformly without generating cell voltage imbalance, achieving an improved energy utilization ratio compared with conventional techniques. A five-cell module prototype was built for experimental verification. While the module voltage varied between 1.04 and 2.83 V, all cells discharged from 2.5 to 0.3 V. The result is equivalent to a 98.6% energy utilization ratio.