The concept of vehicle-to-grid has received considerable attentions over the past decade. Electric vehicle (EV), a common load of DC microgrids, can serve as distributed energy storage system to improve the reliability and stability of the DC microgrid, support integration of renewable energy source, and improve overall system efficiency. In order to achieve bidirectional power transmission between the DC microgrid and EV, this paper proposes a novel three-level CLLC resonant converter for off-board EV charger. By adding resonant CLLC components and by combining the working modes of the two three-level full bridges, the proposed converter adapts to the wide voltage range of EV, from 200 V to 700 V. An equivalent circuit model with first harmonic approximation approach is established to analyze the frequency characteristics of the resonant converter. Moreover, an algorithm of working mode selection is proposed on the principle of minimum transformer RMS current. Finally, a 3.5 kW hardware prototype was built to verify the feasibility and the advantage of the three-level CLLC resonant converter. The efficiency of the proposed converter changes little over a wide output voltage range, and the flying capacitor voltages of the proposed converter are balanced well.

Magnetically coupled resonance wireless power transfer systems (MCR WPT) have been developed in recent years. There are several key benefits of such systems, including dispensing with power cords, being able to charge multiple devices simultaneously, and having a wide power range. Hence, WPT systems have been used to supply the power for many applications, such as electric vehicles (EVs), implantable medical devices (IMDs), consumer electronics, etc. The literature has reported numerous topologies, many structures with misalignment effects, and various standards related to WPT systems; they are usually confusing and difficult to follow. To provide a clearer picture, this paper aims to provide comprehensive classifications for the recent contributions to the current state of MCR WPT. This paper sets a benchmark in order to provide a deep comparison between different WPT systems according to different criteria: (1) compensation topologies; (2) resonator structures with misalignment effects; and, (3) electromagnetic field (EMF) diagnostics and electromagnetic field interference (EMI), including the WPT-related standards and EMI and EMF reduction methods. Finally, WPT systems are arranged according to the application type. In addition, a WPT case study is proposed, an algorithm design is given, and experiments are conducted to validate the results obtained by simulations.

This paper analyzes the problem of instability in enhancement-mode gallium nitride (GaN) transistors based half-bridge circuits. The instability may cause sustained oscillation, resulting in overvoltage, excessive electromagnetic interference (EMI), and even device breakdown. GaN devices operate in the saturation region when they conduct reversely during the dead time. Under the influence of parasitic parameters, the GaN-based half-bridge circuit exhibits positive feedback under certain conditions, thus, resulting in sustained oscillation. A small-signal model is proposed to study this positive feedback phenomenon. Like the second-order under-damped system, damping ratio is defined to determine the system's stability. Based on the model, the influence of circuit parameters on instability is investigated and guidelines to suppress the oscillation are given. Reducing the common-source inductance, increasing the gate resistance of the inactive switch or connecting a diode in parallel to the inactive switch are some effective ways to suppress the oscillation. Finally, the analyses are verified by both simulation and experiment.

One promising topology for solid state transformer (SST) is modular multilevel cascaded converter, in which sub-module is composed of dual active bridge (DAB) and H-bridge. For SST application in PV system, the efficiency could be severely affected especially for DAB due to the wide voltage and power range of PV panels. Thus, the motivation of this paper is to deal with the control strategy to improve DAB efficiency inside SST for PV application. Instead of utilizing time-domain based analysis method, which requires complex modeling process, this paper models DAB under frequency domain by fully considering the effect of both fundamental and harmonic frequency component. To achieve high efficiency operation, multi-order reactive-current suppression (MRS) strategy is proposed for DAB by controlling phase shift angle as well as the duty cycles of primary and secondary side H-bridges. In terms of DAB controller design inside SST, the small signal model under MRS is established and notch filter is implemented to suppress the second order line frequency fluctuation in the control loop. Finally, 10kV/1MW SST prototype is introduced along with the system control structure and implementation method. The experiment of sub-module and SST confirms the effectiveness of proposed method.

DD planar coil with ferrite core is one popular coupling structure in wireless power transfer (WPT), especially for electric vehicle applications. The coil design is critical for improving the performance of WPT system. However, the coupler design purely relying on finite element analysis (FEA) software is time-consuming. Literature on the modeling of DD coil is lacking. Therefore, the model of DD coil with a ferrite core for calculating self-inductance, mutual inductance, and ac resistance is proposed in this article. The condition when primary and secondary coils have different sizes is also considered. The case that there is misalignment is also analyzed. Meanwhile, a genetic algorithm based on the proposed coil mathematical model is proposed to optimize the coil structure. For verifying the accuracy of the proposed model, the simulation model based on the FEA and a 1.2-kW laboratory prototype are built simultaneously. The calculation values based on the proposed model is highly consistent with the simulation results and experimental values. Error for calculating self-inductance and mutual inductance is within 5% when coils on the primary side and secondary side are in alignment. The error for calculating the ac resistance of the coil is within 10% when the frequency range is 0–200 kHz.

Semiconductor devices based solid-state circuit breakers (SSCBs) are promising in the DC power distribution system as protective equipment for their ultra-short action time. This letter proposes a topology of SSCB using series connected silicon carbide (SiC) MOSFETs, which only requires a single isolated gate driver. The SSCB has very low cost and high reliability because it only has 13 components including passive components and diodes apart from two SiC MOSFETs to achieve both balanced voltage distribution during short-circuit interruption duration and reliable positive gate voltage during on-state. The SSCB prototype is built and experimentally verified to interrupt 75 A short-circuit current under the DC bus voltage of 1200 V within 1.5 μs.