The CLLC resonant converters are drawing more and more attention for their superiority in soft switching, wide output range and symmetrically bidirectional operation. However, CLLC converter still suffers the problems of unsatisfactory voltage regulation and low efficiency under light-load conditions. Therefore, this paper proposes an optimization for extended phase-shift (EPS) control in full-bridge CLLC resonant converter to improve the light-load efficiency. In order to study the relations between phase-shifts of EPS control and converter efficiency, a detailed circuit model is first established to solve the voltage gain, time-domain expressions of main circuit variables and root mean square (RMS) values of resonant currents. Then, a comprehensive loss evaluation is conducted by calculating and analyzing the main power losses, including conduction loss, switching loss and core losses of magnetic components. Besides, zero voltage switching (ZVS) conditions of primary and rectifier switches are derived to define the range of soft switching. Finally, based on these analyses, the optimal combination of phase-shifts for EPS control is determined to achieve the maximum conversion efficiency of full-bridge CLLC converter at light-load conditions. The validity of the proposed optimized EPS control is verified on a 18~25V/400V, 200W GaN based CLLC resonant converter prototype.

Dual-active-bridge (DAB) based dc–dc converter is usually chosen for energy conversion. In this article, there exist many coupled relationships such as the controversy between local minimum current stress and transmission power, which influence efficiency promotion greatly. To analyze complicated coupled relationship and obtain the global minimum values, after unifying backflow power and transmission power into one integral function, a novel extended-phase shift (EPS) control based on complete differential algorithm is proposed, which can not only eliminate the backflow power completely, but can also realize a type of multiobjective optimizations including improvement of transmission power and decrease of current stress. Through the proposed method in this article, the efficiency can be improved gradually. Moreover, the coupled relationships among variables are clarified clearly, which can avoid power loss increase during optimization. A DAB-based prototype is established and the experimental results verify the correctness and effectiveness of the proposed method.

With inhomogeneous irradiance on photovoltaic (PV) units significantly deteriorating the effectiveness of solar energy harvest, distributed maximum power point tracking (DMPPT) concept is considered as an effective solution against resultant mismatch. A comparative analysis of full power processing and differential power processing structures, as two main branches of DMPPT solution, is provided first. Advantages and disadvantages of widely used structures are analyzed and compared from different aspects. To harvest more power and realize better utilization of PV units, this paper presents a submodule level differential power processing PV system with a flexible double stage time-sharing maximum power point tracking (MPPT) control strategy. The proposal can achieve the accurate maximum power points (MPPs) of all PV submodules regardless of mismatch cases with only one adaptive MPPT controller, facilitating system integration and largely reducing system cost and conversion loss. The efficacy of the scheme is analyzed using Simulink and validated by experimental results.

Inertia emulation in inverter control strategies benefits in improving dynamic frequency performance in microgrid. Therefore, some inverter control strategies introduce virtual inertia, and the corresponding performance is investigated. These strategies mainly focus on improving frequency dynamics from the perspective of inverter, while their dynamic influence to ac bus and the interaction among different inverters are still unclear. This paper aims to investigate the dynamic frequency performance of multiple generation units in both grid-connected and islanded mode considering inertia interaction. A transfer function model of paralleled inverters with emulated inertia is set up for theoretical analysis. With this model, frequency and phase response of ac bus is studied under several cases, which reveals the interactive frequency performance of voltage-controlled inverters. Furthermore, inertia interaction among different inverters is analyzed in detail, which indicates the reaction from grid to inverter. Consequently, the effectiveness and ineffectiveness of virtual inertia in improving frequency dynamics are demonstrated, which is practical and helpful for virtual inertia imitation in microgrid. Simulation and experimental results are provided to validate its effectiveness.

The active power control of increasing renewable energy resources is a growing concern. For example, solar energy exploitation is highly dependent on the central controller and other resources. Previous research has introduced some solutions for this problem, but the performance is usually unsatisfactory in power allocation, communication dependence, mode switch, and algorithm complexity. This article proposes a straightforward but effective strategy for the two-stage photovoltaic (PV) inverter, which uses the voltage-control method to adjust the PV inverter’s output power and match the load demand. Moreover, reference power tracking is used instead of traditional maximum power point tracking. The mode detection and switch strategies are proposed to solve the power shortage problem, making the PV inverter maintain the voltage-control method even in the power shortage state. Even without adequate support from other facilities, the proposed method can control the PV inverter to organize an islanded microgrid if PV arrays’ maximum power is larger than the load demands. Therefore, solar energy can further participate in grid support and ensure that the microgrid is more stable, robust, and economical. Finally, the proposed methods are verified in simulation and experiment.

Dual active bridge (DAB) is a popular interface between energy sources and electric vehicle storage systems because of its galvanic separation, bidirectional power transmission, and extensive voltage adjustability. To overcome the drawbacks of conventional modulations, an optimal primary-side duty modulation (OPDM) is proposed in this article, which combines phase-shift modulation and asymmetrical duty modulation on the primary side to achieve a compromise between high efficiency performance and lower control complexity. To begin with, under traditional DAB modulations, modes attaining all devices zero voltage switching (ZVS) are frequently associated with high peak current stress or indirect power transfer, which can be avoided by OPDM. Second, under OPDM, all six typical operation modes can realize all semiconductors ZVS, where full-power-range ZVS can be realized simply to reduce switching loss. Third, under fixed transmission power, the minimum peak-to-peak current stress can be obtained through Lagrange multipler method. Peak current, rms current, and soft-switching characteristics under OPDM outperform conventional control methods. Eventually, a DAB prototype is established to verify the effectiveness of proposed OPDM method.