A series resonant DAB converter (SRDAB) is taken as the research object in this paper, and an efficiency optimization strategy for reducing the power return is proposed to improve the converter's operation efficiency under the phase shift modulation strategy. First, a mathematical model of the transmission efficiency of SRDAB is established under phase shift modulation, the mathematical expression for power return during its operation is derived, and the parameters that affect the power return are obtained. Second, the mathematical expressions for the transient and on-state losses generated during the operation of the converter are formulated, a mathematical model for its operation efficiency is constructed, and the relationship of its efficiency with the phase shift ratio and the ratio of switching frequency to resonant frequency is obtained, thus optimizing the two key parameters to further improve the operation efficiency. Finally, the correctness and effectiveness of the proposed efficiency optimization strategy were verified by performing simulations and building experimental circuits.

With the rapid development of microprocessors, more and more attention is paid to the advantages of digital control of microprocessor power supplies as the output capacitor and its equivalent series resistance (ESR) are gradually reduced, and sufficient stability is required for these digital multi-phase Buck controllers. Digital constant on-time multi-phase controllers are studied. First, the stability conditions of the system are derived through a digital compensation ramp (i.e., a ramp with a fixed slope in one switching cycle), the total inductor current information and the charge changes at both ends of the output capacitor. Then, the requirements for the slope are obtained using the stability conditions, where the effects of analog-to-digital converter (ADC) sampling delay and circuit propagation delay are considered. Conclusions can be drawn by classifying and analyzing the overlapped and non-overlapped duty cycles. Based on SIMPLIS simulations and by designing and changing the minimum compensation slope and the actual slope parameters, it is found that the simulation results are consistent with the theoretical analysis results, showing the accuracy of stability analysis for the digital multi-phase Buck converter.

A multi-objective optimal control method based on extended phase shift (EPS) is proposed for a dual active bridge (DAB) converter in order to simultaneously reduce the backflow power and improve its dynamic performance. First, the output power and backflow power characteristics of EPS in each mode are comprehensively analyzed, mathematical models are developed, and the optimal combination of the backflow power shift angles is solved according to the Karush-Kuhn-Tucker (KKT) condition. Second, a virtual voltage compensation scheme is used to improve the dynamic performance of the system by rapidly changing the transmission power at that moment. Finally, the proposed scheme was compared with the traditional single phase shift and EPS schemes through experiments, thereby verifying its effectiveness and advantage.

A double integral sliding-mode (DISM) controller is introduced into the control strategy for a four-switch Watkins-Johnson (WJ) converter, which can improve the stability of the converter under bipolar voltage output and wide resistive-inductive load conditions. However, the direct application of its controller has a problem that the state variables cannot be measured unless two current sensors and one voltage sensor are used. Aimed at this problem, a nonlinear resistance is used to approximately replace the output load impedance of the converter, a large-signal reduced-order model suitable for the control of a sliding-mode variable structure is established, and an improved reduced-state DISM controller is designed accordingly. The improved controller utilizes the matching of sliding-mode coefficients to eliminate the influence of equivalent nonlinear resistance, thus avoiding the problem that the state variables cannot be directly measured unless a large number of voltage and current sensors are used. Simulation and experimental results show that the reduced-state DISM controller only needs to use one voltage sensor to make the converter work stably under the condition of bipolar voltage output and wide resistive-inductive loads, and the dynamic response performance of its output is less affected by changes in the operating point, indicating a strong control performance.

Load resonance soft-switching can effectively reduce the switching loss of single-phase grid-connected inverters, which is a research hotspot in high-frequency inverter applications. However, the traditional PI control strategy based on linear models is easy to be unstable under large signal disturbance, and the system output waveform is poor. Since the passivity-based control strategy based on the nonlinear theory can effectively improve the output and stability of the system, a passivity-based control strategy for a load resonant soft-switching inverter based on an Euler-Lagrange model is proposed. A single-phase full-bridge grid-connected inverter in unipolar BCM (boundary conduction mode) modulation mode is modeled in detail, and a passivity-based controller based on the damping injection method is designed. To verify the effectiveness of the proposed control strategy, a simulation model of the resonant soft-switching inverter is built, and this strategy is compared with the traditional PI control strategy under steady-state and transient conditions.

Under the traditional phase disposition (PD) modulation strategy, there is a power imbalance problem among the units in a cascaded H-bridge inverter. Aimed at this problem, the control principle of freedom degrees is analyzed, and a novel power equalization modulation strategy is proposed. On the basis of ensuring the output power equalization and voltage harmonic characteristics, the power equalization time is shortened, and the complexity in digital control is reduced. On the one hand, the freedom degrees of triangular carriers are recombined based on the PD modulation method to distribute them evenly in different carrier layers. Then, by fully utilizing the switching redundancy state at each output level, the power equalization is achieved within a carrier cycle to shorten the time needed for achieving the equalization. On the other hand, the complexity in digital control is reduced by restructuring the vertical freedom degree of the modulated wave. Finally, the theoretical analysis and feasibility were verified by simulation and experimental results.

In a grid-connected system of non-isolated photovoltaic (PV) inverters, the electromagnetic interference causes the problem of leakage current. Aimed at this problem, an improved model predictive control strategy is proposed for a three-level inverter. First, a mathematical model of the grid-connected inverter system is established to analyze the generation mechanism of leakage current and its relationship with the common-mode voltage. Second, the common-mode voltage variation rate is controlled to improve the model predictive direct power control algorithm, thereby optimizing the optimization result to achieve the suppression of leakage current. Finally, the conventional strategy and the proposed improved model predictive control strategy are compared and analyzed through simulations in terms of output power, harmonic distortion of grid-connected current, DC-side neutral point potential balance and leakage current amplitude. Results show that the proposed strategy performs well in all the above four aspects, and its leakage current suppression effect can reach 99.5% compared with the conventional model predictive control strategy.

To solve the problem of large leakage current in a photovoltaic (PV) grid-connected inverter, a low-leakage current HERIC transformerless single-phase PV grid-connected inverter topology is proposed. This inverter topology only needs eight power switching tubes, four of which constitute four quasi-bidirectional voltage switching units. The four quasi-bidirectional voltage switching units are divided into two groups, and they alternate conduction with grid frequency to realize continuous current, thus reducing the loss of switching tubes and effectively suppressing the common-mode leakage current. Finally, the proposed topology was verified by simulation and experimental results.

Harmonic pollution is one of the key problems restricting the grid connection of new energy sources. There are many factors that lead to an increase in current harmonics of grid-connected inverters, including the harmonic voltage of the grid and the sampling error of voltage and current signals in the grid-connected inverter system. On the basis, the principle of voltage sampling error and current sampling error of the grid-connected inverter system affecting the 2nd-order harmonic current of the inverter is analyzed, and a 2nd-order harmonic current suppression method based on high-precision harmonic sampling and 2nd-order harmonic current three-phase double-frequency rotation dq feedback is studied. In addition, a model of the 2nd-order harmonic current suppression loop is established, a design method for the parameters of the 2nd-order harmonic current suppression loop is given, and the expression of suppression effect is deduced. Finally, the proposed method was verified by experimental results of a 100 kW prototype.

To reduce the calculation amount in the model predictive current control (MPCC) algorithm for a three-level inverter model, eliminate the weighting factor, achieve a fixed switching frequency and improve the steady-state performance of the system, a three-vector fixed frequency MPCC algorithm based on vector set selection is proposed. First, through the analysis of the relationship between current and voltage, the control of current under the rotational coordinate system is converted into the control of voltage under the stationary coordinate system by using the beat-free idea, and the candidate vector set is selected according to the sector where the reference voltage is located, thus reducing the calculation amount. Second, to achieve the midpoint potential control, according to the different effects of different switching vectors of a neutral point clamped (NPC) three-level inverter on the neutral point and the principle of switching smoothness, different candidate vector sets are reasonably selected, and the switching vector action time is obtained by using the principle of volt-second balance. Afterwards, the optimal switching sequence is selected according to the evaluation function. Finally, the traditional MPCC and the proposed algorithm are compared, and experimental results show that the proposed algorithm has advantages such as small calculation amount, fixed switching frequency and low current harmonic content.