The calculation of high-frequency transformer winding losses is crucial for optimizing the efficiency of CLLC converters. However, there remains a lack of accurate and efficient methods for calculating the winding losses, particularly the winding losses in Litz wire. A mutual-impedance equivalent circuit-based method for calculating the winding losses in Litz wire high-frequency transformers is proposed. First, the analytical approaches for calculating the Litz wire winding losses based on a one-dimensional Dowell model and a two-dimensional Bessel function method are comparatively analyzed. Then, a mutual-impedance equivalent model is established to simplify the calculation of winding losses. A specific case study is given, and the values calculated using the proposed method are compared with the impedance matrix results derived from two-dimensional finite element simulations. Results demonstrate that the relative error between the equivalent series resistances calculated using the proposed method and the results based on the finite element method does not exceed 6%, validating the effectiveness and accuracy of the proposed method. Finally, a high-frequency transformer prototype for the CLLC converter was constructed, and experimental measurements verified the feasibility of the proposed method.

A low-ripple DC-DC converter with quasi-Z-source coupled inductance is proposed, which integrates a quasi-Z-source network, a coupled inductor and voltage doubler cells. The quasi-Z-source network has functions of Boost voltage and passive clamp. While the converter achieves a high gain, the switching tube realizes passive clamp and recovers leakage energy from the coupled inductors. When the duty cycle is in the range of 0–0.5, the high gain, high efficiency and low ripple of input current required by new energy power generation can be achieved, avoiding the influence of an extreme duty cycle. The steady-state analysis, efficiency analysis and experimental verification for the converter were carried out, and its performance was compared with those of similar converters. A 200 W prototype was fabricated in a laboratory, and experimental results were basically consistent with the theoretical analysis.

An enhanced extended phase shift modulation strategy for an LCL-type full-half bridge resonant converter is proposed, which can achieve zero voltage switching (ZVS) for all the switches while eliminating the backflow power on the primary side. Meanwhile, the half-bridge structure on the secondary side enhances the cost-effectiveness of the converter under low-power conditions. The steady-state analysis of the converter is performed by using fundamental harmonic approximation, so as to derive a power model and the corresponding ZVS conditions. In addition, simulations are conducted using the software PSIM. Finally, a 300 W experimental prototype was built, and results verified the feasibility of the theory.

A SiC/Si hybrid five-level inverter and its optimized modulation strategy are proposed. The proposed topology consists of four SiC MOSFETs and four Si IGBTs, and it utilizes DC-link split capacitors to achieve a five-level structure. Under the proposed modulation strategy, all the high-frequency switching loss is distributed to the four SiC MOSFETs, while the four Si IGBTs only generate conduction losses and part of the junction capacitor charging and discharging losses. The analysis results of loss modeling show that the proposed SiC/Si hybrid modulation strategy can optimize the loss distribution, fully utilize the low-switching-loss advantage of SiC MOSFET and the low-conduction-loss advantage of Si IGBT under large current conditions. Compared with the full Si scheme, the proposed SiC/Si hybrid scheme significantly improves the conversion efficiency while only slightly adding the hardware cost. Compared with the full SiC topology, the proposed SiC/Si hybrid scheme achieves a similar efficiency while obviously reducing the hardware cost. In addition, under heavy load conditions, the proposed SiC/Si hybrid scheme achieves a higher efficiency than the full SiC scheme. An experimental platform of 4 kW SiC/Si hybrid five-level inverter was built to validate the feasibility and effectiveness of the proposed SiC/Si hybrid five-level inverter topology and its optimized modulation strategy.

An improved power balance modulation strategy based on carrier stacking modulation is proposed to address the issue of output power imbalance caused by different modulation methods used in low-voltage units in hybrid cascaded H-bridge inverters under the traditional modulation strategies. Based on the topology of an asymmetric hybrid cascaded H-bridge 11-level inverter, the symmetrical characteristics of the low-voltage unit control pulse signal under carrier stacking modulation were analyzed. Moreover, the low-voltage unit pulse control signal was segmented using T/4 or T/2 as cycles, and all the control pulse combinations under each segmentation method were given. The optimal working mode was selected based on the minimum carrier shift times and the shortest power balance time. As a result, the output voltage between low-voltage units was symmetrical, and the output power of low-voltage units was self-balanced. Finally, the proposed modulation strategy was validated through simulation and experiments.

Aimed at an offshore wind power flexible DC transmission project, the problems of the large mass and volume of modular multilevel converters (MMCs) and the high construction cost of offshore converter stations need to be solved urgently. Therefore, the lightweight research of MMC is one of the research hotspots at present. First, an optimized configuration MMC (OC-MMC) is proposed, and its configuration method is given. In addition, a voltage balancing strategy is designed based on the nearest level modulation. The novel converter improves the number of output voltage levels on the AC side by introducing two submodules whose capacitance values are the same but the rated voltage values are proportional by a factor of K. Then, through the simulation results on a MATLAB/Simulink platform and experimental results, it is found that the novel converter can successfully complete the voltage conversion process. The steady-state operation characteristics of the system are similar to those of a conventional MMC, and the dynamic characteristics are good. On the premise that the numbers of AC voltage levels of the two converters are the same, the proposed OC-MMC can indirectly reduce the number of submodule assembly and reduce the mass, volume and construction cost of the converter stations compared with the conventional MMC, so as to achieve the goal of lightweight.

A typical configuration of a DC solid-state transformer is formed by connecting standardized power conversion cells in an input-series output-parallel (ISOP) arrangement to achieve DC transformation from medium voltage to low voltage. To provide redundancy for the power conversion cells, a thyristor bypass circuit is typically required. While DC solid-state transformers generally employ SiC MOSFET devices, the high dv/dt characteristic of SiC MOSFETs can readily false triggering in the thyristor bypass circuit. The issue of false triggering in thyristor bypass circuits is focused on, and a co-design methodology involving the thyristor snubber circuit and the gate-parallel resistor is explored. The proposed approach aims to enhance the operational reliability of the thyristor bypass circuit in high dv/dt environments, and experimental validation is provided.

The high common-mode voltage (CMV) generated by pulse-width modulation in a power converter will cause overvoltage on the motor shaft and further lead to the high-frequency high-amplitude current therein, thereby damaging the motor insulation and increasing the electromagnetic interference. To reduce the CMV of a neutral point clamped (NPC) three-level converter, a CMV suppression strategy based on the framework of algebraic modulation was proposed. First, the issue of duty-cycle assignment was transformed into a problem of solving non-homogeneous linear equations. Second, the degrees of freedom for modulation were properly selected to realize the objective of neutral point voltage (NPV) balance. Third, according to the duty-cycle constraints, the offset signals were selected to obtain the final duty-cycle. Finally, the switches in the phase with intermediate voltage values adopted inverse carrier to obtain a new combination of switching states, so as to reduce the CMV. The proposed modulation strategy can be easily implemented without additional controllers. On the premise of ensuring that the NPV of the NPC three-level converter has good balance characteristics, the CMV can be reduced to the range of one-sixth of the DC-side voltage. The effectiveness and feasibility of the proposed modulation strategy were validated by experimental results.

In an islanded microgrid, the inverter filter circuit parameter perturbation and the insufficient steady-state performance of a PI controller will cause steady-state errors in the output signal, and the unbalanced loads, nonlinear loads and load switching will cause the degradation of dynamic performance. A performance compensation control structure based on a residual generator is proposed, and the gradient descent method is used to optimize the compensation controller in real time to enhance the system’s steady-state and dynamic performances. First, the transfer function model of an inverter under double closed-loop control is established, and the effects of filter circuit parameter perturbation, insufficient steady-state performance of the proportional-integral (PI) controller and load disturbance on the inverter’s output voltage performance are analyzed. Second, the performance compensation controller Q(s) in a quasi-resonant form is designed based on the real-time residual values obtained from the residual generator. Third, an optimization objective function is constructed based on the PI controller current-voltage error and the control input signal, and the compensation controller is optimized in real time using the gradient descent method. Finally, experiments were conducted using a DSP control board with a PXI semi-physical simulation platform to verify the effectiveness of the proposed method.

LLC converters are widely applied in the fields of household energy storage and distributed energy storage owing to their excellent features such as a good soft-switching range and a high power density. In the modulation design of LLC converters, to achieve a high efficiency, it is necessary to meet the conditions for resonant soft-switching based on the design of hardware parameters with a low overall power loss of the converter. The commutation process during the dead-time is analyzed to explore the influences of the parasitic parameters of switching devices on the realization of soft-switching in the converter. By optimizing the switching timing sequence during the dead-time and utilizing the circuit mechanism where the parasitic parameters of switching devices participate in resonance, the realization of soft-switching is facilitated, ensuring a higher operation efficiency. Considering the loss distribution during the dead-time, the impact of not changing the modulation under bidirectional power flow conditions on the efficiency is analyzed. Finally, verification was performed by building a 4 kW LLC prototype to carry out a bidirectional power transmission experiment.