A passive current sharing method for paralleled LLC resonant converters is proposed in this article, and the current sharing effect of paralleled common capacitor LLC resonant converter is analyzed. The passive current sharing method is achieved by adding coupled impedance between the LLC resonant converters, and the coupled impedance can be inductive, capacitive, infinitely small, and infinitely large. Among them, it is traditional paralleled LLC resonant converter without current sharing ability when the coupled impedance is infinitely large. First, the mathematical model of paralleled common capacitor LLC resonant converter with coupled impedance is built based on fundamental harmonic analysis. Second, according to the mathematic model, the current sharing effect of the paralleled common capacitor LLC resonant converter of different coupled impedance under different parameter difference is analyzed, it is easy to see the current sharing effect is improved after the couple impedance is added, and in general, inductive coupled impedance has a better current sharing effect for paralleled common capacitor LLC resonant converter. Last, the theoretical analysis results are verified by simulation and experimental results.

Variable switching frequency sinusoidal pulsewidth modulation (VSFSPWM) method does well in many aspects such as lower harmonic peak and losses compared with constant switching frequency sinusoidal pulsewidth modulation (CSFSPWM) for DCAC converters. This article presents a passive VSFSPWM (PVSFSPWM) method for spectrum improvement, filter inductor reduction, and losses reduction, obtaining similar output current quality as that of the CSFSPWM method. Because the PVSFSPWM method does not need any feedback information compared with the model-predictive-based method, it is an open-loop method and is called the passive method. In order to obtain the law of harmonic distribution, the mathematical expression of harmonic spectrum distribution is derived. The required filter inductor under the same current ripple and the losses between CSFSPWM and PVSFSPWM methods have been compared. Simulation and experimental results show that the PVSFSPWM method achieves excellent performance in EMI suppression, inductance, and losses reduction without impairing the output quality.

Two-stage single-phase inverter using dual-mode control can match wide input voltage variation range and increase efficiency. That is studied by many researchers. In addition, using dual-mode control can improve the line-to-line output voltage waveform to be more easily filtered into sine wave. Thus, the traditional method to determine the LC filter parameters is unreasonable and will cause over filtering. The redundant design of filter will bring unnecessary volume, weight and loss on inductor and capacitor of the inverter, while decrease the power density. To solve this issue, this article derives the line-to-line output voltage harmonic components calculation formulas for the conventional two-stage inverter and the dual-mode inverter by using double Fourier integral analysis. By comparing the harmonic components calculation results around switching frequency, a new LC filter parameters design method for dual-mode inverter is proposed. Through the proposed method, the LC filter parameters are declined by 50%, which significantly reduces the volume, weight and cost of the inverter under other conditions of constant performance. Calculation and experimental results based on a 1 kW prototype verify the validity of the proposed method.

In this paper, a hybrid control method with adaptive phase-shifting modulation (PSM) and pulse-frequency modulation (PFM) is proposed to optimize the steady-state performance of an LLC resonant converter in wide voltage-gain range application. For the primary-side switches under the hybrid control method, zero-voltage switching (ZVS) performance is maintained over wide voltage-gain range and the turn-off loss is reduced; therefore, the converter reconciles wide voltage-gain range and high efficiency. Mode characteristics under different phase-shifting-angle-θ and switching-frequency-fs combinations is analyzed, and then the soft-switching characteristic is revealed. By introducing time domain analysis, the turn-on current of the primary-side switches is calculated, and thus the ZVS boundary of different θ and fs combinations is inferred. In addition, to acquire the optimum steady-state operation of the converter, the turn-off current is calculated; by making the converter work near to the minimum turn-off current operating point, the turn-off loss can be reduced greatly. With the principles of achieving ZVS performance and minimizing turn-off current, the phase-shifting angle θ of PSM is designed to be adaptive to the reference output voltage, no additional circuits are needed, and the two control degrees (θ and f_s) are simplified to one (f_s). The simulation and experiment are developed to verify the feasibility and effectiveness of the hybrid control method; the results show that ZVS performance is maintained at wide voltage-gain range and the turn-off current of the hybrid control method is reduced to that of the single PFM, and thus the turn-off loss is reduced. The efficiency comparison validates the fact that the hybrid control method has less power loss than single PSM and single PFM.

For the conventional single-phase current source inverter (CSI), a large inductor is needed to stabilize the input current, which increases system volume, cost and losses. In this paper, a single-phase single-stage CSI with its operation principle is proposed. Different from conventional CSIs where only voltage-boost operation is supported, the input DC-link voltage of the proposed CSI can be higher than the peak value of output voltage (buck mode), and it can also be lower than the peak value of output voltage (boost mode). So, it is very suitable for occasions with a wide input voltage range. Moreover, the input inductor current can be balanced within per switching cycle using the proposed operation strategy. As a result, the input inductor and output filter capacitor are decreased. First, the derivation and operating modes of the CSI are given in detail. And then, the design guidelines of the components are presented, and a comparison with some existing CSIs has been made. A 1 kW prototype is also built and tested in the laboratory, the experimental results are presented to verify the effectiveness of the proposed CSI and control strategy.

Virtual space vector modulation (VSVM) is capable of suppressing neutral-point voltage (NPV) ripples in a coupled ten-switch three-phase three-level inverter. However, it presents a low NPV control (NPVC) speed. In this article, an enhanced VSVM (EVSVM) with the improved NPVC speed for balanced and unbalanced NPVs is proposed. By analyzing the features of the defined virtual vectors under the unbalanced NPV, the region division and duty ratio calculation are properly modified. Two coefficients are configured in the EVSVM. One distributes the duty ratios of two virtual small vectors to improve the NPVC speed, whereas another adjusts two pairs of redundant small vectors considering the NP current polarity to achieve accurate active NPVC. The highlight of this method is the improved NPVC speed and zero NP current in each switching cycle in a steady state. Theoretical analysis shows that the EVSVM increases the available NP current by 4.5% at least under a higher modulation index. Besides, the EVSVM could regulate the NPV without obvious ac ripples and dc offset under a full range of modulation index and power factor for both balanced and unbalanced NPVs. The feasibility and performance of the proposed modulation and control strategies are verified by a 3-kVA experimental prototype.