In high-power applications, parallel operation is required to improve both efficiency and reliability. In an LLC converter, it is difficult to achieve parallel operation due to the gain sensitivity to component tolerance. This paper proposes a new structure of a common resonant capacitor for a multiphase LLC converter, which can achieve very good current-sharing performance with no additional components or control. To evaluate the current-sharing performance, a new first harmonic approximation (FHA) model is built in this paper to analyze the LLC converter with coupled resonant tanks. Both FHA analysis and powersim (PSIM) simulation results demonstrate the load-sharing improvement of the proposed common capacitor LLC resonant converter over the conventional two-phase LLC converter. The current-sharing performance from the FHA analysis and the PSIM simulation are compared, and the error of the FHA analysis is evaluated and explained. A 600 W two-phase LLC converter prototype based on the proposed method is also built to verify the feasibility. Excellent current-sharing performance (6% resonant current sharing error at a wide load range) has been achieved.

This paper introduces an efficient strategy to improve the voltage quality of sensitive loads with the optimal utilization of a dynamic voltage restorer (DVR). Traditional control strategies mainly focus on the voltage compensation stage to reduce the voltage rating of the DVR or minimize the required capacity of the energy storage device and the phase jump correction in the early stage of the compensation has attracted more attention as well. In reality, phase jump issues may occur in the initial and final stages of compensation, and they must be avoided for most loads, but few works focus on how a DVR can smoothly exit the system after the fault elimination. With the main objective of mitigating the phase jump in the load side voltage while improving the overall sag compensation time, this study involves the following: 1) Based on energy-optimized compensation strategies, the proposed approach aims at completing the smooth transition of the transient process during the voltage compensation and recovery stages and ensuring effective linkage from the former to the latter. 2) The mode operation boundaries in the two stages are derived and compared, and the updated procedure of a new injected reference voltage is activated to prevent the system from going outside its operating limits. Furthermore, the operation logic and the overall control scheme are elaborated, which ensure that the proposed approach preserves superior controllability to provide flexible voltage support. Finally, a combination of simulation and experimental results is used to validate the performance of the proposed method.

Multilevel inverters are becoming more and more popular in photovoltaic applications because of lower total harmonic distortion, lower switching stress and lower electromagnetic interference. In this article, a single-phase five-level transformer-less PV inverter is proposed for the purpose of leakage current reduction. The inverter is based on a flying capacitor (FC) configuration, the voltages of FCs are balanced at V _dc /4 through the coordination of the two half-bridge at the switching frequency, helps to reduce investment cost in passive elements. Two bidirectional circuits are added to guarantee a constant common-mode (CM) voltage. The leakage current elimination mechanism is analyzed in detail with operational states and CM equivalent model. The losses are analyzed, the control strategy and the FCs are detailed designed. A comparison is also made in terms of the number of elements, voltage stress and CM voltage. Experiment results under grid-connected condition show that the proposed topology achieves both excellent differential-mode and CM performance.

In data center power supplies, the front end DC-DC stage is required to operate with wide input voltage range to provide hold up time when AC input fails. Conventional LLC converter is not suitable for this requirement, as the normal operation efficiency (at 400V input) will be penalized once the converter is designed to achieve high peak gain. This paper examined the operation of LCLC converter, and revealed that LCLC converter could be essentially equivalent to a set of LLC converters with different magnetizing inductors that are automatically adjusted for different input voltages. For nominal 400V input, LCLC converter behaves like an LLC converter with large magnetizing inductor, thus the current stress is small. In hold-up period, as the input voltage reduces, the equivalent magnetizing inductor will reduce together with switching frequency, so the converter achieves high peak gain. In this paper, a new design methodology is proposed to achieve optimal capacitor utilization for high power applications. To verify the effectiveness of the LCLC converter for hold up operation, comprehensive analysis has been conducted; a detailed design example based on capacitor voltage stress is introduced; an experimental LCLC prototype of 250V-400V input and 12V/500W as output has been presented.

The single-phase five-level transformer-less inverters have been gradually applied in photovoltaic (PV) grid-connected systems due to low voltage stress and excellent waveform quality. However, they also suffer from serious leakage current as conventional three-level inverters. In order to reduce the leakage current, a single-phase five-level transformer-less inverter is proposed in this paper. The proposed inverter guarantees the common-mode (CM) voltage is clamped to a half of the dc-link voltage when outputting ${\pm V_{dc}/2}$ and zero voltage levels. Consequently, the high-frequent CM voltage is avoided and the leakage current is suppressed. The voltage stresses of eight switches and two flying capacitors are only Vdc/4, thus the system investment cost and total losses are reduced. The conduction and switching losses are analyzed detailly and a comparison with some existing topologies is also made. Finally, the simulations and experiment are carried out based on a 1 kW prototype. The results show that the proposed single-phase five-level inverter can obtain both excellent differential-mode (DM) and CM performance.

The bridge arms of traditional three-phase multilevel converters are independent of each other; thus, more active switches and passive diodes are required. In order to reduce the number of devices, a coupled three-phase converter architecture, which is suitable for both dc-ac and ac-dc converters, is presented. Based on this architecture, a ten-switch three-phase three-level (10S-3P-3L) inverter is proposed in this article. The proposed topology employs only ten active switches and no additional diodes, which helps to cut size, weight, and costs of system. The proposed space vector modulation enables 10S-3P-3L inverter to operate full modulation indexes. A comparison between the proposed inverter with some existing topologies in terms of device voltage and current stresses, conduction loss, switching loss, and other parameters is made to highlight its excellent performance. Moreover, the experimental results from a 3-kW experimental prototype reveal that the proposed inverter offers superior power quality.