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Current-fed bidirectional dc–dc converters have attracted much attention in battery energy storage system applications due to their zero-voltage-switching performances and greater degree of control freedom. However, high efficiency cannot be achieved for both light and heavy load ranges with the present modulation schemes for a current-fed bidirectional dc–dc converter, especially with low battery voltages. A new modulation scheme is proposed to improve the conversion efficiency over a wide load range and with variations in the battery voltage. The relationship between the duty cycle of the secondary full bridge and the phase-shift angle was investigated to lower the power losses with the wide load variations. Based on the operation mode analysis, the control loop with the proposed pulse wide modulation plus phase-shift modulation scheme was developed. Comparisons of the switch conduction loss and the core loss in the series inductor for the previous modulation schemes and the proposed modulation scheme were carried out. The results show that lower power losses occur with the proposed scheme. The experimental results verify the theoretical analysis and effectiveness of the proposed modulation scheme on loss reduction. [ABSTRACT FROM PUBLISHER]

This paper presents a three-phase back-to-back transformerless inverter for online uninterruptible power supply applications. The inverter consists of a three-phase three-wire input rectifier stage and a three-phase four-wire output inverter stage with their ground wires tied together. Both of the two stages are with sinusoidal pulse width modulation based division-summation (D-∑) digital control. The input stage controls the power flow between utility grid and dc link, regulates the dc-link voltage, and fulfills power factor correction. While the output stage tracks three-phase ac reference voltages. The proposed control laws derived based on the D-∑ digital approach take into account the effects of dc-link voltage fluctuation, grid-voltage distortion, and inductance variation due to different current levels. Thus, distortion of input current and output voltage, and filter inductor core size can be reduced significantly. Moreover, circulating current through their common ground can be suppressed effectively. Simulated and experimental results from a three-phase back-to-back transformerless inverter have confirmed the analysis and discussion of the proposed control approach. [ABSTRACT FROM PUBLISHER]