Flattop Modulation Technique for a Low-Device Count Single-Stage AC-DC DAB Converter to Improve Its Overall Efficiency

Abstract

This article proposes a flattop modulation technique for a single-stage, single-phase bidirectional ac-dc dual-active-bridge (DAB) converter for achieving a flattopped DAB inductor current. The flattopped DAB inductor current has a low root mean square (rms) value, and it improves the soft-switching performance of the converter. The novelty of the proposed technique is that it guarantees flattopped DAB inductor current throughout the ac line cycle and has direct control over the ac current. In this technique, sinusoidal pulse width modulation (PWM) is performed at the ac side bridge for direct ac current control. Sinusoidal phase shift modulation is performed at the dc side to generate a bridge output voltage with a pulsewidth identical to the ac side bridge. The high-frequency (HF) transformer turns ratio is appropriately selected to match the voltages at the DAB inductor’s ends. Thereby, flattop DAB inductor current is achieved throughout the line cycle. The proposed modulation technique ensures a regulated dc bus at the ac side and provides a ripple-free ac current by interleaving. The operation of the converter under the flattop modulation technique is analyzed in detail, and parameter design guideline is provided. Experimental results from a 2-kW converter prototype support the theoretical claims.