A Four-Level Modulation Strategy for NPC DAB Converter With Extended ZVS Range and Reduced Voltage Stress

Multilevel dual-active-bridge (DAB) converters utilize low-voltage-rating power switches with reduced channel resistance, provide additional degrees of freedoms for control, and exhibit improved efficiency. This article proposes a four-level modulation strategy for a neutral-point-clamped (NPC) DAB converter to address the performance limitations of NPC-DAB topologies operating under conventional modulation schemes—particularly in applications with wide voltage gain variations, such as renewable energy and battery energy storage systems. The proposed modulation scheme preserves compatibility with passive flying capacitor voltage balancing, while extending the zero-voltage switching (ZVS) range and mitigating over-voltage stress across NPC switches through adjusting two independent control parameters: duty-cycle and phase shift, without the need for complicated calculations or prestored lookup table of optimized control trajectories. The four-level voltage waveform significantly reduces inductor peak and rms currents under extreme voltage gain conditions, alleviating current stress and conduction losses in both the inductor and the switches. Unlike conventional modulation strategies where unequal voltage distribution is observed across the switches under varying voltage gains, the converter ensures safe operation by maintaining voltage balancing across switches and split-capacitors even under high voltage gains without requiring any additional active switches or complicated control methods. Theoretical analysis, validated through experimental results, demonstrates reduced turn-off and conduction losses, reduced transformer turns-ratio, and enhanced efficiency compared to conventional modulation techniques, particularly with significant voltage gain deviations.