6. Three-Level Bidirectional DC–DC Converter with an Auxiliary Inductor in Adaptive Working Mode for Full-Operation Zero-Voltage Switching

This chapter proposes a three-level bidirectional DC–DC converter with an auxiliary inductor for full-operation zero-voltage switching (ZVS) in high-output voltage applications. The auxiliary inductor is connected across the middle node of the split flying capacitors and the center tap of the secondary winding in the transformer. In this topology, the outer and inner switches in the three-level stage can generate two independent 50% duty-cycle square waveforms, which is used to control the current in the auxiliary inductor to extend ZVS range from no loads to full loads condition. Considering the phase shift angle in three-level stage, the ZVS range of the converter is analyzed, and the modulation trajectory to maintain the full-operation ZVS range with low conduction loss is proposed. A flowchart implementation can guarantee the seamless transfer in different working modes. Then, the conduction loss in the proposed converter is compared with the previous three-level bidirectional solution, which illustrates that the conduction loss in the proposed converter only increases in light loads. Finally, the experimental results verify the theoretical analyses and ZVS performance across the whole power and voltage range, and the efficiency curves demonstrate the efficiency improvement.