Analysis and Optimization of 3/2LNPC-DAB Converters to Minimize Current Stress With Four-Degree-of-Freedom Modulation

The three-to-two-level neutral-point-clamped dual active-bridge (3/2LNPC-DAB) DC-DC converter offers higher voltage blocking capability and more control degrees of freedom (DoFs) compared to the conventional two-level DAB converter. However, the screening of modes and optimization of modulation in the time-domain analysis of the 3/2LNPC-DAB converter is highly challenging due to the increase of the control DoFs. In this paper, a visualized modal optimization method is proposed to minimize the current stress. For the 3/2LNPC-DAB converter, five optimization principles for minimizing current stress (FOP MCS) are summarized to screen the optimal modes, and the equational constraints among the four control DoFs in the optimal modes are derived. On this basis, the current stress minimization modulation with an analytical solution in the entire power range can be solved by combining the Karush–Kuhn Tucker (KKT) conditions. Finally, the effectiveness of the proposed modulation is validated by a 1.6 kW prototype. With the proposed modulation, the current stress of the converter is significantly reduced while the efficiency is improved.