Optimized Quadruple Phase-Shift Modulation of 3/2LNPC-DAB Converters for Full-Power-Range Soft-Switching and Minimum Peak Current

The symmetrical phase-shift modulation of the three-to-two-level neutral-point-clamped dual-active-bridge (3/2LNPC-DAB) DC-DC converter has more controllable control variables than the conventional two-level DAB converter. The increase in control variables leads to an increase in the number of operating modes and inequality constraints, significantly complicating the modulation optimization process. To enhance the efficiency of the converter, this paper proposes an optimized quadruple phase-shift (OQPS) modulation considering both conduction losses and switching losses. Five effective operating modes that have the potential to achieve zero-voltage-switching (ZVS) for all switching devices are screened based on the ZVS inequality constraints. The inductor peak current is used as an optimization objective to minimize the conduction losses while satisfying the ZVS inequality constraints. The boundary of the feasible region satisfied by the optimal solution is derived by numerical optimization. Based on this, an improved Lagrange function and Karush-Kuhn-Tucker (KKT) conditions are proposed to solve the analytical solution of OQPS modulation. Finally, the proposed OQPS modulation is applied to a 1.6kW prototype to verify the improvement of efficiency. The designed prototype can achieve 96.9% peak efficiency at 300V input and 150V output voltages. Compared with previous modulations, the proposed OQPS modulation can achieve the highest efficiency in the full power range.