A hybrid 4-DoF control for bidirectional series half three-level CLLC converter with minimum RMS current in solid state transformer

The bidirectional series half three-level CLLC (SHBCLLC) converter is an attractive option for solid state transformer due to its high efficiency and bidirectional power flow capability. However, existing variable-frequency and pulse width modulation (PWM) modulation exhibit large inductor RMS current. Additionally, PWM control methods fail to achieve ZVS across the entire operating range. To address this issue, a four-degrees-of-freedom control (4-DoF) modulation strategy is proposed in this paper. The proposed modulation scheme achieves lower RMS current and full-range ZVS operation, thereby reducing switching and conduction losses. A unified frequency-domain model is employed to indirectly analyze the time-domain characteristics of the SHBCLLC converter under the four degrees of freedom (4-DoF) modulation. Subsequently, the optimal combinations of modulation variables are obtained using an offline particle swarm optimization (PSO) algorithm. Finally, the effectiveness and advantages of the proposed 4-DoF modulation are experimentally verified on a 1-kW SHBCLLC converter prototype.