Design of Three-Phase Staggered LLC Resonant Converter with Flexible Transition Control for Wide Voltage Gain

The three-phase staggered LLC resonant converter has the advantages of high power density and small output current ripple, which has been widely used in various electrical fields. In this paper, a flexible transition control strategy is designed to realize the wide voltage gain based on multimode switching. For different input voltages, the converter can work in three-phase interleaved mode, single-phase full-bridge mode or single-phase half-bridge mode according to the switching frequency. And the change between two modes become smooth by adding a transition process. Meanwhile, the dead time setting of converter switches is analyzed in detail. The minimum dead time limit is obtained by investigating the MOSFET parasitic parameters. On the other hand, the maximum dead time limit is obtained by the accurate calculation of the resonant current during the switching process. So that the MOSFET can be turned on before the resonant current returns to zero. The wide voltage gain and full-range soft switching are realized while improving the converter’s efficiency. Feasibility and superiority of proposed control strategy for the LLC resonant converter are presented by theoretical analysis, computer simulation and experiments.