A Novel Fourth-Order CLCL Current-Source Resonant Converter With Continuously Enhanced Pulse Density Modulation

Pulse density modulation (PDM) can reduce switching losses by achieving zero voltage switching (ZVS) and zero current switching (ZCS), enabling high-efficiency operation of resonant converters. However, the pulse density in PDM is defined as the ratio of the number of minimum energy transfer cycles to the total number of control cycleN. This fixed minimum energy unit restricts the ability to continuously regulate the input current and, due to the discrete nature of the PDM, often requires a large value of N to achieve regulation, thereby increasing resonant voltage ripple. To address these limitations, a continuously enhanced pulse density modulation (CEPDM) strategy is proposed, which integrates half-cycle enhanced pulse density modulation (EPDM) with phase shift modulation (PSM) by embedding PSM within the passive mode of half-cycle EPDM. This strategy enables continuous current regulation within a fixed and limited total control cycle N, thereby decreasing voltage ripple. Furthermore, a fourth-order CLCL resonant circuit is proposed to solve the DC offset issue associated with half-cycle EPDM in a third-order CLC resonant circuit.