LLC resonant converter with wide voltage gain range based on a CSPWM hybrid modulation strategy

Due to its inherent high efficiency, the LLC resonant converter has been extensively studied by numerous researchers. However, when operating in the step-down range, the LLC resonant converter’s gain changes slowly, widening the switching frequency and resulting in higher losses. To address this issue, this paper proposes an LLC resonant converter that combines Center-Symmetric Pulse Width Modulation (CSPWM) with PFM modulation. This effectively narrows the converter’s operating frequency range and increases the converter’s voltage gain range. This paper first analyzes the operating principles of PFM and CSPWM, respectively. Secondly, the principles of the proposed control method are demonstrated in detail, introducing a hysteresis mechanism during the CSPWM and PFM switching process to avoid frequent switching. Finally, the proposed theory is verified using a 6.6kW LLC experimental prototype.