Analysis of a Hybrid Variable Frequency-Duty Cycle Modulated low-Q LLC Resonant Converter for Improving Light-Load Efficiency for Wide Input Voltage Range

Light load efficiency and output voltage regulation of a low- $Q$ LLC resonant converter is a critical problem for wide input voltage and load range applications. Parasitic capacitances such as rectifier diode junction capacitance ( $C_j$ ) degrade the soft switching performance. Compact size, high density, and high transformer turns-ratio requirements for microinverter applications add significant distributed capacitance ( $C_d$ ) of the low-profile transformer, worsening the output regulation and zero-voltage-switching (ZVS) capability at light loads. Wide switching frequency requirement for regulation at light loads, which increases core losses and turn- off switching losses in power MOSFETs, further degrades the power conversion efficiency. The conventional phase-shift modulation causes a high circulating current and loss of ZVS at light loads. Therefore, a hybrid adjustable switching-frequency-duty-cycle modulation technique for improving the light load efficiency is proposed and analyzed for a full-bridge $LLC$ resonant converter. Accurate loss analysis for the proposed modulation scheme, including the effect of parasitic capacitances, is performed using time-domain equations. The proposed methodology precalculates the optimal duty cycle at light load conditions for the required input voltage range such that minimum power losses are incurred. Variation in switching frequency at the preselected duty-cycle value regulates the output voltage. ZVS over a wide range of operating conditions is observed. An experimental prototype for a 20–40 V input, 380-V/300-W output $LLC$ converter is tested for the validation of theoretical analysis.