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Accurate Light Load Loss Analysis of Hybrid Modulation Strategy for ZVS Operation of Low-Q LLC Resonant Converter for Wide Input Voltage Range Applications

Light load efficiency and output regulation of LLC resonant converter is a critical problem for wide input voltage and load range applications due to converter’s parasitic capacitances such as rectifier diode junction capacitance (Cj). Compact size, high density and high transformer turns-ratio requirements for micro-inverter applications adds significant distributed capacitance (Cd) of low-profile transformer, worsening output regulation and zero- voltage switching (ZVS) capability at light loads. Magnetic core losses and turn-off switching losses in power MOSFETs further degrade power conversion efficiency at light loads. Therefore, an improved loss analysis for a hybrid modulation technique is proposed for full-bridge LLC resonant converter. The proposed methodology calculates duty cycle offline such that minimum power losses are incurred at different light loading condition. Variation in switching frequency at selected duty cycle value regulates output voltage. Time domain analysis of proposed technique including effects of Cd and Cj is performed to ensure accurate loss calculation. An experimental prototype for 20-40V input, 380V/300W output LLC converter is tested for validation of theoretical analysis.