A Fixed Switching Frequency Dual-Input LLC Converter with PWM Controlled Semi-Active Rectifiers for PV Applications

This paper proposes a new modulation for a dual-input LLC resonant converter with semi-active rectifiers that interfaces with two PV panels. This topology is a dual half-bridge LLC circuit with four active switches on the primary and a full-bridge rectifier with two diodes and two switches on the secondary. Instead of using the traditional pulse-frequency modulation (PFM) to control the converter voltage gain; in this new modulation, the duty cycle of the secondary switches is varied to maintain the output voltage at the desired value while the switching frequency is always kept at the same value as the resonant frequency, so that the switching loss can be reduced. A phase-shift pulse-width modulation (PWM) control for primary switches is still employed to implement maximum power point tracking (MPPT) for PV panels. Soft switching is realized in all switches over the entire range of source voltage and load conditions. A 500-W prototype was built to validate the behaviors of the proposed modulation. The input voltage range of each PV source was selected to be 25 V to 40 V and the output voltage was regulated at 200 V DC. Experimental results are closely matching the theoretical analysis.