Design and Implementation of a High-Gain Bidirectional CLLC Resonant Converter for Photovoltaic Energy Storage Systems

In residential photovoltaic (PV) energy storage systems, the bidirectional DC–DC converter between the DC bus and the battery must operate efficiently over wide input- and output-voltage ranges. This paper presents the design and implementation of a high-gain bidirectional CLLC resonant converter rated for 4.8 kW. The converter operating principle under pulse frequency modulation (PFM) is described, and its voltage gain is derived using first harmonic approximation (FHA). Resonant tank parameter selection considers voltage gain, soft switching, and gain monotonicity. To improve modulation accuracy and coordinate register updates under PFM, a high-precision implementation based on high-resolution pulse-width modulation (HRPWM) is proposed. Global loading synchronizes supported PWM shadow registers, while high-resolution period updates are coordinated separately. A converter loss model guides the forced-air cooling design of a parallel-fin heatsink, assessed by electrothermal co-simulation at 6 kW output power and long-duration tests at 4.8 kW battery-port power. Experimental results show that the peak efficiency of the prototype reaches 97.45% in charging mode and 96.66% in discharging mode.