Ultralocal Model-Free Pulse-Frequency-Modulation of a Bidirectional Resonant DC-DC Converter for V2G System

CLLLC-type dc-dc converter (abbreviated as CLLLC converter) has drawn significant attention due to its capability to step-up and step-down voltage in both forward and reverse operation directions, while maintaining consistent operating characteristics, soft-switching across the full-load range, and high efficiency. Conventional control methods, such as Proportional-Integral (PI) regulator, are widely used due to their simple structure and straightforward principles. However, these approaches heavily depend on the model accuracy, which may lead to suboptimal performance in the presence of parameter mismatches and disturbances. To overcome this limitation, this paper proposes a comprehensive control framework for voltage closed-loop regulation and introduces an ultra-local model-free pulse-frequency modulation (UMF-PFM) approach. In this framework, the frequency and output voltage are used as variables in frequency modulation. This enables the identification of the unknown nonlinear term of the system, which encapsulates all relevant structural information. By adjusting the frequency, the controller's performance remains robust, reducing its dependence on model accuracy. Simulation and experimental results demonstrate that, the UMF-PFM shows better stability against parameter mismatches and perturbations on the primary side of the dc-dc converter compared to existing PFM and PSM methods, which are affected by less than 1 V of the output voltage under the same conditions.