Enhancing Dynamic Performance of Asymmetrical CLLC Resonant Converters by Optimal Trajectory Control and First Harmonic Approximation

In this paper, an innovative control method using optimal trajectory control (OTC) and First Harmonic Approximation (FHA) is proposed to improve the dynamic performance of an arbitrary CLLC resonant converter during load variations. Firstly, mathematical derivations have been proposed to decouple the five state variables of CLLC converter with an arbitrary resonant tank into only two normalized independent state variables, which enables state-plane analysis (SPA) to visualize the load variations in a 2D state-plane. By manipulating the gate signals of the switches during transition using OTC and FHA, the converter can temporarily disregard the traditional PI controller for frequency modulation and track the optimal trajectory from the initial steady-state to the final steady-state. Load steps have been simulated for various scenarios in PLECS to confirm that the proposed control method is effective to enhance the dynamic performance by up to a factor of 10.