A Unified Reduced Order Modeling and Dual-Loop Frequency Control Design for CLLC Resonant Converter

This paper proposes a unified small-signal model of a dual active bridge (DAB)-based CLLC resonant converter, considering the internal phase shift in the primary bridge, the external phase shift between the two bridges, and the switching frequency as the model parameters. The extended describing function (EDF) concept and fundamental harmonic approximation (FHA)-based equivalent circuit simplifications are utilized to arrive at a third-order model of the converter, which provides all necessary control transfer functions. A 22 kW DAB-CLLC resonant converter, with input and output voltage ratings of 750 V and 400 V, respectively, verifies the proposed model. The control architecture features a dual-loop setup that comprises of internal current and external voltage loop, offering precise and efficient control. The external loop compensator generates a current reference for the internal loop based on the desired output voltage. The internal loop compensator modifies the switching frequency to match the current reference. The transient response of the designed compensator is analyzed, giving a settling time of 5 ms for both step loading and step change in output voltage reference. The control scheme is implemented using MATLAB Simulink.