Small Signal Modeling of LLC Converter based on Time Domain Analysis and Equivalent Circuit Reduction

Resonant converters, notably LLC resonant converters, are gaining popularity and are being used in various applications due to their inherent soft-switching ability. In existing literature, various modeling methods have been derived based on the extended describing function, which uses the first harmonic approximation. This first harmonic approximation-based small-signal model provides accurate results in terms of gain and phase when operating at the resonant frequency. However, operating away from the resonant frequency yields inaccurate results, particularly in gain. This paper presents an accurate small-signal model derived from time-domain analysis for gain determination and circuit reduction techniques for pole location determination. The accuracy of the proposed analytical model has been confirmed through validation using MATLAB/Simulink and experimental verification.