Advanced Modeling Approach for LLC Resonant Converters: From Multiharmonic Analysis to Small Signal Model

This article proposes an innovative approach for accurate modeling of a dc-dc LLC resonant power converter based on extended describing functions. Unlike most of the models available in the literature, which have been developed under the first harmonic approximation, the proposed model allow to take properly take into account the contribution of higher harmonics, providing accurate results even for operating conditions far from resonance. The inclusion of higher harmonics cannot be directly performed using the superposition principle due to the nonlinear nature of the converter. As a consequence, the inclusion of higher harmonics is a nontrivial problem. For this reason, the procedure for including multiple harmonics in the classical extended describing functions (EDFs) approach (which typically works by considering only the fundamental harmonic) is presented. The EDF approach also allows to derive the small-signal model of the converter, which is useful in designing the control circuit of the converter. An analysis to identify the minimum number of harmonics necessary to achieve a target accuracy and obtain the best compromise between simplicity, computational cost and accuracy is presented. The improvements with respect to models available in the literature are evaluated by extensive experimental validations.