Small Signal Modelling of Hybrid Frequency and Phase-Shift Control Full-Bridge LLC Converter using Extended Describing Function Method

This paper investigates small signal modelling of a hybrid frequency and phase-shift controlled full-bridge LLC converter to facilitate the design of a feedback compensator for wide variation in load and gain. By using Generalized Harmonic Approximation (GHA)-enabled Extended Describing Function (EDF) method, a small signal equivalent circuit is developed. In comparison to the conventional frequency-controlled LLC converter, the proposed methodology can achieve a wider gain range with a lower frequency sweep and hence improved efficiency by including an optimized phase-shift control. A proof-of concept of 500W LLC converter, generating 20–28 V output from the 270-V dc link is designed and tested to verify the small signal model accuracy for hybrid phase-frequency modulation implementation.