Small-Signal Dynamic and High-Bandwidth Design of LLC Resonant Converters

Abstract

For high-frequency power conversion, resonant converters are gaining increasing popularity. However, the lack of an accurate small-signal model becomes an impediment for performance optimization. The existing equivalent circuit model and its simplified were based on fundamental approximation where the resonant tank was deemed as a good band-pass filter. These models work well for series resonant converter with high Q (quality factor) while inadequately for LLC resonant converter. The recent modeling breakthrough was based on the extended describing function method and was demonstrated to be accurate at all frequency. Following this method, a simplified third-order equivalent circuit model was proposed which predicted accurately the small-signal behavior up to 1/2 switching frequency. Based on the simplified model, this paper discusses the small-signal dynamics and the corresponding physics of LLC converter in detail. These insights lead to a new guideline in designing resonant tank with additional consideration for transient performance.