Automatic Resonant Frequency Tracking Scheme for $LLC$ Resonant Converter Based on Adaptive Extended State Observer

This paper proposes a novel automatic resonant frequency tracking (ARFT) scheme based on adaptive extended state observer (AESO) for unregulated $LLC$ converter. A second-order $LLC$ converter small-signal model using extended describing function (EDF) method with simplified resonant circuit is presented. A new adaptive scheme that automatically updates the inner model of extended state observer (ESO) are provided. The proposed AESO can well track the resonant frequency with limited sensors, and is easier to implement compared with the conventional ARFT scheme. Finally, simulation and experimental results demonstrate the effectiveness of the proposed ARFT scheme.