Accurate Small-Signal Model for LLC Resonant Converters

Resonant converters are the preferred topologies of a wide range of applications. The major benefits, especially for LLC converters, are zero-voltage switching (ZVS), low turn-off current of primary-side switches, zero-current switching of rectifier, and circuit simplicity. Despite the popularity, no simple and accurate small-signal model is available. In most applications, a feedback loop is incorporated into the control system to provide a regulated output. Therefore, small-signal models for those converters are critical for an optimal controller design. Based on describing function method, this paper proposes an accurate small-signal modeling for LLC converters. The study on spectrum shows the non-linear portion in LLC converters consisting of voltage-controlled oscillator (VCO), inverter, resonant tank, and rectifier has to be modeled as a whole. The describing functions of the non-linear block are derived. The model is accurate beyond switching frequency, which establishes a solid base for further development of a simplified equivalent circuit model.