The LLC resonant converter, as a mainstream high-efficiency DC-DC converter, suffers from a narrow voltage regulation range. By employing a hybrid modulation technique combining Pulse Frequency Modulation (PFM) and Phase Shift Modulation (PSM), the voltage regulation range can be effectively expanded. However, traditional single controllers cannot simultaneously ensure system stability and responsiveness in both operating modes. To address this issue, a unified model for LLC resonant converters with hybrid modulation is proposed. Initially, the small-signal model of the LLC converter under PFM and PSM modulation modes is established using extended describing function method. Then, by introducing an intermediate control quantity and aiming for consistent model gain variation rates, the intermediate function is solved to obtain the unified model of the LLC resonant converter. Finally, based on this unified model, a PI controller is designed, and an experimental platform is constructed to validate its effectiveness.