Hybrid PWM and PFM Control Strategy for LLC Resonant Converter with Hold-up Time Operation Requirement

Hold-up time operation is crucial for some applications, such as distributed power systems and server power supply, which requires voltage regulation capability when the DC bus voltage decreases in a wide range. For conventional LLC resonant converter, to satisfy the hold-up time requirement, a small magnetizing inductance value is required to boost the system output voltage, which will lead to large circulating current and efficiency degradation. To solve this problem, a novel hybrid pulse width modulation (PWM) and pulse frequency modulation (PFM) control strategy for LLC resonant converter is proposed, a bi-directional switch and auxiliary inductor is connected in parallel with the transformer. For the proposed topology, the PFM is adopted in the normal operation stage to achieve the output voltage regulation; while for the hold-up time operation, by controlling the duty cycle of the bi-directional switch, the equivalent magnetizing inductance value can be modified to satisfy the voltage gain requirement. Therefore, the LLC resonant converter can be designed only at normal operation stage. Operation principles, design considerations, and simulation and experiment results are presented in this paper.