Hybrid Control Strategy for LLC Converter With Reduced Switching Frequency Range and Circulating Current for Hold-Up Time Operation

Hold-up time operation is a crucial requirement for distributed power systems and server power supplies. LLC resonant converter is widely used in these applications due to its high efficiency and power density characteristics. Conventional pulse frequency modulation (PFM)-controlled inductor-inductor-capacitor (LLC) resonant converter requires a wide switching frequency operation range to satisfy the hold-up requirement. In this article, a novel control scheme is proposed for stacked structure LLC resonant converter. During the normal operation, the frequency doubler modulation strategy is adopted to reduce the switching loss and driving loss, and the output is regulated by PFM. During the hold-up time operation, the pulsewidth modulation is adopted to increase the system voltage gain, where the switching frequency is fixed during this operation stage. For the proposed converter, the resonant tank parameters are designed at the nominal operation point, and the only design consideration for the magnetizing inductor is the zero-voltage switching for primary switches. Therefore, compared with PFM-LLC, both the switching frequency operating range and the circulating current are reduced, and high efficiency feature is obtained. Finally, a 100-W experimental prototype with input voltage range of 160-340 V was built to validate the advantages of the proposed converter.