Control Strategy of Half-Bridge Three-Level LLC Resonant Converters With Wide Output Voltage Range

A half-bridge three-level LLC (HBTL-LLC) resonant converter has low voltage stress, high power density, and a large range of soft switching, so it is widely considered and applied, but there are problems such as a small adjustable range of output voltage and difficulty in implementing soft switching. In this article, based on the detailed analysis of the working principle of an HBTL-LLC resonant converter, a three-stage hybrid control strategy is proposed: frequency modulation (FM) control is used when the gain is high, and phase shift control is used when the gain is low. If the phase shift angle achieves the critical value of the soft switch and the gain is still high, variable-frequency burst control is used to expand the adjustment range of the output voltage and improve the power efficiency at low voltage output. Finally, a 500-V, 15-A prototype was designed and analyzed and experimented. The results verified the feasibility and effectiveness of the proposed three-stage hybrid control strategy.