Hybrid PFM and PWM Modulation Strategy for Stacked Structure LLC Resonant Converter With Wide Input Voltage Range Application

In this article, a hybrid pulse frequency modulation (PFM) and pulse width modulation (PWM) modulation strategy is proposed for the stacked structure LLC resonant converter. Three-level operation of the resonant tank input voltage is achieved by introducing the half of input voltage level due to the adopted PWM modulation strategy. Therefore, two control freedoms, switching frequency and duty ratio, are available for the proposed converter. To optimize the system efficiency performance, a power loss model based on the time-domain analysis method was built. By setting the converter power loss as the optimization objective, the optimized combination result of switching frequency and duty ratio can be found. In addition, the proposed hybrid modulation strategy can be applied for wide input voltage range application. Compared with traditional PFM LLC resonant converter, the proposed converter has better efficiency performance and the required switching frequency operating range is reduced significantly. Compared with the PWM LLC resonant converter, the resonant inductor current peak value and root-mean-square (RMS) value are reduced greatly. Finally, a 115 W experimental prototype with a wide input voltage range of 160 V - 340 V was built to validate the effectiveness of the proposed hybrid modulation strategy for the stacked structure LLC resonant converter.