Optimization of Stacked Structure LLC Resonant Converter with Hybrid Modulation Strategy

In this work, a new modulation strategy that combines duty ratio control and switching frequency control is proposed for the stacked structure (two half-bridge in series) LLC resonant converter with three-level operation. Three-level operation is achieved by introducing half of the input voltage level due to the adopted pulse width modulation (PWM) strategy. Therefore, two control freedoms, switching frequency and duty ratio, are available for the proposed operation mode. To optimize the system efficiency, a power loss model based on the accurate time domain analysis method was developed. By selecting the converter power loss as the optimization objective, the optimal combinations of switching frequency and duty cycle under different scenarios can be found. Compared with traditional PFM or PWM modulation strategy for the system, the proposed converter with hybrid modulation strategy has better efficiency performance and the switching frequency operating range is narrowed significantly. Thus, the converter voltage gain operation range can be extended effectively. Experimental results under various conditions by using different modulation strategies are discussed and compared to evaluate the advantages of the developed hybrid modulation strategy.