A Switching-State Equivalent Gain Method for the Design of Dual-Transformer Wide Gain Range LLC Resonant Converter

Dual transformer resonant tank (DTRT) structure has been increasingly applied in the LLC resonant DC-DC converters to meet the demand for ultra-wide gain range in the utilization of renewable energy. Due to the structural particularity, the analysis of its gain characteristics and the design of topology are still rather complicated. In this article, the parameter equivalent model of DTRT structure is derived from the traditional LLC converter to optimize the design of resonant elements. On this basis, the port theory based on mathematical model is proposed to quantify the relationship between the gain characteristics and the switching-states of the front-stage inverter. Inductively, the switching-state equivalent gain (SSEG) method was abstracted to broaden the total gain range within limited frequency range by mode splicing, and to guide the design of topology and operation strategy. To verify the theoretical analysis and the performance of proposed method, a 40–320 V DC input, 96 V DC output, 300 W prototype was built up and applied in various inverter configurations. Analysis and experimental results indicate that the SSEG method is applicable to all kinds of DTRT based converters, representing the operating characteristics consistent with of the traditional converter but with an ultra-wide gain range.