Control Strategy For Input-series Output-parallel System Based On Boost+LLC Resonant Converter

The input-series output-parallel(ISOP) converter features reduced switch voltage stress and modular design, and has been widely used in high-input voltage low-output voltage high-power applications. This thesis focuses on the ISOP system based on boost+LLC resonant converter.In the proposed boost+LLC resonant converter, the LLC resonant converter works with constant switching frequency and the output voltage is regulated by boost converter. The operating principle of the boost converter and LLC resonant converter is analyzed. It is revealed that the LLC resonant converter can implement zero-voltage-switching in full load range and have the constant voltage gain with appropriately designed parameters.The small-signal model of the boost+LLC resonant converter is derived in this thesis. Especially, the small-signal model of the LLC resonant converter is simplified based on the fundamental wave approximation method and Thevenin-Norton branches’ equivalent transformation. The expression of small-signal model of LLC resonant converter is deduced and this provides theoretical basis for regulator design.Due to the right-half-plane zero of the boost converter and beat frequency characteristic of the LLC resonant converter, the band width of the boost+LLC resonant converter is very low, leading to a poor dynamic performance. A method for Proportional-Integral(PI) regulator plus lead compensator is designed to improve the system dynamic performance. In order to depress the output voltage’s change resulted by the input voltage’s disturbance, this thesis proposes an input voltage feedforward control strategy, and the expression of feedforward function is derived.This thesis investigates the input voltage sharing and output current sharing of the ISOP system. Due to the DC transformer characteristic of LLC resonant converter with constant switching frequency, the ISOP system based on boost+LLC resonant converter can naturally realize the input voltage sharing and output current sharing.The prototype of the ISOP system consisting of two 6kW boost+LLC resonant converter has been constructed and tested in the lab. The experimental results verify the correctness of theoretical analysis.