Analysis And Optimal Design Of Asymmetry Current In Secondary Side Of Buck-Boost LLC Resonant Converter

The cascade converter Buck-Boost LLC topology is widely used in operating conditions with a wide input voltage range and in systems sensitive to power supply efficiency because of its advantages of widerange input voltage regulation,high efficiency,and high power density.However,there is an asymmetry problem in the current waveform of the front-stage inductor current,which causes the asymmetry of the work waveform of the rear-stage resonance circuit and the current waveform of the secondary rectifier tubes.These problems limit the improvement of Buck-Boost LLC’s voltage regulation range,and also limit the further improvement of efficiency and power density.This thesis proposes an optimized design scheme based on the traditional design method of excitation inductance.Firstly,the embodiment of the asymmetry of the cascade converter’s operation is analyzed.The asymmetry of the front-end inductor current waveform determined by the topology structure,which results in the asymmetry waveforms of midpoint voltage and excitation current during the operation of positive and negative half-period.Secondly,the equivalent circuit model of the working mode during the dead time is established.The different value of excitation inductance has different influence on the completion of the charging and discharging of parasitic capacitance under certain working conditions.The calculation module is built on MATLAB,so as to determine the design range of the excitation inductance value,and alleviate the inconsistency of the charging and discharging implementation time of parasitic capacitance.Then,a topology simulation platform for cascaded converters was built on the Pspice.The simulation verification was carried out under different input voltages on the platform.It was verified that the symmetry of the waveforms of the midpoint voltage and the excitation current can be promoted,and the system efficiency is also improved.The test prototype based on Buck-Boost LLC topology is built.Under the working conditions of 1MHz and 300W,the proposed design scheme is verified to optimize the symmetry of the key waveforms of Buck-Boost LLC topology.The test results show that the optimal design scheme of the excitation inductance value proposed in this thesis reduces the current asymmetry of the secondary side synchronous rectifier tube from 9%-12.7%to less than 5%under the working condition of the input voltage range 250V400V and output load 25A;the output power is increased from 264W to 300W;the peak efficiency has increased by 1.4 percentage points compared to traditional design methods,reaching to 96.9%;the average efficiency has increased from 94.5%to 95.5%.The measured results meet the requirements of the design indicators.