MOSFET输出电容对CLLLC谐振变换器模型的优化

Optimization of CLLLC Resonant Converter Modeling by MOSFET Output Capacitance

在CLLLC谐振变换器基波等效建模方法中,未虑及变压器二次电流为零及死区时间内MOSFET输出电容对输出侧H桥桥臂中点电压的影响,当开关频率低于谐振频率(欠谐振)时,变换器输出侧H桥桥臂中点电压模型的准确性有待进一步提高。该文通过解析变换器欠谐振区域模型准确度较低的成因,得到变压器二次电流为零及死区时间内MOSFET输出电容电压方程,对基波等效建模方法进行了更为深入的研究,提出了状态变量(输出侧H桥桥臂中点电压、变压器二次电流)与变换器增益的优化计算方法。该方法计算式中包含开关频率、谐振频率、输出电压和MOSFET输出电容的信息,减小了状态变量稳态值求解误差,使等效输出负载与谐振腔参数更加准确,解决了欠谐振时变换器输出侧H桥桥臂中点电压模型精度相对较低的问题。最后,搭建CLLLC谐振变换器电路仿真模型与实验系统对所提方法进行验证,结果验证了该文理论分析的正确性。

The fundamental harmonic approximation(FHA)modeling method is commonly used to analyze CLLLC resonant converters because of its simple operation and high accuracy.The FHA method uses the fundamental wave component of the input and output voltage of the resonator,so the circuit topology analysis of the CLLLC resonant converter is transformed into the analysis of the sinusoidal steady-state circuit.The voltage gain model of the CLLLC resonant converter established by the FHA modeling method can be used to analyze the characteristics or design the parameters of the CLLLC resonant converter.However,in the traditional FHA modeling method,the influence of MOSFET output capacitance on the midpoint voltage of the H-bridge arm on the output side is not considered when the secondary current of the transformer is zero and MOSFET is at dead time.When the switching frequency is lower than the resonance frequency(under-resonance condition),the accuracy of the midpoint voltage model of the H-bridge arm needs to be further improved.The reason that the model accuracy of the traditional FHA modeling method is lower with the decrease of switching frequency is analyzed when the CLLLC resonant converter operates under the under-resonance condition.Two operating states of the CLLLC resonant converter are provided:the current in the secondary winding of the transformer is zero,and the MOSFET is at dead time.The transient equations of the two operating states are established using the time-domain analysis method.The voltage expression of the MOSFET output capacitance is derived by solving the transient equations,and the voltage at the middle point of the H-bridge on the output side of the CLLLC resonant converter is expressed as a complete function.Furthermore,a more precise method for calculating the RMS current on the secondary side of the transformer is given,and a more accurate formula for calculating the output equivalent load is obtained.The optimized calculation formula of the voltage at the midpoint of the H-bridge arm on the output side of the CLLLC resonant converter and the output equivalent load contains the variables of switching frequency,resonant frequency,output voltage,and MOSFET output capacitance.The obtained voltage gain model can describe the actual gain of the converter more accurately.Therefore,the accuracy of the FHA model can be improved when the CLLLC resonant converter is in under-resonant conditions.Based on the optimized voltage gain model,an accurate parameter design method is presented to provide theoretical support for the parameter design of the CLLLC resonant converter.Finally,the circuit simulation model and an experimental system of the CLLLC resonant converter are set up with an input voltage of 380 V and power of 1 kW.By comparing the relative errors of output voltage,the accuracy of the proposed optimization method is verified.The results show that:(1)Under simulation conditions,the maximum relative error of the traditional FHA method is 3.2 times the optimized FHA method.(2)Under experimental conditions,the maximum relative error of the traditional FHA method is 1.42 times the optimized FHA method.