Optimization of CLLLC Resonant Converter Modeling by MOSFET Output Capacitance
Zhang Xinwen1, Liu Wenze1, Yang Shude2, Liu Bailin1
1. School of Electrical and Information Engineering North Minzu University Yinchuan 750021 China; 2. College of Electrical Energy and Power Engineering Yangzhou University Yangzhou 225127 China
Abstract 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.
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2024, Vol. 39 
