The Research On High Step-down LLC Resonant Converter Combined With Switched Capacitor

With the low carbonization of the energy structure,the DC microgrid has developed rapidly.In the low voltage DC distribution network,there are different levels of bus voltage,through a number of converters step-down mode for load power supply.In order to improve the energy transmission efficiency in the DC microgrid,the research of DC/DC converter with higher step-down ratio and higher efficiency has important significance.In this paper,to overcome the shortcomings of the existing high step-down ratio DC/DC converter,a new switched capacitor LLC resonant converter combined with Ga N semiconductor and planar transformer structure is proposed.It has the characteristics of reliable soft switching,high efficiency and high step-down ratio.This paper analyzes the topology structure and the working principle of the designed converter.Under the condition of high frequency and high step-down ratio,the secondary side leakage inductance of the transformer cannot be ignored.The paper discusses the influence of secondary side leakage inductance on the resonance process,studies the working modes of the converter in detail and calculates the voltage and current equations in each mode.This paper establishes an optimization model of the converter with the method of First Harmonic Approximation to analyze the gain characteristic s of the resonator.The condition of soft switching is studied under high frequency condition,and a more accurate dead time range is given.The loss existing in the circuit is analyzed and the loss of charge and discharge between switched capacitors is ca lculated with the limit frequency method.The component parameters of the resonant cavity and the switched capacitor circuit are optimized,so that the converter can reliably realize soft switching and reduce the capacitor charge and discharge losses.In this paper,the Extended Description Function method is used to consider the effect of transformer secondary leakage inductance on the system,and a more accurate and effective small signal model is established.The open-loop transfer function of the system is calculated,and the Bode diagram and the pole-zero distribution diagram are drawn.To compensate the output characteristics of the system,a 4P2 Z digital compensator is designed.By adopting PWM and PFM hybrid control method,the problem of insufficient DSP frequency resolution is overcome,which improves control accuracy and enables the system to achieve good closed-loop control performance.This paper introduces hardware circuit design and device selection.The structure and winding arrangement of the planar transformer are optimized.Simulation experiments are conducted to verify the correctness of the design scheme.Under limited experimental conditions,an experimental prototype with a switching frequency of 500 k Hz and an output power of 48 W is bui lt and debugged.The rated output voltage can achieve 12 V,with ripple 500 m V.The system efficiency reaches 94.8% at full load,which reaches the design target.