Research On Half-Bridge Three-Level LLC Converter

Compared with hard-switching circuit,the soft-switching circuit has the advantages of small switching loss,low electromagnetic interference,and ensures the reliability and efficiency of the system.In all topologies,LLC resonant converter can achieve a wide range of soft-switching without adding any auxiliary circuits,and the power density is high,which is one of the main topologies in the mid-high power DC-DC converter.And half-bridge three-level converter is widely used in power electronic transformers,HVDC and other fields for the reason that the voltage stress across the power switches is only half of the input dc voltage.The topology of half-bridge three-level LLC converter combines the above advantages,so that it not only breaks through the limitation of the voltage level of the power device,but also further improves the efficiency and power density of the converter,has important research value.This paper research on the half-bridge three-level LLC converter and the main work is as follows:(1)The basic working principles of the half-bridge three-level LLC resonant converter in frequency-modulating(FM)control and phase-shifting(PS)control are respectively discussed.And the steady-state working process and commutation analysis are presented in detail.(2)The half-bridge three-level LLC converter is modeled by fundamental wave analysis method,the gain expression of the converter in FM mode and PS mode is deduced,and the soft switching and efficiency characteristics of the two modes of operation are analyzed.Based on this,a method of designing the resonator parameters through the mapping and utilizing the analytical constraints is proposed.(3)Firstly,aiming at the mid-point balance problem caused by the half-bridge three-level structure,a novel neutral-point balancing digital control method is presented in this paper to realize the voltage-sharing control of input dividing capacitors,which regulates the duty by amending the modulating wave in the single-carrier,doublet trigger mode.Then in order to meet the wide range of output voltage requirements designed in this paper,a hybrid control method is proposed,which combines the advantages of FM and PS control modes to greatly increases the voltage regulation range while ensuring the soft-switching.Finally,the feasibility and effectiveness of these two control methods are verified through simulation.(4)By building a prototype of 7.5kw experimental platform,the proposed theoretical analysis is verified.