Research On Full-bridge LLC Resonant Converter

Having the advantages of the low stress, high efficiency and lowelectromagnetic radiation in high power DC/DC converter applying, full-bridgeLLC resonant converter has became a research hotspot. But the operatingcharacteristics and reliability of the LLC converter is very sensitive to the loadcondition and the operating frequency. Especially in low input voltagecircumstances，when load condition is changed from idle to full-load, the operatingfrequency decreases rapidly, if the parameters of resonant cavity are not designedproperly, the converter will run in capacitive mode and burn down in the end.In order to improve the reliability, the basic working principle of thefull-bridge LLC converter is analysed firstly, according to the intervals that theworking frequency of the converter lies in, the work phases of converter aredescribed in detail respectively. After simplifying the converter structure, theprinciple circuit is obtained and a FHA model is constructed, with that model, a gainformula is derived and a set of curves are drawn, then the influence of the cavity’sparameter on gain features of the converter are analysed in details. After discussingall the working stage of the converter in capacitive mode, the safe workingrequirment of the LLC converter is found, then the relationship between thatrequirment and the parameters of the resonant cavity is gained. In discussing theefficiency of the converter, the loss of the swith and the rectifer diode is analysed indetail, then the switch loss formula with respect to magnetizing inductnce isobtained, finding out the relationship between the parameters of the resonant cavityand the efficiency of the converter. Then, by discussing the processing that the loadconditon is changed from idle to full-load, the output equivalent circuit of theconverter is achived, by applying transient analysis on that circuit, the formula forcalculating the equivalent output load value is derived. Taking the formula as adesign consideration and combining the factors of requirement for soft-switchingimplementation and the relationship between parameters of the resonant cavity andoperating characteristics of the converter，a set of design methods for full-bridgeLLC resonant converter resonator parameters is found to improve the reliablity ofconverter whose efficiency is also taken into account.According to the design method presented in this article, a48V/15A LLC fullbridge converter is designed, by using the saber to simulate and producing anexperimental prototype，finally，theory is verfied.