Research And Design Of Wide Range CLLC Converter In Bidirectional On Board Charger

Due to the clean,safe and sustainable development advantages of electric vehicles,the electric vehicle industry has been rapidly developed with the encouragement of national policies.What followed was a higher demand for the On Board Charger(OBC).In more and more occasions,the on-board charger not only needs to implement the traditional charging function,but also reverse-discharges the lithium battery as an energy storage system,and at the same time can adjust the power and provide stable power supply.The on-board charger that realizes the bidirectional flow of energy has a broad application space.OBC generally consists of a pre-stage PFC and a post-stage DC/DC converter.In the study of bidirectional OBC,the latter stage DC/DC converter has become the focus of research.The research of bidirectional DC/DC converters of wide range,high efficiency,high reliability,and high power density is of great significance.Bidirectional DC/DC converters have many topological classifications,each with different advantages and disadvantages and different applications.By comparing various topologies,the topology of the DC/DC converter is finally used as a CLLC bidirectional resonant converter.The CLLC topology evolved from the LLC topology.By adding a resonant capacitor on the secondary side,it has excellent characteristics of bidirectional buck-boost and soft switching while maintaining the excellent characteristics of the LLC.Since the characteristics of CLLC are not exactly the same in two directions,this brings some difficulty to the design.it requires detailed working state analysis and accurate gain curve to optimize the design.Fundamental Harmonic Analysis(FHA)is a relatively common traditional analysis method.However,when it deviates from the resonance point,it will produce a large deviation,and it is difficult to give an accurate gain result.In addition,FHA analyzes the circuit in the frequency domain,which cannot obtain the time domain characteristics of the circuit.These limit the application of FHA.This paper introduces the time domain analysis method,which uses numerical calculation method to solve the state equation,and obtains the accurate gain of the CLLC converter.In this paper,the forward and reverse running states of the circuit are analyzed respectively,and the precise waveform expression and gain curve of each mode in whole working range are obtained.On the one hand,the precise gain curve is especially suitable for a wide range of circuit parameter design;on the other hand,the accurate waveform expression of the component can accurately calculate the voltage and current stress of the component,and can perform circuit loss analysis to optimize system efficiency.Based on the CLLC time domain analysis,the key parameters of CLLC are designed.The components selection,loss analysis and hardware design methods are introduced.A 3.3Kw CLLC bidirectional resonant converter prototype is also built.The maximum efficiency of the forward and reverse transformations is over 98%.The experimental results verify the feasibility and effectiveness of the design method.