Research On High Frequency DC-DC Converter For Locomotive Auxiliary Converter

The auxiliary converter is an important part of the electric locomotive.The optimization of the front stage DC-DC converter of DC-DC-AC auxiliary converter is very important to improve the power density of the auxiliary converter.This paper takes the Buck+LLC converter as the research object,mainly studies the topopogy comparison of LLC Resonant Converter,the application comparison of Si C MOSFET and Si IGBT in LLC Resonant Converter,and the Control strategy of Buck+LLC converter.Firstly,based on the specific application conditions of the auxiliary converter,the structural characteristics,waveform characteristics and losses of four topologies: full bridge LLC Resonant Converter,asymmetric half bridge LLC Resonant Converter,symmetric half bridge LLC Resonant Converter with split capacitor and symmetric half bridge LLC Resonant Converter are compared.The results show that the full bridge LLC Resonant Converter is beneficial to reduce the current stress and the conduction loss of the switch,and the symmetrical half bridge LLC Resonant Converter with split capacitor is beneficial to reduce the cost and the effective current value of the resonance capacitor.Secondly,the application of SiC MOSFET and Si IGBT in LLC Resonant Converter is compared.Firstly,the static parameters are compared,and then the switching characteristics are compared by double-pulse experiment.Finally,the resonance parameters and dead time of LLC Resonant Converter are designed to ensure the realization of soft switch.On this premise,the application comparison of Si C MOSFET and Si IGBT in LLC Resonant Converter is carried out.Si C MOSFET can achieve higher switching frequency due to the higher switching speed,at the same switching frequency or the highest switching frequency that can be achieved separately,LLC Resonant Converter has lower switching loss and higher efficiency when using Si C MOSFET.Finally,the control strategy of Buck+LLC converter is studied.The rear stage LLC Resonant Converter fixed width and fixed frequency control fixed transformation ratio to reduce the voltage,the digital peak current control is used to adjust the output voltage of Buck converter.The CCM and DCM of Buck converter are considered in the control.The possible sub-harmonic oscillation problem of digital peak current control is analyzed,the method to solve the sub-harmonic oscillation by adding slope compensation is given,and the small signal model of digital peak current control is established.Finally,the input voltage fluctuation and load switching conditions of Buck converter and Buck+LLC converter are tested.