Research On Dual Transformer Based Wide Operating Range Current-fed Bidirectional DC-DC Converter

Renewable energy generation has been a research hotspot and widely used in new energy microgrid and transportation electrification in recent year.In order to ensure the stability of the system,auxiliary energy storge devices are required,where the bidirectional DC-DC converter is significant as a mediation module.Current-fed DC-DC converters are more suitable for such low voltage high current application due to lower current ripple and higher step up ratio.Single phase shift or dual phase shift modulation based naturally clamped current-fed DC-DC converters can operate with wide input voltage and realize wide soft switching range without additional auxiliary devices.However,a problem of single phase shift modulation is the high current through transformer leakage inductance at light load condition.Although this problem is sovled by dual phase shift modulation,converter can not operate within wide input voltage and wide output voltage range simultaneously and the practical region of power transfer is limited under wide variation range of voltage.A dual transformer based wide operating range current-fed bidirecitional DCDC converter is proposed in this dissertation to make up these deficiencies.The high voltage side of proposed converter is a hybrid topology,which contains full bridge and half bridge circuits.Another control variable is added to achieve hybrid phase shift modulation,and converter's performance is improved by the complication of topology and control method.The leakage inductance current of proposed hybrid converter is changed according to the duty cycle and overlap of primary and secondary voltage across transformer.It maintains the same advantages as dual phase shift modulation,namely controllable peak leakage inductance current and wide range of soft switching,thus reducing the peak and rms value of current under light load conditions.In addition,the regulation of three phase shift angles can extend the range of voltage gain and power transfer,meeting the application requirements of wide input and output voltage variation range and meanwhile improving the maximum transferred power capacity.As a result,the practical operation range of the converter is extended.For reverse power transfer,a hybrid control strategy with synchronous rectification on low voltage side at higher loads and active modulation on low voltage side at lower loads is utilized to maintain wide oprating range of soft switching.The dual transformer structure brings more possibility and flexity for operating modes of converter as well.Various operating modes can be realized by adjusting the turns ratio and voltage phase relationship of transformers.After theoretical derivation and analysis of single phase shift modulation based half bridge converter,dual phase shift modulation based full bridge converter and proposed hybrid converter,simulation and experimental verification are implemented using PSIM software and laboratory prototype.The feasibility of the proposed dual transformer based wide operating range converter is verified by simulation and experimental results.