Research On The Modulation Strategy Of A Modified Current-fed Bidirectional Isolated DC-DC Converter

Since the 20 th century,due to the depletion of fossil fuel reserves and the adverse effects of carbon dioxide on climate,the demand for renewable energy development is increasing.Solar and wind energy are the fastest growing renewable energy sources.Their main drawback,however,is the intermittent nature of the power output,which strongly depends on weather conditions.In order to overcome this limitation,the intervention of energy storage unit is needed to provide stable power output.The bi-directional isolated DC-DC converter transfers the energy between the energy storage power supply and the DC bus.It is widely used in electric vehicles,photovoltaic systems and fuel cell systems.This paper is based on a current-mode bidirectional isolated dc-dc converter.The low-voltage side of the converter is the low-voltage side structure of the traditional boost half-bridge bi-directional isolated dc-dc converter.The inductance in series on the high voltage side and the leakage inductance of the transformer provide the reverse current for the zero-voltage switching(ZVS).The traditional modulation strategy of the converter use hybrid phase shift plus pulse width modulation to control the converter.The practical ZVS is realized in the wide input voltage and full power range of the high voltage side switch of the converter,and the voltage matching on both sides of the transformer is made by PWM control,which is more favorable to the realization of ZVS.In order to analyze the converter characteristics more comprehensively,the duty cycle is added to a new degree of freedom on the basis of traditional two degrees of freedom,and the original modulation strategy is improved.At the same time,the practical ZVS can be realized,and the efficiency of the converter is improved.Firstly,the topology of the converter is introduced in this paper.In order to simplify the analysis of the converter,the equivalent circuit of T-type and ?-type of converter is deduced.Then,based on the analysis of the original two degrees of freedom,the analysis of three degrees of freedom is put forward.Combining with the working principle of the converter,the power transmission characteristics and the total current RMS characteristics of the converter are obtained.Then the zvs conditions of all switches of the converter are analyzed.Based on the characteristics of the total current RMS and the actual zvs condition of the switch,a new modulation strategy is obtained based on the traditional modulation strategy.Then an experimental platform of 30-60 V input and 400V/2.5A output is designed,including the design of transformer and inductor,the choice of switchs and the realization of digital control,etc.Finally,the feasibility of the proposed converter and its control strategy and the correctness of the theoretical analysis are verified by experiments.