High Step Up-down Isolation Bidirectional DC-DC Converter And Control Strategy Research

Bidirectional DC-DC converter is widely used in new energy power generation system,aviation power,electric vehicle battery management system and other fields.It is functionally equivalent to two unidirectional converters.The bidirectional converter can effectively reduce the volume and weight.It has important research significance and practical value.The bidirectional converters are divided into non-isolated and isolated.Non-isolated bi-directional DC-DC converter is simple and easy to implement,but cannot achieve electrical isolation.Compared with the non-isolated converter,the isolated bidirectional converter can effectively reduce the interference between the input and output,so that the power supply system works more stable.In this paper,a new type of isolated bidirectional DC-DC converter is proposed.By adjusting the duty cycle and transformer turns ratio,the proposed converter can obtain a higher voltage ratio.The main work of this paper is listed as followed:(1)A new type of isolated bidirectional DC-DC converter is proposed,which converts energy through two capacitors and high frequency transformer.The zero-voltage conduction of some switches is realized,which reduces the switch loss.By adjusting the duty cycle and transformer turns ratio,the proposed converter can obtain a higher voltage ratio.The working principle,the working time and the key current waveform of the converter in buck and boost mode is analyzed.The steady-state equations of each stage are obtained,and derives the voltage ratio of the converter,the voltage stress of the device and the boundary condition are derived.(2)According to the state of the four switches,the converter is divided into two state.The steady state equation of the converter is deduced in the two operating modes.Using the steady-state equation,obtain the state space averaging equation.Based on the small signal modeling method,the input-output transfer function and the duty cycle-output transfer function are derived.(3)The open-loop transfer function of the closed-loop system under the two modes of the converter is derived.The stability of the open-loop transfer function of the converter is analyzed according to the frequency domain method.And a passive advance correction network is designed according to the characteristics of the converter,which increases the phase margin of the converter and improves the stability of the converter.In summary,a novel high-gain isolated bidirectional DC-DC converter is proposed,and the working principle of the circuit is analyzed.The small-signal model and the transfer function of the system are obtained by using the state space averaging method.Based on the system open-loop transfer function,the passive pre-compensation network is designed to improve the stability of the system.A 20 W prototype is be designed,and the process of the key device design is introduced in detail.The experimental platform is set up based on the prototype.The experimental results verify the correctness of the converter design.The efficiency of the test prototype in the Buck and Boost mode respectively is 92% and 90%.The simulation results show that the converter has good anti-interference ability after adding the compensator.