Development Of Isolated Bidirectional DC/DC Converter Without Reactive Power Circulation

Owing to its advantages of two-way power flow,voltage transformation,electrical isolation and easy control,isolated bi-directional DC/DC converter is widely used.As converters are used in high-power application such as micro-grid and energy storage systems,electric vehicles and power electronic transformer,its power and voltage grade are constantly improving.So,there are higher requirements for its performance like output voltage stability and efficiency.For dual active bridge topology which is widely used,although it has prominent advantages in various aspects,its issue of the reactive circulation is the main factor that hinders the improving of performance.Adopting a current-source type topology without reactive circulation,this thesis develops an experimental prototype of isolated bi-directional DC/DC converter with 1500W rated power and nominal voltage 160V and analysis the principle,efficiency and control strategy of prototype.Combining with pulses of switch tube,this thesis analyzes four work modes of converter in one switching cycle,and illustrates the working principle.Aim at complex application condition,Buck-Boost extended topology is proposed based on this circuit,and control strategies of two topologies are proposed.Simulation and experimental results verify that the proposed topology has no reactive circulation,can realize two-way power flow and the control strategy can validly realize control targets.Futhermore,Buck-Boost converter can realize switch of two kinds of modes.To build the experimental prototype of converter,components parameters of main circuit is designed.According to the ripple requirements,capacitance and inductance are calculated,then the magnetic core and winding number of isolation transformer is design,and the switching devices are selected according to the voltage and current conditions.Considering the influence of transformer leakage inductance in the practical application,this thesis gives the setting scheme of RCD absorption circuit and designs the specific parameters of absorbing capacitor,resistor and diodes.Through the analysis of experiment waveforms,it is concluded that the device parameters meet the design requirements,RCD absorption circuit can achieve leakage energy absorption and reduce voltage surge.In view of the converter efficiency,this thesis establishes loss mathematical models of switching devices and magnetic element based on the operating characteristics of the devices;Combined with the actual operation conditions of the converter,the loss value of switch tube and magnetic components is concretely analyzed,then the whole power loss and efficiency value are obtained;By efficiency experiment,the bidirectional efficiency curve is gotten and the comparison verifies the correctness of the loss analysis method.In order to study converter system stability under small signal disturbance,according to the average switch model of equivalent Boost circuit and closed loop control block diagram,this thesis deduces the transfer function of closed-loop system,analyzes the system stability when individual parameter changes,and gives the parameter selection range.Simulation verifies that change the parameters of converter can effect system stability,and the experimental results prove the effectiveness of stability analysis method.