Design And Implementation Of A Three-Level DC-DC Converter With Soft Switching Characteristics In Full Load Range

With the development of power electronics technology,high-voltage and high-power converters are widely used in energy storage power stations,photovoltaic power generation and other fields.The power supply of their control and drive circuits depends on the auxiliary power supply.The voltage stress of the switching tube of the isolated three-level DC/DC converter is only half of the input voltage,which can be directly drawn from the high voltage of direct current generatrix,suitable for auxiliary power applications.This paper mainly focuses on the related technology of high voltage DC input isolated three-level converter.Firstly,the working principle of isolated three-level DC/DC converter is studied.The reasons for the loss of secondary duty ratio and the unbalance of input capacitor voltage of isolated three-level DC/DC converter are described.Aiming at the problem that the converter can not realize the soft switching in the full load range,the realization conditions of the soft switching in different bridge arms are analyzed,and a new active commutation auxiliary circuitis proposed,which can realize the soft switching in the full load range.The parameter design method of the active commutation auxiliary circuitis is given,the current and voltage stress and loss of the active commutation auxiliary circuitis are studied,and the simulation study of the isolated three-level DC/DC converter based on the active commutation auxiliary circuitis carried out.It is proved that the proposed active commutation auxiliary circuitcan realize the soft switching of the isolated three-level DC/DC converter in the full load range.Secondly,the control strategy of isolated three-level DC/DC converter system is studied.In order to improve the dynamic control effect of the system,the PIMPC control strategy of voltage outer loop as PI controller and current inner loop as model prediction controller(MPC)is adopted in this paper.The equivalent prediction model of isolated three-level DC/DC converter is established,the value function is designed according to the control objective,and the optimal current control duty ratio is obtained.Through simulation study,the dynamic performance of PIMPC cascade control strategy and the traditional cascade PI control strategy is compared and analyzed in the startup process and the input voltage changes.The dynamic performance of the adopted PIMPC cascade control strategy is better than the traditional cascade PI control strategy.Finally,the software and hardware of the system are designed and the experimental platform is built to verify the isolated three-level DC/DC converter researched in this paper.An experimental prototype with input voltage of 500 V,output voltage of 24 V and rated output current of 10 A is designed and made.Soft switch experiment in full load range and comparison experiment between PIMPC cascade control strategy and traditional PI cascade control strategy are carried out.Experimental results show that the proposed active commutation auxiliary circuitcan realize the full load range soft switching of isolated three-level DC/DC converter and improve the efficiency of the converter.The dynamic performance of the PIMPC cascade control strategy is better than that of the traditional PI double closed-loop control strategy.