Research On Zero Current Switching Full Bridge DC/DC Converter

To develop new energy power generation is an important supporting mean for our country to achieve the “double carbon” goal.Whether it is new energy power generation such as photovoltaics,wind power,or loads such as data centers,the DC characteristics of the ports are becoming more and more obvious.The DC characteristics of the source load will greatly increase the demand for DC converters in the future.It also puts forward higher requirements for its power density,efficiency and reliability.In particular,the intermittent and random characteristics of new energy make traditional power equipment,grid structure and operation technology unable to withstand a large amount of new energy being connected to the grid.The new energy DC grid connection technology is one of the effective solutions to solve the above problems.The dissertation studies the high-power DC converter in the medium-voltage DC collection system of new energy power generation from the aspects of circuit topology,soft-switching technology and optimal design.This dissertation summarizes and analyzes the circuit structure of full-bridge converters and the development regular pattern of soft switching technology commonly which used in high-power DC conversion applications.The dissertation points out that zero voltage switching full bridge converters are suitable for applications using MOSFET as power devices.The zero voltage and zero current switching full bridge converter is suitable for IGBT as a power device,but it still achieves zero voltage switching beyond the front bridge arm.And finally,the zero-current switching full-bridge converter is further developed,and all its main power devices can realize zero-current switching,which is more suitable for high-power DC conversion occasions using IGBT as power devices.In response to the issue of a type of ZCS full bridge converter requiring high-voltage filtering inductance in existing literature research,this dissertation proposes a multiplexed bridge arm combination type ZCS full bridge converter.It uses low-voltage small inductance to achieve energy transmission,and the output side adopts capacitive filtering,which is more suitable for medium-voltage DC collection of new energy power generation.This dissertation analyzes the soft-switching characteristics of the converter,which can realize zero current switching turn-on and turn-off of four main power devices in the full load range,and zero voltage zero current switching turn-on and zero voltage switching turn-off of two auxiliary switches.At the same time,the transformer turn ratio is optimized and designed.However,the converter also has some shortcomings,such as the existence of a certain circulating current on the primary side,the auxiliary transformer does not participate in energy transmission,and the capacity of the main transformer is higher than that of the converter.For this reason,the above full-bridge converter is improved in Chapter 3.It adjusts the same terminal and rectification method of the secondary winding of the auxiliary transformer,and then connects a blocking capacitor in series in the primary winding of the auxiliary transformer.In this way,on the basis of retaining the original soft-switching characteristics of the converter,the auxiliary transformer can also participate in the energy transmission to reduce the capacity requirement of the main transformer,and at the same time,the circulating current on the primary side is removed,and the conduction loss is reduced.In Chapter 3,the dissertation provides a detailed analysis of the working principle of the ZCS full bridge converter,discusses the selection principles of blocking capacitors,optimizes the turn ratio selection of the main transformer and auxiliary transformer,and conducts simulation and experimental verification.In order to adapt to the development trend of the higher and higher voltage of the new energy power generation port,combined with the neutral point clamp three-level technology,a circulating current-free ZCS three-level composite full-bridge converter is proposed in Chapter 4,which consists of a half-bridge three-level circuit and a full-bridge circuit are formed by multiplexing two switches.Therefore,the voltage stress of all switches is only half of the input voltage,which is beneficial to the application of the converter to high input voltage occasions,and at the same time,the soft switching characteristics of all switches remain unchanged and the primary side circulating current is eliminated.In order to further reduce the peak current of all switches and the turn-off circuit of auxiliary switches,LC series resonance technology is introduced in Chapter 5,which can realize that the turn-off current of auxiliary switches is lower than its peak current and further reduce switching losses.