Reseach On Key Technologies And Applications Of Push-Push Converters

Recently,the solar,wind and fuel cell,etc.,renewable energy source generations are in the stage of rapid development and large-scale applications,in which DC-DC and DC-AC converters are taken the important parts.The push-pull converters are suitable for these low-input-voltage applications due to attributes of simple circuitry,galvanic isolation and better transformer utilization,etc.The push-pull converter includes voltage-fed and current-fed types and both of conventional ones suffer from the problems of hard-switching,high switching loss and high voltage stress.The voltage-fed topology has lower voltage stress and smaller filters compared to the current-fed type under the same specifications,but it requires a larger transformer turns-ratio.Although the current-fed one has lower transformer turns ratio and smooth input current,the switch stress is high.Therefore,many solutions have been proposed in the past literatures regarding these drawbacks.However,some of them still work at hard-switching,high electrical stress,un-regulation output,complexity clamping-circuit and high circulating current,etc.In this dissertation,key technologies and applications of push-pull converters have been researched and figured out regarding these issues,which provides a considerable solution for high efficiency,high reliability and high power density power conversion.The improvement and capacity expansion of voltage-fed push-pull converter(VPPC)have been investigated.The voltage-fed ZVS three-switch push-pull converter is derived firstly with the fix switching frequency modulation and two kinds of auxiliary net are proposed with respect to the limitations of high circulating current,secondary voltage ringing and narrow soft-switching load range.Basically,two families of improvement ZVS VPPC topologies are presented.In order to suppress the voltage stress,a wide ZVS voltage-clamped VPPC with voltage-stress less than twice input voltage is deduced,as well as discussing a list of its enhanced topologies.Besides,the input-parallel output-series(IPOS)concept and three-phase VPPC are adopted in view of the capacity expansion of VPPC.The ZVS improvement three-phase VPPC are proposed and discussed at length.Furthermore,theoretical analysis,key features,design guidelines and experimental verification have been fully conducted by three typical VPPC enhanced converters.In addition,the enhancement and capacity enlargement of current-fed push-pull converter(CPPC)have been studied.One improvement CPPC structure is derived in accordance to the equivalent power flow path principle.All switches can realize ZVS turn-on and rectifier diodes can achieve ZCS turn-off by utilization of magnetizing current solving the drawbacks of clamping circuit complexity and hard-switching under light load in the previous CPPC types.Moreover,via employing an inductor in the secondary,all switches and diodes can achieve ZVS turn-on and ZCS turn-off over a wide load range,respectively.Similar to VPPC,this chapter further discusses the IPOS and three-phase strategy to enlarge the capacity of CPPC and presents a simple ZVS three-phase CPPC structure interfacing the low input-voltage applications.Also,theoretical analysis,key features,design considerations and experimental verification have been fully discussed by three typical solutions.Concerning the requirement of bidirectional power conversion and single stage high-efficiency DC-AC conversion in the renewable energy system,the proposed VPPC/CPPC improvement topologies are expanded to bidirectional DC-DC conversion and high-frequency-link DC-AC inversion.A family of bidirectional push-pull bridge DC-DC converters and that of high-frequency-link DC-AC inverters are presented and discussed.Especially,the push-pull full-bridge and push-pull full-wave types are in-depth analyzed,designed,compared and verified by two prototypes,respectively.Results reveal that the proposed topologies have lower switching loss,lower circulating current and simpler circuit structure in comparison with the similar characteristics converters such as dual-active-bridge(DAB)and full-bridge high-frequency-link DC-AC converters in the past.The higher efficiency,higher reliability and lower cost can be obtained with the proposed solutions in real applications.Finally,the RGD application of the improved topologies has been also expanded and investigated and a list of ZCS isolated push-pull RGD topologies(PPRGD)are presented,including the single-channel and dual-channel,respectively.The operation principle,loss breakdown,design guidelines and experimental verification of typical dual-channel PPRGD are full-scale presented and its applications are discussed.