Research On Step-down Three-phase PFC Converters And Its Nonlinear Control Strategy With Robust Analysis

Harmonic current pollution of power system is becoming more and more serious,with wide utilization of power electronic equipments,which contain many uncontrolled or phase-controlled rectifiers.Nowadays,active power factor correction(APFC,named with PFC if not specified)technology is widely used in single-phase electrical facilities,effectively reducing the harmonic current level.The coupling of three-phase power system complicates the application of PFC technology that has well developed in the single-phase.Compared with the boost-type,three-phase PFC circuits based on buck have lower voltage stress,wide step-down output range,and strong control capability of input and output current,which are of critical importance in high input voltage or power level,such as power supplies in data centers,chargers of electric vehicles boomed recently.Meanwhile,with the development of superconducting magnetic storage,buck-type three-phase PFC technology is becoming a promising technology.Especially,the step-down three-phase PFC circuits based on harmonic-current-injection(HCI)concept have attracted much interest featuring low voltage stress,low switching loss,simple control,etc.It has important theoretical value and practical significance to carry out the research on the topologies and control strategies of this class of three-phase PFC converters.(1)Focusing on the HCI step-down three-phase PFC converters,one kind of buck-type three-phase converter,termed as Swiss Rectifier is introduced.To reduce the input current distortion due to switching dead time of HCI,an enhanced HCI network is proposed,which allows a small overlapping in switching,improving input current quality and system reliability.The operating principles,steady state characteristics and device stresses of improved converter are analyzed in detail.The differences between synchronous and interleaved driving patterns of the two high-frequency switches are studied,concerning on input current distortion,inductor current ripples and system efficiency,and a design guideline is given.All above are validated by simulation and prototype experiment.(2)The dynamic modeling based on energy balance is adopted.The EL models of HCI step-down three-phase PFC converter are constructed in ideal and non-ideal condition.According to the EL models,the equivalent circuits of averaged switching period are derived.The EL models have the characteristics of general and systematic,without any approximation.During modeling process,all the nonlinear information of system retains.(3)The input current quality of the proposed buck-type three-phase PFC circuits is sensitive with current ripples of DC side inductors.Based on one-cycle control(OCC)technology,a nonlinear control strategy is proposed,which effectively reduces input current THD.According to the equivalent DC-DC circuit gained,the control model is deduced,with excellent parameter control performance(robust performance).Lyapunov second law proves the nonlinear control has wide operation stability(robust stability).The control circuit design considers the phase shift between the input current and voltage due to input EMI filters.All the above analyses are confirmed by simulation and experimental results of a prototype.(4)In some low output voltage power supplies without need for galvanic isolation,the switches in buck-type three-phase PFC circuits should work at a small duty cycle for large step-down capability,which will result in high device stresses,poor efficiency,or even make the control failure.A large step-down three-phase PFC circuit is proposed,with the combination of modified HCI network and switched-inductor structure.The duty cycle is extended,and the equivalent DC-DC model is deduced,voltage transfer ratio and ripples are studied,which are verified by experimental results.What's more,the current sharing feature of multiple modules in parallel is discussed.(5)In three-phase PFC converters featuring high voltage and current,the soft-switching technology is investigated for efficiency improvement.Focusing on low stresses passive soft-switching strategies,a passive soft-switching cell is proposed featuring minimum voltage and current stresses,which is suitable for all six basic PWM DC-DC converters.Its operating principles are analyzed in detail by example of buck;furthermore a general circuit model is deduced.Design guidelines of soft-switching cell are given on the basis of the general circuit model.An efficiency comparison between soft and hard switching is deployed in Swiss Rectifier prototype.