Study On Sensorless Current And Voltage Balancing Technology Based On Zero-voltage Soft-switching In Near-CRM Mode

In the battery energy storage system based on DC microgrid,the power electronic equipments represented by grid-connected converter and DC power converter are its core component.To achieve high efficiency,low cost and small volume of power electronic equipment has always been the unremitting pursuit of the industry,especially for energy storage applications that need high power and economy.In order to reduce the cost of energy storage system,at present,the non-isolated converters are generally used.And the traditional technology is to control the converters working in CCM mode with low ripple mode,which leads to the problems of large inductor,difficult to realize soft-switching and low efficiency.In addition,the interleaved and multi-level technologies are widely used in high-power converter application so as to increase converter power and reduce the volume of passive components.However,there are current balancing problem in interleaved topologies and voltage balancing problem in multi-level topologies,which lead to some challenges in using interleaved and multi-level technologies.In order to solve these problems,based on the new points of ZVS in near-CRM mode,this dissertation studies the new sensorless current balancing and voltage balancing technology applied to interleaved and multi-level topologies,and puts forward the corresponding control methods.The main points of this dissertation are as follows:(1)Compared with the traditional CCM mode with hard-switching,the near-CRM mode not only reduces the inductor volume,but also eliminates the turn-on loss and diode reverse recovery loss completely.In this dissertation,ZVS is reviewd from the resonance of near-CRM mode.Firstly,the working mode of near-CRM and ZVS process are analyzed in detail,and the analytical expressions of resonance process are established.It is realized that resonance occurs in the turn-off process and zero-voltage turn-on is spontaneous,and based on these,strict ZVS boundary conditions are proposed.Then,based on the analyses of the turn-off energy transfer and loss mechanism of ZVS switch,it is realized that the turn-off energy is not completely lost,and a test method of real turn-off loss is proposed.Finally,the experimental results verify the correctness of the resonant process analysis and boundary conditions of ZVS,and the reduction of the real turn-off loss of IGBT and Si C MOSFET are measured and compared.(2)Based on the new review of the resonant transition time of ZVS,the sensorless current balancing mechanism applied to interleaved topology is found.Firstly,the inverse function relationship between the actual equivalent output duty cycle and the offset current is established.Therefore,the current self-balance mechanism and its feedback mechanism for interleaved DC-DC converter based on the near-CRM mode are analyzed,and the effectiveness of current self-balance is evaluated.Furthermore,a variable switching frequency control method to realize ZVS and current self-balance is proposed.The new method has the advantages of low cost of current balancing,high efficiency of soft-switching and near-CRM to reduce the volume of inductor.Finally,the experiments verify the mechanism of current self-balance,current balancing effectiveness and variable switching frequency control,and compare the difference with fixed switching frequency control.The highest efficiency of the 20 k W experimental prototype reaches 99.1%.(3)Furthermore,it is found that the feedback compensation mechanism of the actual equivalent output duty cycle is also suitable for voltage balancing of multilevel topology.Specifically,based on a three-level DC-DC converter,the resonant transition and ZVS process are analyzed in detail.Therefore,the functional relationships between the actual equivalent output duty cycles of the upper/lower half-bridges and the capacitor voltages are derived.Then,the feedback mechanism of voltage self-balance is explained and the feedback model of voltage self-balance mechanism is established.Furthermore,the corresponding variable switching frequency control method is proposed to realize the capacitor voltage self-balance in a wide operating range of the converter.The experimental results show that the new method not only realizes ZVS turn-on,but also realizes the voltage balancing without voltage sensors.(4)In order to make use of the sensorless current balancing and voltage balancing methods,the core mechanism of realizing the current and voltage self-balance is analyzed.According to this mechanism,for an interleaved three-level DC-DC converter,in order to realize voltage and current self-balance simultaneously,the working principle of the converter,the ripple and cancellation of inductor current are analyzed in detail.A unified method of calculating the peak value of the inductor current ripple is put forward,and a unified expression is derived,so as to realize the variable switching frequency control.By the proposed variable switching frequency control method applied to the interleaved three-level DC-DC converter,the experiments verify the analyses of ripple current and cancellation,and the self-balance of current and voltage can be realized simultaneously.(5)The realization method to extend the principle of variable switching frequency control and current self-balance to DC-AC inverter is studied.This dissertation analyzes the near-CRM working mode of interleaved DC-AC inverter,and studies the calculation method of interleaved current ripple cancellation for the variable switching frequency.Furthermore,the feasibility of current self-balance in interleaved DC-AC inverter is analyzed and evaluated.Compared with the traditional fixed switching frequency,the proposed variable switching frequency control makes the interleaved inverter not only realize all switches ZVS turn-on,but also realize the inductor current self-balance,so that no additional current balancing sensors are needed.In general,this dissertation makes use of near-CRM mode to realize the new idea of high efficiency,current balancing and voltage balancing of power electronic converter.Not only the strict ZVS boundary condition and the measurement method of real turn-off loss are put forward,but also the self-balance mechanisms of current and voltage are found firstly,and their effectivenesses are analyzed in detail.By the proposed variable switch frequency control method,ZVS soft-switching and sensorless current and voltage balancing are realized in the full working range.