Study On Topology And Control Of Single-stage Electrolytic Capacitor-less LED Driver Operating In Universal-line Range

With the development of LED lighting technology,the LED driver with long life,high efficiency,and cost effective is the main direction.The conventional solution for this branch includes two categories,two-stage and single-stage configuration.The two-stage configuration is easier to design and optimize,and it can achieve high power factor and tight output current regulation.However,it exhibits a poor efficiency,low power density,and high cost for more stages.In contrast,single-stage LED driver has the merit of higher efficiency and lower cost,but it has component stress problem under light load and it is hard to design.We believe single-stage topology has a brighter future.Therefore,the single-stage forward converter is taken as the object,to study the wide-range input technique,the electrolytic-capacitorless technique,and soft-switching technique,to realize the industrial development of single-stage LED driver.Thus,this thesis is dedicated to the study and design of single-stage LED drivers with long lifetime,and high efficiency.The main contents and contributions of the thesis are listed as follows.(1)A systematic analysis is made to the present situation of LED driver,It is pointed out that the main problems we encountered in LED driver is bad performance under wide-range input,short lifetime,low efficiency.Then the steady-state and dynamic characteristic of the single-stage boost-forward converter is given.The origin of voltage stress problem under light load is analyzed,and the corresponding solution in LED application is presented,that makes it available to LED driving.(2)A novel valley-fill single-stage boost-forward LED driver is proposed.The main merit of the proposed LED driver is that its design can be optimised under wide-range input due to the equivalent dc input and duty cycle varies in a narrow range.Comparing to the conventional voltage-doubler solution,it has two more merits.The first one is lower voltage stress under low input voltages.The second merit is the voltage stress on MOSFET,storage capacitor,and the diode in series with the third winding of the main transformer is reduced.(3)A novel filter-type electrolytic-capacitorless single-stage boost-forward LED drive technique is presented,to promote the lifetime of the single-stage forward LED driver.The bidirectional converter is used as an active filter,to construct the electrolytic-capacitorless single-stage boost-forward LED driver.Three work modes of the combined converter is analyzed,to give the rule for the storage capacitance selection.Study shows that this electrolytic-capacitorless technique has the merits of erasing the line-frequency ripple effectively,stablizing the output voltage,and a simpler constrction.(4)A low-ripple type electrolytic-capacitorless single-stage forward LED drive technique is proposed,to make the LED driver more cost-effective with long life.Present low-ripple circuit combined with single-stage flyback has some flaws due to the limitation of flyback converter.It has to work in deep discontinuous conduction mode and the output power is too small,or there is too much switches and the control is complex.The novel LED driver has the advantages of high power,less switches,simple control,and cost effective.(5)The soft-switching single-stage LED driver technique is proposed,to promote the efficiency of single-stage LED driver.After analyzing the efficiency of the single-stage forward LED driver,the single-stage LLC LED driver is studied.The study shows that,the valley-fill technique,the active filter technique,and the low ripple technique proposed for the single-stage forward converter,can all be applied to the single-stage LLC LED driver.Together with the soft-switching technique,the single-stage LLC LED driver will be the best single-stage LED drive technique.At the same time,the optimal design process based on the voltage stress of the storage capacitor,and the improved PWM dimming control scheme is presented for the soft-switching single-stage LED driver,and the problem that component stress might increases dramaticly under light load is resolved.The component stress can be refined to a certain range;therefore,a single-stage LED driver can be achieved with higher efficiency,higher power,and higher performance,based on the proposed control method.The study to sinlge-stage converter in this paper has good theoretical and realistic meaning to LED driving application.It offers great help in the developing of LED lighting.