| In today's advocating "low-carbon and energy-saving" society, LED lighting technology has caught more and more people's eyes in the lighting area due to its energy saving, high efficiency and environmental protection. In the application of high power lighting and backlighting, usually it's necessary to put LED strings in series-parallel structure to achieve the required power level. The brightness of LEDs is directly related to their forward current. However due to its inherent characteristic of voltage-current, it is necessary to adopt the current balancing method to ensure the LED strings connected in series-parallel structure have the same current. The research content of this paper is the current balancing method of multi-output LED driver and the related application issues.This paper firstly reviews the existing current balancing method of multi-output LED driver, which can be divided into two catalogs:active method and passive method and compares their advantages and disadvantages. To do more research on passive current balancing method, a new hybrid passive current balancing method with coupled inductor and balance capacitor based on SRC resonant converter is proposed in this paper. The basic operation principle is analyzed and the design consideration of the key parameters is described in detail, and finally the proposed method is validated by the experimental results.Then this paper points out that the original proposed current balancing method will have problem when the cathodes of all LED strings are connected together. The cause of current unbalancing is studied in detail and is that the coupled inductor will be short for a while during the switching period. In order to overcome the topology's defect, two new modified methods are proposed, theoretically deduced and simulated in detail in this paper. However, these two new methods can not ensure totally current balancing of all LED strings in theory. Then, another new method with voltage-doubler rectifier structure at secondary side is proposed and the paper analyzes this new passive current balancing method based on SRC&LLC resonant converter. Through the theoretical analysis and simulation, the topology structure is verified that it can ensure all the LED strings achieve current balancing. The passive current balancing method based on resonant converter topology structure when the cathodes of all LED strings are connected together is studied and summarized in detail in this section.This paper also proposes a capacitor-isolated LED driver with inherent current balancing capability based on SRC resonant converter. In this section, the concept of the capacitor isolation is described briefly, the basic operation principle is analyzed and the design consideration about the key parameters of the circuit is proposed. Finally, an experimental prototype is built and the proposed method is validated by the experimental results. Besides, this paper proposes the application of synchronous rectifier (SR) technology in LED driver with two current balancing outputs based on LLC-full bridge type circuit. The proposed driving scheme is the hybrid method which combines the voltage driving scheme and current driving scheme with current transformer (CT). The driving scheme's structure is simple and easy to be implemented. The use of synchronous rectifier (SR) technology will improve the efficiency of LED driver greatly when the biggish output current of the LED driver is required.At last in this paper, all the research work is summarized and the direction of future research is pointed out. |
