A Precise Passive Current Balancing Method for Multioutput LED Drivers

The multistring LED in parallel is a popular structure in backlight and lighting applications. The current balancing for each string is required to achieve uniform luminance and reliable operation. This paper proposes a simple and precise lossless passive current balancing circuit for multiple LED strings application. With a dc block capacitor in series with the transformer secondary side and the proposed two outputs rectifier structure, the currents of the two outputs can be automatically balanced due to the charge balancing of the capacitor. Compared to the conventional active current sharing technique using switching converter or linear switch, the proposed methods can achieve high efficiency and low cost. The proposed method can be applied to both current-fed topology and voltage-fed topology, which can also be extended to even outputs application. The basic two balanced output structure with a voltage-fed half-bridge topology is analyzed in this paper to show the detailed operation principle. The design considerations for continuous conduction mode and discontinuous conduction mode operations are also presented. Performance of the proposed circuit is validated by the experimental results from a 60 W prototype with 400-V dc input.