Use of Daisy-Chained Transformers for Current-Balancing Multiple LED Strings

A modular and scalable structure that can perform current-balancing multiple LED strings is presented. The current-balancing function is accomplished by connecting multiple transformers in a daisy chain-the primary and secondary windings of each transformer are connected to two different modules, and thus the two module currents are mutually coupled. The modules are shared with the same input, derived from the output of a switching network, and each module is used to drive an LED string. When there is a fault occurred in a string, such as open- or short-circuit of any LED, such string will be disconnected from the corresponding module. As the output voltage of the module will then increase under such situation, an energy-recycling circuit with each module having an OR-ing diode connected to its input will be activated to clamp the module voltage by recycling energy back to the source. Thus, the faulty string(s) will not affect the normal operation of the healthy strings. A prototype with seven 1 W LED strings has been built and evaluated. Experimental results show that the string current has less than ±1% variation under different dimming levels, string voltage variation of ±30%, and faulty operations in some LED string(s).