Daisy-chain transformer structure for current-balancing multiple LED strings

This paper proposes a modular and scalable structure that can balance the currents through multiple LED strings. It is based on using daisy-chained transformers, in which the primary and secondary windings of each transformer are connected to two different modules, so that the currents through the two modules are mutually coupled. Moreover, the proposed structure can handle any faulty condition, such as open circuit or short circuit condition, in LEDs. Any faulty string(s) will not affect the normal operation of the other strings. A prototype with seven 1W LED strings has been built and evaluated. Results show that the string current has less than ±1% variation at different dimming levels, even if the string voltage has ±30% variation and there is an abnormal condition in the LED string(s).