A comparison between open- and daisy-chain transformer structures for current-balancing multiple LED strings

A comparison between the open-chain (OC) and daisy-chain (DC) transformer structures for current-balancing multiple LED strings will be presented. The methodology starts with deriving a general analytical expression for describing the current distribution in both structures. The derived model is then used to study the differences in the string currents to the variations of the LED string voltages, magnetizing and leakage inductances of the transformers. Results show that the DC structure gives better current-balancing properties than the OC structure. The required whole size of the transformers for DC structure is smaller than that for OC structure with achieving the same level of current balancing, even if the DC structure requires one more transformer than the OC structure. The findings have been verified successfully on a 21W prototype with seven strings.