Transformer-Isolated Resonant Driver for Parallel Strings With Robust Balancing and Stabilization of Individual LED Current

This paper presents a transformer-isolated resonant driver for parallel strings with robust balancing and stabilization of an individual LED current. The methodology is based on connecting multiple transformers in a daisy chain to balance the string currents and using them to form part of the resonant tank in a series-resonant converter. The input source and output strings are electrically isolated by transformers. Such structure behaves like a constant current source with multiple outputs for multiple LED strings. The string currents are found to be insensitive to the string voltages. In order to ensure service continuity of LEDs, each LED in a string has a bypass thyristor, which is triggered upon an open-circuit failure, so that the rest of the healthy LEDs in the string would still be operated. A 21 W prototype with seven strings has been built. Each string consists of three 1-W LEDs connected in series. Experimental results show that the LED current can still be regulated at the reference value within ±1% error even if the whole lamp has only one piece of LED in operation and the rest of LEDs fail short or open. Modeling, analysis, and design of the proposed driver will be given.