Capacitor-isolated multistring LED driver with daisy-chained transformers

By extending a prior-art transformer-isolated structure for driving multiple LED strings, a scalable and modular capacitor-isolated architecture is presented in this paper. The proposed idea is based on first utilizing a daisy-chained transformer structure to share equally the current delivered from a half-bridge switching circuit and supply the shared currents to multiple string networks and then using a capacitor network in each string network to derive the string current and separate the grounds of the input source and the strings. The capacitor networks give further advantage of balancing the string currents, as compared to the prior-art transformer-isolated structure. Dimming function is achieved by adjusting both the switching frequency and duty cycle of the switching devices in the switching circuit. A 21-W prototype with seven strings has been built and evaluated. The lamp can be dimmed to 10% of full output, and the LED string current can be regulated at less than ±1% error, even under significant variations in the string voltages and some LED failures.