A Ring Diode–Capacitor Network for Current-Balancing Multiple LED Strings

A ring diode-capacitor network for current-balancing multiple LED strings is presented. The concept is based on utilizing the charge-balance property of capacitors to balance the currents of two adjacent LED strings in each switching cycle. Such current-balancing mechanism propagates over a ring network. The network has several merits, including 1) high current-balancing accuracy, 2) a simple structure, 3) modular and scalable, 4) current-balancing property being less dependent on the string voltages and the values of the capacitors, 5) inherent galvanic isolation between the driver and the LED strings, and 6) operation of the healthy LED(s) being unaffected by the failure LED(s). In addition, the capacitors in the network can be used as resonant tank components for the front-stage driver. An 80-W prototype for balancing the currents of ten LED strings has been built and evaluated. Each string has eight pieces of 1-W LED connected in series. The prototype is driven by a series-resonant converter. Results reveal that the string current variation is less than ±0.5%, irrespective to the variation of the string voltages and capacitor values. The total LED power can be dimmed to 10% of the rated output by a hybrid switching frequency and duty-cycle control scheme. The overall efficiency from the driver input to all LED strings is around 95% over the operating range. Modeling, design, and analysis of four possible configurations will be given.