Accurate Capacitive Current Balancing in Multistring LED Lighting Systems Based on Switched-Capacitor-Controlled LCC Resonant Network

The combination of high-frequency ac drive and capacitors offers an attractive passive current balancing solution for multistring light-emitting diodes (LED) lighting systems due to the advantages of compactness, high efficiency, and ease of modularity. Given that frequency-controlled resonant converters are commonly used as high-frequency ac drivers, the conventionally used switching frequency modulation could impair the effectiveness of capacitive balancing as the impedance of the balancing capacitors is a sensitive function of the switching frequency which is constantly varied for the purpose of output voltage regulation. In this paper, a constant-frequency capacitive impedance current balancing method is proposed. Compared to the conventional method, output voltage regulation of resonant converter is achieved by modulating the resonant frequency of resonant tank by switch-controlled capacitor (SCC) technique, which gives rise to constant balancing capacitor's impedance and constant LED current over a wide range of load conditions. Its implementation in an LCC resonant network is studied in this paper and a systematic design procedure of a SCC-based constant-frequency LCC resonant converter is presented. The effectiveness of the proposed method is experimentally verified by a 70-W experimental prototype.