LLC resonant converter with variable resonant inductor for wide LED dimming range

In this paper, LLC resonant converter with direct output current controlled variable resonant inductor (VRI) is proposed and studied. The output LED current is directly flowing through the auxiliary windings of VRI as the DC bias current to control the inductance of the VRI and no additional control circuit is needed. The VRI is smallest under rated LED current and increases as LED current decreases. Therefore, the resonant frequency also decreases with LED current and the switching frequency variation range is reduced. In this way, the dimming range of LLC converter with VRI can be extended. Because the same optimized parameters with conventional LLC converter can also be used, the proposed LLC converter with VRI can achieve high efficiency especially under heavy load. Detailed design considerations on this proposed method were presented. Finally, an 110W LLC LED laboratory prototype with 83V/1.32A output was developed. The proposed LLC converter and the conventional LLC converter were tested on the laboratory prototype. Experimental results show that the peak efficiency of the proposed converter is 95.9% and the LED current can be dimmed up to 5% of the rated current.