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LEDs are potential successors of incandescent lamps with high luminous efficacy and long lifetime. To improve the overall luminair efficacy and lifetime, the power efficiency and lifetime of LED ballasts become important factors. Efficiency gain in transformerless power converters appears attractive for applications without isolation. Driving solid-state LED bulbs in an existing lighting fixture such as PAR30 style housing from universal mains necessitates a high-voltage step-down ratio in order to produce an output voltage of about 10-20 V, which is very common in LED lighting applications. Traditional nonisolated step-down pulse width modulation buck converters may suffer from poor efficiency due to the long diode freewheeling time at small duty cycles. In this paper, we propose a resonance-assisted buck converter to achieve a high-voltage step-down ratio and high converter efficiency, whilst maintaining durability and compatibility with existing incandescent dimmers. The performance of the proposed LED driver is verified experimentally..