In LED lighting systems, consideration about dimming and flicker is necessary. It is found that conventional DC-mode technique produces flicker-free light and high luminous efficacy but poor dimming function, whereas pulse-width-modulation (PWM) technique offers better dimming flexibility but inherent flicker in comparison to DC technique. Although increasing the PWM dimming frequency could make the flicker invisible to the human eyes, the flicker still exists, and the potential health risks should not be ignored. In this paper, a cost-effective DC driving technique is proposed to improve dimming flexibility of existing DC technique while retaining flicker-free and high luminous efficacy. A current-controlled buck converter is operated in discontinuous conduction mode (DCM). LED current is regulated by the number of inductor current pulses during the dimming period. A method to distribute PWM pulses over the dimming period is utilized to ensure a small output voltage ripple without a large output capacitor for the buck converter. A RGB LED lamp was used in the experimental verifications. The results show that linear constant DC through LEDs could be obtained and flicker could not be found. The main tradeoff is the reduction of control precision, which is acceptable for LED outdoor lighting systems.
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