Abstract
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.
© 2014 IEEE
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