| Summary: | 碩士 === 國立臺灣科技大學 === 電機工程系 === 107 === PWM dimming method has been widely used for driving LEDs, and the average current through LEDs can be linearly controlled by adjusting the duty cycle of the dimming signal. However, most illuminance-current characteristic curve of LEDs is nonlinear. The smaller the current is, the larger the ratio of illuminance to current is, and will gradually decrease as the current increases. Hence, optimal overall electrical-to-light conversion efficiency cannot be fully achieved using PWM dimming method.
In order to improve the luminous efficacy of LEDs, two dimming methods are proposed which are varied current sensing resistance method and varied dimming voltage method. The determination of the optimal transition points is first investigated in this thesis. Next, an 115W LED driver is constructed and the proposed dimming methods are realized using dsPIC33FJ16GS502 digital signal controller (DSC). Finally, illuminance-power curves, CIE1976 and CIE1931 chromaticity diagrams are presented and discussed with the aid of experimental results. Compared with PWM dimming, the average output power reduction of the LED driver under the same illuminance are 17.08% and 13.17%, respectively; the average illuminance improvement under the same output power are 13.66% and 11.17%, respectively.
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