This paper proposes a nonlinear approach of controlling the luminous intensity and correlated color temperature (CCT) of white light-emitting diode (LED) systems with dual color temperatures. This LED system is made up of a warm color LED source (2700 K) and a cool color LED source (5000 K). The luminous intensity of each of these LED sources is individually controlled by pulsewidth modulation. The overall intensity of the LED system is due to the combined emitted flux of both LED sources. Its overall CCT is the mixed average CCT of both LED sources. This proposed method is based on the nonlinear empirical luminous and CCT models of the LEDs, which take into consideration the thermal effect of LEDs on its luminance and CCT properties. With reasonable approximation, the theoretical models are simplified into practical solutions, which are translatable into real-life applications. It is experimentally validated that the proposed approach is considerably more accurate than existing linear approaches that do not consider color variations of LED sources. The idea is applicable to LED systems with multiple color temperatures and is not limited to white LEDs.

This paper proposes a closed-loop nonlinear method for precisely controlling the luminosity and correlated color temperature (CCT) of a bi-color adjustable light-emitting diode (LED) lamp. The objective is to achieve a precise and fully-independent dimming and CCT control of the light mixture emitted from a two-string LED lamp comprising warm-white and cool-white color LEDs, regardless of the operating conditions and throughout the long operating lifetime of the LED lamp. This control is formulated using the non-linear empirical LED model of the bi-color LED system. Experimental results show that with the proposed closedloop nonlinear control, both CCT and dimming control of the bi-color lamp is significantly more accurate and robust to ambient temperature variations, ambient light interference, and LED aging than the conventional linear control used in existing products. The maximum error in luminous flux employing the proposed nonlinear control method is 3%, compared with 20% using the closed-loop linear method. The maximum deviation in CCT is only 1.78%, compared with 27.5% with its linear counterpart.1 For future correspondence, please email: sctan@eee.hku.hk.

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CitationThis article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication.

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