Precise luminous flux and color control of dimmable multi-string light-emitting diode systems

Abstract

A generic LED system involves complex photometric, electrical, thermal, and chromatic interactions, all of which make precise control of luminance and color mixing in high-quality lighting applications challenging. Multi-string LED drivers such as single-inductor multiple-output (SIMO) converter for precise dimming and current regulation are proposed. However, the relationship between the electrical and optical properties remains ambiguous. Therefore, new empirical models relating the SIMO circuit and color perception are necessary for accurate LED color control.

The thesis will first introduce new nonlinear empirical models of a practical RGB LED system with closed-loop control. They enable precise prediction of luminous flux and color coordinates by using three distinct reference voltages as control variables for independently regulating the current flowing through the red, green, and blue LED strings. The proposed empirical models are experimentally validated using a DC-DC single-inductor three-output (SITO) LED driver with proportional-integral (PI) compensators and a time-interleaving control scheme. The measured values of luminous flux and color coordinates are very close to the predicted values from the models.

In the subsequent study, new nonlinear empirical models are extended for practical bi-color white LED systems with feedback control. A hardware prototype of a single-inductor dual-output (SIDO) boost converter with a time-multiplexing digital control scheme is implemented to construct and experimentally verify the nonlinear luminous flux and correlated color temperature (CCT) models. Similar to nonlinear models of the RGB system, the proposed models enable accurate prediction of luminous flux and CCT based on the control variables in the feedback loop for independently regulating the current across the warm-white and cool-white LED strings. The experiment results confirm that the measured values of luminous flux and CCT agree closely with the corresponding predicted ones.