Development of driver and color control system for high-power RGB LED

Abstract

A bilevel driving approach was proposed in the literature for mitigating the energy wastage associated with driving LEDs at high peak current. The main idea is to introduce two drive parameters, i.e., high/low PWM current levels and duty cycles, that give rise to a 2-D luminosity control capability. The same idea was later generalized to an n-level driving approach for maximizing the electrical-to-light conversion efficiency of LEDs. Although previous uses of the n-level driving approach have been focusing on improving the luminous efficacy of LEDs, it is shown in this thesis that its 2-D luminosity control feature can also lead to a significant improvement in color resolution when applied to driving RGB LEDs, hence making it well suited for applications in large-area LED display panels. In comparison to the first prototype driver proposed by the inventor of the driving approach, a more practical implementation of the complex 2-D driving approach with digital microcontroller is demonstrated. The various technical aspects of the driving approach, including luminous efficacy, color resolution, and color stability over dimming, are discussed in this thesis with the aid of experimental results. This thesis also presents a method of controlling the white color point in red/green/blue (RGB) LED driver system. In contrast to conventional systems where the average driving currents of the primary-color LEDs can become saturated when the LEDs have sufficiently aged, and causes the resulting white color point to go out of regulation, the proposed method avoids this problemby adjusting the color set points when a pre-defined threshold current is reached by one or more of the primary-color LEDs. It is shown that the method can effectively maintain the white color point of the RGB LED at the desired value when the LEDs are subjected to an accelerated ageing through repetitive current stress cycles.