Modeling and design of current driving circuits for light-emitting diodes

Abstract

The development progress of light-emitting diodes (LEDs) and their related technologies have skyrocketed in recent years. The rapid growth of the LED related technologies can be attributed to the break-through in the LED's luminance efficacy improvement and the development of new materials for making white color LED phosphor. Along with the development of new LED technologies, different LED driving circuits and driving methods have been studied for optimizing the system's performance in terms of power efficiency, luminance efficacy, thermal design, color and light quality, power factor and cost. Recent research has shown that the above mentioned performance aspects can be affected by the choice of the LED driving method. The benefits and drawbacks of different LED driving methods, including amplitude mode (AM), pulse-width modulation (PWM) and bi-level have been extensively studied and analyzed. Notwithstanding the abundant performance studies and analyses of the LED driving methods, there is insufficient fundamental research on the characterization of LED driving systems. Thus, the objective of this thesis is to undergoan in-depth investigation of the circuit design and properties of various LED driving methods. This thesis consists of four parts. In the first part, we examine the problems of the existing LED models, specifically, the effect of junction temperature on the AC and DC resistance of the LED are difficult to predict and the semiconductor parameters required in the model are not readily measurable. Consequently, a numerical model of the LED using laboratory measured data is developed and proposed. The model characterizes the relationship of the forward voltage, the forward current and the case temperature of the LED and the evaluation on the values of DC and AC resistances under various conditions is made possible. In the second part of this thesis, an in-depth survey on the implementation methods, namely AM, PWM and Bi-level current driving methods, are conducted. The LED current waveform and the dimming factor are defined on each of the driving methods. In the third part of this thesis, the large-signal and the small-signal models for LED current driving systems are derived. It can be used to evaluate the steady-state operation point of the system and predict the system loop gain as well as the system stability. In the last part of this thesis, a systematic synthesis procedure for deriving new topologies for DC-DC power converters is proposed. The proposed method applies the fundamental duality principle of electrical circuits to generate new DC-DC power converters. The method can identify the uncovered basic power converter structures including voltage-to-current power converters which are potentially suitable for use in LED driving systems.