| Light-emitting-diode (LED) light sources, which are more compact, capable to change color in real time, less dissipative and more durable, are finding more applications than conventional light bulbs in domestic, commercial and industrial environments. However, LEDs have nonlinear optical, thermal and electrical characteristics, which are major hurdles for widespread lighting applications. Specifically, stabilizing colors of LED lights is a challenging task, which includes color light intensity control using switching mode power converters, color point maintenance against LED junction temperature change, and limiting LED device temperature to prolong LED lifetime. Also, requirements such as high power factor, long lifetime, accurate current control and high efficiency pose challenges to the design of LED ballast circuits.;This thesis is devoted to the study and design of effective LED lighting systems to overcome the inherent deficiencies and provide solutions for modern lighting applications. The main contributions of this thesis are summarized as follows.;1) To stabilize the colors of LED lights, we propose an LED junction temperature measurement technique for a pulse-width-modulation (PWM) diode forward current controlled red, green and blue (RGB) LED lighting system. The technique has been automated and can stabilize the color effectively without the need for using expensive feedback systems involving light sensors.;2) To drive an LED lighting system durably and efficiently, we design a first-end isolated power-factor-correction (PFC) pre-regulator for different power levels. The usage of low-voltage capacitors and resonant tank ensure the long lifetime and high efficiency of the whole LED lighting system. Some post-end LED current regulators are non-cascaded after the PFC converter for the further efficiency improvement with little additional cost.;3) To replace the incandescent lamps, we present the LED replacement lamp compatible with the existing incandescent dimmers. The LED replacement lamp uses a high-step-down-ratio and high-efficiency resonant assisted buck converter without using isolation transformers, which has the properties of long lifetime, low cost and high efficiency. |
