Development and design of light-emitting-diode (LED) lighting power supplies

Abstract

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 present 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. Performance in terms of chromaticity and luminance stability for a temperature compensated RGB LED system will be presented. 2. LED light sources have a long life expectancy. The lifetime of LED luminaires are thus determined by their electronic ballasts. It is found that the lifetime of power converters is limited by the high-voltage electrolytic capacitors which normally have the shortest lifetime compared to other components in the system. A first-stage isolated power-factor-correction (PFC) pre-regulator is proposed, which allows the storage capacitor to be located in the secondary side of the isolation transformer. Low-voltage large capacitors of longer lifetime can therefore be used to extend the overall lifetime of the LED luminaires. Steady-state state-space averaging analysis is performed for designing the converter in discontinuous conduction mode (DCM). A prototype converter is built to verify performance of the proposed PFC LED pre-regulator. 3. The concept of secondary capacitors is extended to operate the same PFC pre-regulator in continuous conduction mode (CCM) for high-power applications (>300 W). The arrangement of power delivery is identified as the first non-cascading structure. Two secondary transformer windings and a post LED current driver are arranged to form a second non-cascading structure to improve efficiency. Additional buck current drivers can be connected in parallel to drive more LEDs independently. Since the energy from one output winding can directly be used to drive the LED without going through another power stage, a higher efficiency can be guaranteed. 4. In applications with power lower than 25 W, the current harmonics requirements are less stringent as stated in some international standards. A high-step-down-ratio and high-efficiency resonant assisted buck converter for LED light sources is proposed, which also has the lifetime of capacitor extended. The circuit is simple and efficient, without using isolation transformers. Analysis and experimental verification are presented.