Research on driver circuit design for sustainable LED lighting systems : from low to high-power applications

Abstract

Light-emitting diodes (LEDs) have been gaining popularity in various daily lighting applications due to the advantages of longevity, high efficiency, non-toxic and wide color gamut. However, the unique device characteristics and driver circuit requirements have also drawn attention from the academia and industry to the topics of proper driver circuit design. Among them, the conventional use of electrolytic capacitors is considered as one of the critical design issues that could severely limit the reliability of the overall LED lighting system. Additionally, for high-power applications, current balancing and independent control of paralleled LED strings are also indispensable issues that have to be addressed by the driver circuit. Despite a number of feasible solutions proposed for handling these issues over the years, the solutions available for low-power applications still suffer from different shortcomings such as high component count, high control complexity, low driver efficiency or excessive flicker. On the other hand, even with the advantages offered by the existing approaches for driving high-power multi-string lighting systems using resonant converters, they often neglect the current accuracy problem arising from electrolytic capacitor-less or string failure condition as the converter is regulated by conventional switching-frequency modulation. Also, performing both functions of current balancing and independent control could complicate the existing approaches with additional circuit/control complexity or power loss. In this thesis, beginning with the review and classification of existing driver circuit structures, several durable driving solutions without the use of electrolytic capacitors are proposed for low- to high-power applications. For low-power applications, high-voltage driving is first proposed as an alternative and simple approach to LED driving which enables the use of non-electrolytic capacitors without additional control complexity. Furthermore, high efficiency and low component count can also be achieved concurrently because of the use of single-stage LED drivers without voltage step-down. A systematic design procedure for meeting specific flicker and storage capacitance requirements is discussed and a single-stage, boost PFC LED driver is built to verify the idea. For high-power applications, the switch-controlled capacitor (SCC) is proposed as an electronically-variable capacitor for the regulation of resonant LED drivers. Compared with the conventional switching-frequency modulation method, the output of the converter can be regulated by modulating the resonant frequency of the tank under constant switching frequency. As a result, accurate capacitive current balancing of paralleled LED strings can be achieved due to constant capacitive impedance under any input or output voltage variation induced by electrolytic capacitor-less or string failure condition. The idea is confirmed by an LCC resonant LED driver. After that, the idea of the electronically-variable capacitor is further extended by substituting all fixed capacitors of the resonant tank with SCCs such that current balancing and independent control of LED strings can be performed simultaneously by varying the effective capacitances of SCCs under constant switching frequency. By doing so, the respective advantages of the existing passive and active approaches can be enjoyed, which are beneficial to some applications requiring exible control of LED strings to obtain different desirable brightness and/or color spectra in different areas or time periods. Finally, in view of the possible degradations of output current regulation and control stability in conventional converters due to the parametric variations of LEDs under various working conditions or aging after long operating hours, LED drivers based on current-source converters, which can be obtained readily from the existing circuit topologies by duality principle, are discussed and considered as suitable potential candidates for LED driving. For maintaining reasonable power density and efficiency of LED drivers, particularly in low-power applications, an electronic-smoothing inductor (ESI) is introduced to replace the passive storage inductor in PFC current pre-regulator. The idea is verified by a single-stage, current-boost PFC LED driver.