Single-stage AC/DC single-inductor multiple-output (SIMO) LED drivers

Abstract

With LED products receiving a growing amount of interest and their applications extended to higher lumen and multi-color types, a variety of AC/DC LED driver topologies aimed at achieving a compact, efficient, low-cost, and robust multi-string LED lighting system, have been proposed. These LED drivers, in general, employ a two-stage topology to realize the functions of AC/DC rectification and independent current control of each LED string. The choice of having two stage conversions, however, involves additional hardware components and a more complicated controller design process. Furthermore, such two-stage topologies suffer from a higher system cost, increased power loss, and large form factor. In the first part of this thesis, a single-stage AC/DC single-inductor multiple-output (SIMO) LED driver that is applicable to the multi-string LED system is proposed. The proposed driver requires only a single inductor, N capacitors, and N+1 active power switches and diodes as compared to existing drivers which requires N more inductors and 1 more capacitor (N being the number of LED strings) in its topology. The proposed AC/DC SIMO driver thereby achieves component count reduction and a smaller form factor as compared to existing drivers. Furthermore, the proposed driver can achieve both functions of AC/DC rectification with a high power factor and a relatively good independent current control of each individual LED string simultaneously. Despite the aforementioned merits, achieving full range and high performance dimming at small duty ratio can still be challenging with the proposed single-stage SIMO topology. The LED currents cannot be controlled precisely at small duty ratio due to the presence of small undesired oscillations caused by the linear dimming scheme employed. In addition, the dimming ratio of the driver is substantially limited and the energy efficiency drops quickly as the dimming ratio becomes low. To tackle these problems, in the second part of this thesis, the single-stage SIMO LED driver previously discussed is enhanced with the addition of N active power switches that is complemented with the pulse width modulation (PWM) dimming approach. Two new dimming schemes, i.e., Instant-Duty-Restoration (for string-level dimming) and Coordinated Low-Frequency and Time-Sharing (for system-level dimming) dimming schemes are proposed. Based on the coordination of the string-level and system-level dimming schemes, the modified SIMO LED driver can overcome the practical constraints of limited dimming range and precise current control. The proposed modifications enable the driver to perform dimming within the dimming range from 1% to 100%. It has the flexibility of using either phase-shift or synchronous PWM switching for the dimming control. Two AC/DC single-inductor triple-output (SITO) prototypes have been built to verify the functionality of the two proposed driver topologies, namely, the single-stage AC/DC SIMO LED driver with linear dimming scheme and the modified AC/DC SIMO LED driver with PWM dimming scheme. Results of the first set of experiment corroborate that good and independent current regulation of each individual LED string is achievable by linear dimming, and a power factor of above 0.99 and a peak efficiency of 89% at 30W rated output power are attainable. On the other hand, in the second set of experiment, the improved driver circuit and control operations are also practically verified with a 25 W off-line SIMO driven LED system. Practical evaluations of its power quality and energy efficiency are provided at the end of the thesis.