OFFLINE SINGLE-STAGE ELECTROLYTIC-CAPACITOR-FREE FLICKER-FREE HIGH POWER LED DRIVER

Abstract

The use of white LED bulbs has increased exponentially in recent years because of their low energy consumption, long life time, high luminous efficacy and environmental friendly features. The illumination LED bulbs require a controlled constant amount of DC current to emit a constant light. Therefore, the LED power supplies (LED drivers) determine the lighting performance. Moreover, it impacts the sales price, daily expense as well as the lifespan of the lamp. Conventional single-stage topologies for grid-connected LED drivers with high power factors (PFs) require large capacitances at the output to limit the low frequency (double line frequency, i.e. 120 Hz) LED current ripple that causes flicker. Electrolytic capacitors are commonly used to reduce this low frequency current ripple at the output. LED bulbs have a long lifespan (>50,000 hours), but the electrolytic capacitor has a short lifespan (5,000 hours). Therefore, using electrolytic capacitors will significantly reduce the lifespan of the entire LED lamp. The basic bipolar ripple cancellation technology has been proposed, using a dc-ac inverter, to cancel the low-frequency ac ripple in the LED current and thus minimizing the output capacitance requirement, enabling the use of long-life film capacitors (>50,000 hours). Furthermore, the improved bipolar ripple cancellation technology with a floating dc-ac power structure is proposed to accommodate the usage of low-voltage-rating dc-ac inverter in variable output voltage LED driver applications. The control scheme to achieve both ripple cancellation and the floating auxiliary capacitor voltage regulation is presented. In the above two ripple cancellation technologies, the voltage sensing control method has been used. A new control method, called current sensing control method, is proposed and implemented to achieve ripple cancellation as well. The method keeps using the floating dc-ac power structure while provides a simple and low-cost control solution, compared to the voltage sensing method. Finally, LLC resonant converter is used as the topology of single-stage high-power LED driver application to improve the efficiency. The design rule of LLC resonant converter working in Power-Factor-Correction mode is proposed and the performance is demonstrated.