A Flicker-free Electrolytic Capacitor-less LED Driver

Light Emitting Diode (LED) has the advantages of energy saving, environmental friendliness, safety and long lifetime. It is the fourth generation light source following incandescent lamp, fluorescent lamp and high-intensity discharge lamp. The development of LED lighting technology needs high reliability, high power factor, long lifetime and low cost LED drivers. This thesis is dedicated to the design of a novel flicker-free electrolytic capacitor-less LED driver.When the LED drivers utilize the commercial ac voltage source, the function of power factor correction (PFC) should be realized to achieve a high power factor. In order to balance the difference between the instantaneous input and output power, a large storage capacitor is required, and electrolytic capacitor is often used. The lifetime of electrolytic capacitor is much lower than the lifetime of LED, and electrolytic capacitor is the main component that limits the lifetime of the LED driver. So, electrolytic capacitor should be removed. In this thesis, the operating principle of the electrolytic capacitor-less LED driver based on the flyback converter is analyzed, and the harmonic components in the secondary side of the flyback converter are discussed in detail, base on which, the output filter inductor and capacitor are designed to reduce the peak-average ratio of the output current.The output current in the above electrolytic capacitor-less LED driver is pulsating, and it contains the ac current with twice the line frequency. The pulsating current leads to flicker. A novel flicker-free electrolytic capacitor-less LED driver is proposed. A bidirectional buck/boost converter is introduced into the electrolytic capacitor-less LED driver, and it provides the flowing path for the ac current with twice the line frequency. Consequently, the output current for driving LED is a pure constant dc current, and no flicker occurs. The relationship between the voltage ripple and capacitance of the output filter capacitor in the bidirectional buck/boost converter is discussed, and the value of the input filter inductor in the bidirectional converter is designed based on the trade-off of the tracking speed of the input current and current ripple.Based on the theoretical analysis, a 35 W prototype has been built and tested in the lab. Control strategy and parameter design of the flyback converter and the bidirectional converter are given in details. The experimental results are presented and they verify the feasibility of the proposed methods.