A Resonant Type Of Electrolytic Capacitor-less LED Driver

LED, short for Light Emitting Diode, has great superiority compared with the traditional light source: It has not only a long life, but also can greatly reduce the lighting electricity consumption. It has been hailed as a green light source in 2l century. An AC/DC LED driver converts a commercial AC voltage to a steady DC voltage and produces a constant current for driving LEDs. Electrolytic capacitors should be removed to improve the lifetime and reliability of the LED drivers. The thesis is dedicated to design a two-stage resonant type of electrolytic capacitor-less AC/DC LED driver.The two-stage LED driver consists a power factor correction(PFC)converter and a DC/DC converter in cascaded. The PFC converter converts a commercial AC voltage to a steady DC voltage, and the DC/DC converter converts the DC voltage produced by the PFC converter to a steady DC current for driving LEDs. In this thesis, the boost converter is adopted for the PFC converter. The relationship between the output voltage ripple and the capacity of electrolytic capacitor can be calculated through analysis the relationship between the input power pulse and the the input power factor of the PFC converter.Various solutions to reduce the electrolytic capacitor are compared, and finally, injecting harmonic current to the input current is chosen in the paper. Reducing the storage capacitor significantly at the precondition that the input power factor is higher than 0.9,thus film capacitor can be adopted to replace electrolytic capacitor to prolong the lifetime of the power supply for LEDs. The boost PFC converter is operated in critical current mode, so that the power switch achieves zero-current-switching(ZCS) and the reverse recovery of the boost diode is avoided. The half-bridge LLC resonant converter is chosen as the DC/DC converter. It can not only realize zero-voltage-switching(ZVS) for the power switches and rectifier diodes, but also can achieve a high efficiency. The operating principle of the two-stage electrolytic capacitor-less AC/DC LED driver is analyzed in details, the parameter design of the power stage is discussed,and the control strategy of is also presented.A 140 W prototype has been built and tested in the lab, and the experimental results have verified the effectiveness of the proposed two-stage electrolytic capacitorless AC/DCLED driver.