AN75

AN75 AN75 High Power Factor LED Replacement T8 Fluorescent Tube using the AL9910 High Voltage LED Controller Yong Ang, Diodes Inc. Introduction This application note describes the principles and design equations required for the design of a high brightness LED lamp using the AL9910. The equations are then used to demonstrate the design of a universal, offline, high power factor (PF), 13W LED lamp suitable for use as the replacement for T8 fluorescent tube. A complete design including the electrical diagram, component list and performance measurements are provided. AL9910 high power factor buck LED driver Figure 1 Electrical schematic of a high power factor 13W LED lamp Figure 1 shows the electrical diagram of an offline 13W LED driver. On the input side, CX1, CX2, CX3, CX4, L1 and L2 provide sufficient filtering for both differential mode and common mode EMI noise which are generated by the switching converter circuit. The rectified AC line voltage from the bridge rectifier DB1 is then fed into a passive power factor correction or valley fill circuit which consists of 3 diodes and 2 capacitors. D1, D2, D3, C1, C2 improve the input line current distortion in order to achieve PF greater than 0.9 for the AC line input. The constant current regulator section consists of a buck converter driven by the AL9910. Normally, the buck regulator is used in fixed frequency mode but its duty cycle limitation of 50% is not practical for offline lamp. This problem can be overcome by changi...