AN75

Issue 1 – January 2011

1

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© Diodes Incorporated 2010

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 changing the control method to a fixed off-time

operation.

The design of the internal oscillator in the AL9910 allows the IC to be configured for either fixed

frequency or fixed off-time based on how resistor R

resistor R

now a constant off-time when the power MOSFET is turned off. The on-time is based on the current