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AN75

High Power Factor LED Replacement T8 Fluorescent Tube using the AL9910 High Voltage LED Controller


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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 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 RT is connected. For fixed off-time operation, the
resistor RT is connected between the Gate and ROSC pins, as shown in Figure 1. This converter has
now a constant off-time when the power MOSFET is turned off. The on-time is based on the current
Issue 1 – January 2011
© Diodes Incorporated 2010
1
www.diodes.com



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sense signal and the switching adjusts to be the sum of the on- and off-time. This change allows the
converter to work with duty cycles greater than 50%.
Design Guide – High power factor offline LED driver
In this section the design procedure is outlined according to the schematic shown in Figure 1. First,
the guideline for selecting the components for valley fill power factor correction stage and fixed off-
time buck converter is shown. The power inductor calculation is then demonstrated and finally, the
power losses within MOSFET and free-wheel diode are assessed.
The specifications for the system are:
VAC = 230Vac
VAC(min) = 85Vac
VAC(max)= 264Vac
ILED(nom) = 240mA
VLED(nom) = 54V
VLED(min) = 42V
VLED(max) = 59V
POUT = 12.96W
fswi(nom) = 55kHz
Passive factor correction stage design
The purpose of the valley fill circuit (see Figure 2) is to allow the buck converter to pull power directly
off the AC line when the line voltage is greater than 50% of its peak voltage.
Figure 2 Valley-fill PFC stage and operating waveforms (Green: VIN to LED driver; Orange:
AL9910’s gate voltage)
The maximum bus voltage at the input of the buck converter is,
VIN(max) = 2 × Vac(max) = 2 × 264Vac = 373V
During this time, capacitors within the valley fill circuit (C1 and C2) are in series and charged via D2
and R1. If the capacitors have identical capacitance value, the peak voltage across C1 and C2
Issue 1 – January 2011
© Diodes Incorporated 2010
2
www.diodes.com



Part Number AN75
Description High Power Factor LED Replacement T8 Fluorescent Tube using the AL9910 High Voltage LED Controller
Maker Diodes - Diodes
Total Page 12 Pages
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