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AN75 Datasheet

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

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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
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Diodes Semiconductor Electronic Components Datasheet

AN75 Datasheet

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

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

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is VIN(max) 2 = 186V . Often a 20% difference in capacitance could be observed between like
capacitors. Therefore a voltage rating margin of 25% should be considered.
Once the line drops below 50% of its peak voltage, the two capacitors are essentially placed in
parallel. The bus voltage VIN(min) is the lowest voltage value at the input of the buck converter. VIN(min)
at the minimum AC line voltage Vac(min) is,
VIN(min) = 2 × Vac(min) 2 = 2 × 85Vac 2 = 60V
At 60Hz, the total time of a half AC line cycle is 8.33ms. The power to the buck converter is derived
from the valley-fill capacitors when the AC line voltage is equal to or less than 50% of its peak voltage.
The hold up time for the capacitors equates to tHOLD = 1 3 × 8.33ms = 2.77ms . The valley-fill capacitor
value can then be calculated,
CTOTAL
=
Pout VIN(min) × tHOLD
VDROOP
=
12.96W
60V ×
20V
2.77ms
= 30μF
Therefore, C1 = C2 = 15μF . VDROOP is the voltage droop on the capacitors when they are delivering full
power to the buck converter. Ideally VDROOP should be set to less than VDROOP = VIN(min) VLED(max) in
order to ensure continuous LED conduction at low line voltage. Nevertheless, VDROOP is set to be 20V
in the design example to avoid the need for very large valley-fill electrolytic capacitor.
A 20V VDROOP implies that the bus voltage VIN at the input of buck converter will drop to 40V during
part of the AC line cycle. As the buck regulator requires VIN to be greater than the LED stack voltage
(VLED(max)=59V) for regulation, the LED will be off during part of the AC line cycle. This has the effect of
reducing the actual output LED current at low AC input voltage. In the design example, the LED
current drops by approximately 20% from its nominal value at 85Vac (see Figure 4).
Setting the fixed off-time and switching frequency range
For fixed off-time operation, the switching frequency will vary subjected to the actual input voltage and
output LED conditions.
A nominal switching frequency fswi(nom) should be chosen. A high nominal switching frequency will
result in smaller inductor size, but could lead to increased switching losses in the circuit. A good
design practice is to choose a nominal switching frequency knowing that the switching frequency will
decrease as the line voltage drops and increases as the line voltage increases.
The fixed off-time tOFF can be computed as,
toff
1- VLED(nom)
=
Vac(nom)
fswi(nom)
=
1-
54V
230V
55kHz
= 13.9μs
The off-time is programmed by timing resistor RT as shown in Figure 1. The value of RT is given by,
RT (kΩ) = tOFF(μs)× 25 22 = 13.9 × 25 22 = 326kΩ
A 330kis selected for RT. Next, the two extremes of the variable switching frequency can be
approximated as,
fswi(min)
=
1
VLED(max)
tOFF
VIN(min)
=
1
59V 69V
13.9μs
= 10kHz
fswi(max)
=
1
VLED(min)
tOFF
VIN(max)
=
142V 373V
13.9μs
= 63.8kHz
Issue 1 – January 2011
© Diodes Incorporated 2010
3
www.diodes.com




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