AN-9737 Datasheet, PDF (1/11 Pages) Fairchild Semiconductor – Design Guideline for Single-Stage Flyback AC-DC Converter Using FL6961 for LED Lighting

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AN-9737
Design Guideline for Single-Stage Flyback AC-DC
Converter Using FL6961 for LED Lighting
Summary
This application note presents single-stage Power Factor
Correction (PFC) and focuses on how to select and design
the flyback transformer for 16.8W (24V/0.7A) solution for
universal input for LED lighting applications using FL6961.
The flyback converter using FL6961 operates in Critical
Conduction Mode (CRM) and has functions such as CC/CV
feedback circuit, soft-starting, and the cycle-by-cycle current
limit for LED lighting applications.
Introduction
These days, engineers use various types of LEDs for general
lighting systems because of their long life, excellent
efficacy, price, environmental benefits, and requirements
from end users. At the same time, high power factor (PF),
isolation for safety, and constant current control (CC) for
constant LED color are becoming requirements.
Conventional regulation is the minimum power factor
correction for input power base above 25W, but many want
to reduce power ratings and the new Energy-Star directive
for solid-state lighting requires a power factor greater than
0.9 for commercial applications. Expect PF regulations to
become more stringent.
Basic Operation: High Power Factor
Flyback Converter
The basic idea of achieving high power factor (PF) flyback
converter is to use a Critical Conduction Mode (CRM) PFC
controller. The conventional PFC IC, such as FL6961, has
constant on-time and variable off-time control method,
which means the input average current always follows the
input voltage shape.
Figure 1 shows the typical application schematic of single-
stage PFC topology. The main difference of normal CRM
boost converter is that single-stage PFC doesn’t use a large
electrolytic capacitor after the full rectification diode.
Normally, the single-stage PFC method uses a small
capacitor (C1 in Figure 1) to act as a noise filter to attenuate
high-frequency components and doesn’t use the INV pin for
output voltage regulation.
BR
T1
D3
Fuse
EMI filter
C4
R8
R1
D1
U101
R2
INV
VCC
1
8
C5
R5
D2
C1
R3
C2
COMP
2
OUT
7
MOT
3
GND
6
R6
Q1
R7
CS ₄
ZCD
5
C3 _R4
R8
Feedback
Figure 1. Simplified Schematic of High-Power Factor Flyback Converter with FLS6961
© 2011 Fairchild Semiconductor Corporation
Rev. 1.0.0 • 4/13/11
www.fairchildsemi.com