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Rev. 1.0.2 • 8/5/10

AN-8025

Design Guideline of Single-Stage Flyback AC-DC

Converter Using FAN7530 for LED Lighting

Summary

This application note describes the single-stage power

factor correction (PFC) and presents the design guidelines

of a 75W universal-input, single-stage PFC for LED

lighting applications. Flyback converter topology controlled

by the critical conduction mode control IC, FAN7530 is

applied and several functions; such as CV/CC mode

feedback circuits, cycle-by-cycle current limit, soft-starting

function, and so on, are considered for LED lighting

applications.

Introduction

Despite large output voltage ripple, single-stage AC-DC

conversion is a more attractive solution than two-stage

conversion from the standpoint of the cost and power

density. Especially in applications like battery chargers,

Plasma Display Panel (PDP)-sustaining power supplies, and

LED lighting; low frequency, 100Hz or 120Hz, large output

voltage ripple is inconsequential.

Single-stage AC-DC converter directly converts AC input

voltage to the DC output voltage without a pre-regulator, as

shown in Figure 1.

This application note presents a 75W single-stage AC-DC

converter for LED lighting. As a power-conversion

topology, flyback converter is normally chosen because it

doesn’t need an inductive output filter; the main

transformer works as an inductive filter itself.

Figure 1. Single-Stage AC-DC Converter

Figure 2 shows the circuit diagram of a flyback AC-DC

converter. FAN7530 is used as a controller and both CV

(constant voltage) and CC (constant current) mode feedback

circuits are applied to prevent overload and over-voltage

conditions. In LED lighting, the output is always full-load

condition and the forward voltage drop of LED decreases if

the junction temperature of LED increases. Therefore the

output should be controlled by CC mode in the normal state

while CV mode only works as over voltage protection.

Figure 2. Flyback AC-DC Converter

Figure 3. Block Diagram of FAN7530

Figure 3 shows the block diagram of FAN7530. Its major

features are:

 Fixed On Time CRM PFC Controller

 Zero Current Detector (ZCS) & Valley Switching

 MOSFET Over-Current Protection

 Low Startup (40μA) and Operating Current (1.5mA)

 Totem Pole Output with High State Clamp

 +500/-800mA Peak Gate Drive Current

FAN7530 is a voltage-mode CRM PFC controller; the turn-

on time of switch is fixed while the turn-off time is varied

during the steady state. Therefore, the switching frequency

varies in accordance with the input voltage variation shown

in Figure 4.