Introduction

This application note describes a detailed design strategy for a high-efficiency, compact flyback converter. Design considerations and mathematical equations are presented as well as guidelines for a printed circuit board layout. The highly integrated FAN6751 series of PWM controllers provides several features to enhance the performance for LCDM/TV, NB, and adapters.

Features

■ High-Voltage Startup

■ Low Operating Current: 4mA

■ Linearly Decreasing PWM Frequency to 18KHz

■ Fixed PWM Frequency: 65KHz

■ Peak-Current-Mode Control

■ Cycle-by-Cycle Current Limiting

■ Leading-Edge Blanking (LEB)

■ Synchronized Slope Compensation

■ Internal Open-loop Protection

■ GATE Output Maximum Voltage Clamp: 18V

■ VDD Under-Voltage Lockout (UVLO)

■ VDD Over-Voltage Protection (OVP)

■ Internal Recovery Circuit (OVP, OLP)

■ Internal Sense Short-Circuit Protection

■ External Constant Power Limit (Full AC Input Range)

■ Internal OTP Sensor with Hysteresis

■ Built-in 5ms Soft-Start Function

■ Built-in VIN Pin Pull HIGH (> 4.7V) Recovery Function for Second-Side Output OVP

■ Brownout Protection with Hysteresis

Applications

General-purpose, switch-mode power supplies and flyback power converters, including:

■ Power Adapters

■ Open-frame SMPS

■ LCD Monitor/TV