Datasheet

User's Guide

SLUU434–August 2010

5-V, 25-W Flyback Converter With Secondary-Side

Synchronous Rectification

1 Introduction

The UCC24610EVM-563 evaluation module is a 25-W off-line Discontinuous Mode (DCM) flyback

converter providing an output voltage of 5 V at 5-A maximum load current, operating from a universal AC

input. The module is controlled by the UCC28610 Green-Mode Flyback Controller on the primary side

which uses a cascoded architecture that allows fully integrated current control without an external sense

resistor. Secondary-side synchronous rectification is controlled by the UCC24610 GREEN Rectifier™

Controller. The UCC24610 senses the drain-to-source voltage of the synchronous rectifier MOSFET in

order to drive the GATE signal. The GATE output duty cycle is dependent upon the system line and load

conditions, as well as the programmed minimum on-time and off-time. The converter maintains

discontinuous mode operation over the entire operating range. This innovative approach results in

efficiency, reliability, and system cost improvements over a conventional flyback.

2 Description

This evaluation module uses the UCC24610 GREEN Rectifier™ Controller (TI Literature Number

SLUSA87) in a 25-W DCM flyback converter that exceeds Energy Star™ EPS version 2.0 for efficiency

during active load and no-load power consumption for low voltage AC-to-DC external power supplies. The

input accepts a voltage range of 85 VAC to 265 VAC. The output provides a regulated output voltage of 5

VDC at a load current of up to 5 A. The converter will transition through three operating modes: Green

Mode (GM), Amplitude Modulation (AM), and Frequency Modulation (FM), depending upon the power

level and FB current of the primary-side controller. In FM Mode, which occurs between approximately 25%

and 100% rated load, the on time is fixed, resulting in a fixed peak primary current at each cycle, and the

switching frequency is increased with increasing load. In AM Mode, which occurs from approximately 2%

rated load up to 25% rated load, the switching frequency is fixed at 30 kHz and the peak primary current is

modulated with the on time as with any typical PWM controller. Green Mode operation, at loads less than

2%, consist of burst packets of 30-kHz pulses with a fixed on time and peak primary currents of 33% the

maximum programmed level.

The conduction time of the synchronous rectifier MOSFET (SR-MOSFET) is determined by comparing the

SR-MOSFET’s drain-to-source voltage against internal turn-on and turn-off thresholds. Secondary-side

control features automatic light-load management based upon internal timing conditions and the R

DS(on)

the SR-MOSFET. The UCC24610 requires a minimal amount of simple, low-power external components

to provide a cost effective, efficient solution for low-voltage power supplies.

This user’s guide provides the schematic, component list, assembly drawing, art work, and test set up

necessary to evaluate the UCC24610 in a typical off-line DCM Flyback converter application.

5-V, 25-W Flyback Converter With Secondary-Side Synchronous Rectification SLUU434–August 2010