April 2011

Doc ID 018626 Rev 1

1/18

AN3373

Application note

IEEE802.3at compliant PD + PS design based on PM8803

low power, synchronous flyback PoE converter

Introduction

PM8803 is a highly integrated device embedding an IEEE802.3at compliant powered device

(PD) interface together with a PWM controller and support for auxiliary sources.

Figure 1 is an image of the PM8803 demonstration board. The same PCB can be populated

with different components to support various configurations and topologies: synchronous

flyback with or without active clamp, flyback with diode rectification, synchronous forward

with or without active clamp, forward with diode rectification.

This document focuses on a reference design for a high efficiency, low power PD (up to 7 W)

power converter based on flyback topology with synchronous rectification using the PM8803

as the main controller. This converter can be derived from a PM8803 demonstration board

with a few component changes.

Schematics of the PoE converter are given in Section 2 while the related bill of material is

detailed in Section 3. In Section 4 efficiency measurements, together with the main

waveforms of the PoE interface and the flyback converter, are shown.

Figure 1.

PM8803 FLY demonstration kit

www.st.com