AN4006 Datasheet, PDF (1/19 Pages) STMicroelectronics – using the PM8803 and an external current booster

AN4006
Application note
Designing a high-efficiency (60 W on 4 pairs) PoE converter
using the PM8803 and an external current booster
Introduction
Power over Ethernet (PoE) applications are covered by the IEEE 802.3 working group with
specifications released in 2003 (IEEE 802.3af) and in 2009 (IEEE 802.3at).
Power at the input of the powered device (PD) increased from 12.95 W (of the .af standard)
to 25.5 W (made available by the .at standard). In both cases the power delivery was based
on the “2-pair” system, where 4 wires of the Ethernet cable are used (Tx, Rx pairs or spare
pairs).
Applications requiring more power are constantly emerging and some solutions are already
on the market even though there is no standard fully supporting these applications yet.
Some of the alternatives are based on a 4-pair delivery system that allows doubling the
power delivered along the Ethernet cable with respect to a 2-pair system.
This document focuses on a reference design for a high-efficiency, high-power PD (up to 60
W input) power converter based on an active-clamp forward topology with self-driven
synchronous rectification using the PM8803 as the main controller. The total power is
delivered on the 4 pairs of a single Ethernet cable by a high-power injector.
The PM8803 is a highly integrated device embedding an IEEE 802.3at compliant powered
device (PD) interfaced with a PWM controller and support for auxiliary sources.
To manage the higher input current (up to 1.4 A) of high-power applications, a simple current
booster is introduced in parallel to the PM8803 internal hot-swap MOSFET.
The proposed converter prototype is built from the PM8803 demonstration board, but
several component changes have been introduced in order to manage the higher current on
the input/output section of the converter.
Schematics of the PoE converter are given in Section 2 while the bill of material is detailed
in Section 3. In Section 4 efficiency measurements together with main waveforms of the
PoE interface and power converter are shown.
February 2012
Doc ID 022454 Rev 1
1/19
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