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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Contents
AN4006
Contents
1
2
3
4
High-power PoE converter electrical specifications . . . . . . . . . . . . . . . 4
High-power converter schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Bill of material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Test results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.1
4.2
Efficiency measurements with synchronous rectification . . . . . . . . . . . . . 10
Converter waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
4.2.1
4.2.2
4.2.3
4.2.4
4.2.5
Startup sequence using PowerDsine 9501G injector . . . . . . . . . . . . . . 12
Primary-side MOSFET . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Secondary-side MOSFET . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Output ripple . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
G
loop
measurement and load transient response . . . . . . . . . . . . . . . . . 17
5
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
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AN4006
List of figures
List of figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
Figure 11.
Figure 12.
Figure 13.
Figure 14.
Figure 15.
Figure 16.
Figure 17.
High-power converter schematic: detail of the input section including data transformers,
bridges, protection and optional CM choke . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
High-power converter: detail of the PoE converter based on active-clamp forward topology
with self-driven synchronous rectification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Efficiency measurements at 48 V input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Efficiency of the different circuits on the converter input stage. . . . . . . . . . . . . . . . . . . . . . 10
Booster current characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Booster power dissipation vs. input current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Startup with 0 A load. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Startup with 10 A load. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Primary-side power MOSFET waveforms at 0 A load . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Primary-side power MOSFET waveforms at 16 A load . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Secondary-side power MOSFET waveforms at 0 A load . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Secondary-side power MOSFET waveforms at 16 A load . . . . . . . . . . . . . . . . . . . . . . . . . 14
Output ripple measurement at 0 A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Output ripple measurement at 16 A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Output ripple measurement at 16 A with infinite persistance . . . . . . . . . . . . . . . . . . . . . . . 16
Control loop of the converter at 48 V input and 18 A output . . . . . . . . . . . . . . . . . . . . . . . . 17
Response of the converter to a 8 A - 16 A load transient . . . . . . . . . . . . . . . . . . . . . . . . . . 17
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High-power PoE converter electrical specifications
AN4006
1
High-power PoE converter electrical specifications
Table 1.
Specifications for 3.3 V output
Parameter
Input voltage range
Operative input voltage
Vin rising edge
UVLO
Vin falling edge
Auxiliary input voltage range
Output voltage (Vout)
Output current (Iout)
Peak-to-peak output ripple
Efficiency DC-DC only
Overall efficiency
Switching frequency
Vin= 42 V to 57 V, Iout 0 to Imax
Vin= 42 V to 57 V
48Vin, Iout=Imax
Vin=48 V, Iout=Imax
Vin=48 V, Iout=Imax
30
42
3.25
0
50
91
88
220
48
3.35
54
3.45
18
70
V
V
V
A
mVpp
%
%
kHz
Description
applied at J3 connector
Min
0
42
Typ
Max
57
57
36
Unit
V
V
V
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AN4006
High-power converter schematic
2
Figure 1.
High-power converter schematic
High-power converter schematic: detail of the input section including data
transformers, bridges, protection and optional CM choke
Doc ID 022454 Rev 1
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