dm00041645

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
www.st.com
Contents
AN4006
Contents
1
High-power PoE converter electrical specifications . . . . . . . . . . . . . . . 4
2
High-power converter schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3
Bill of material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4
Test results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
5
2/19
4.1
Efficiency measurements with synchronous rectification . . . . . . . . . . . . . 10
4.2
Converter waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
4.2.1
Startup sequence using PowerDsine 9501G injector . . . . . . . . . . . . . . 12
4.2.2
Primary-side MOSFET . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.2.3
Secondary-side MOSFET . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.2.4
Output ripple . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.2.5
Gloop measurement and load transient response . . . . . . . . . . . . . . . . . 17
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Doc ID 022454 Rev 1
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
Doc ID 022454 Rev 1
3/19
High-power PoE converter electrical specifications
1
AN4006
High-power PoE converter electrical specifications
Table 1.
Specifications for 3.3 V output
Parameter
Description
Min
Input voltage range
applied at J3 connector
Operative input voltage
Typ
Max
Unit
0
57
V
42
57
V
36
V
Vin rising edge
UVLO
Vin falling edge
Auxiliary input voltage range
V
42
48
54
V
3.35
3.45
V
18
A
70
mVpp
Output voltage (Vout)
Vin= 42 V to 57 V, Iout 0 to Imax
3.25
Output current (Iout)
Vin= 42 V to 57 V
0
Peak-to-peak output ripple
48Vin, Iout=Imax
50
Efficiency DC-DC only
Vin=48 V, Iout=Imax
91
%
Overall efficiency
Vin=48 V, Iout=Imax
88
%
220
kHz
Switching frequency
4/19
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AN4006
High-power converter schematic
2
High-power converter schematic
Figure 1.
High-power converter schematic: detail of the input section including data
transformers, bridges, protection and optional CM choke
!-V
5/19
Doc ID 022454 Rev 1
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High-power converter schematic
AN4006
High-power converter: detail of the PoE converter based on active-clamp forward
topology with self-driven synchronous rectification
!-V
AN4006
3
Bill of material
Bill of material
The following table summarizes the bill of material for the high-power PoE converter based
on the PM8803, configured in active-clamp forward topology with self-driven synchronous
rectification.
Table 2.
Bill of material
Reference
Description
Value
EVALPM8803
FWD rev1
Board PCB
C1,C2,C3,C4,
C11C42,C50,
C57
Ceramic capacitor
100 nF
C5,C6,C7,C8
Ceramic capacitor
10 nF
C10,C39,C41,
C53C59
Ceramic capacitor
1 µF
C12
Ceramic capacitor
2.2 nF
C14,C16, C21
Ceramic capacitor
1 nF
C18,C60
Ceramic capacitor
C19,C38
Tol
Voltage
Body
Vendor
50 V
603
Std
10%
100 V
603
TDK
20%
16 V
603
Std
2 kV
1812
TDK
100 V
603
TDK
47 nF
100 V
805
TDK
Ceramic capacitor
22 nF
50 V
603
Std
C23,C54
Ceramic capacitor
470 pF
50 V
603
Std
C26
Aluminium
capacitor
33 µF
20%
100 V
10x10.2
Std Low ESR
C27,C33,C37
Ceramic capacitor
22 µF
20%
6.3 V
805
Std
C28,C29,C30
Ceramic capacitor
2.2 µF
20%
100 V
1812
TDK
C31,C37
Ceramic capacitor
1 nF
10%
100 V
805
Std
C32
Ceramic capacitor
100 nF
10%
100 V
805
TDK
C33,C35
Ceramic capacitor
10 µF
6.3 V
805
TDK
C34,C36
Aluminium
capacitor
330 µF
6.3 V
8x10.2
Std Low ESR
C49, C56
Ceramic capacitor
22 nF
50 V
603
Std
C51
Ceramic capacitor
100 nF
200 V
1210
Std
C55
Ceramic capacitor
100 pF
50 V
603
Std
C58
Ceramic capacitor
1 nF
50 V
603
Std
C61
Ceramic capacitor
2.2 nF
2 kV
1812
TDK
D1, D21
Std diode
STTH302S
200 V
SMC
STMicroelectronics
D32,D35
Zener diode
BZX84C10
SOT23
Std
D4,D7,D8,D9,
D12D13,D14,
D17
Schottky diode
STPS2H100A
SMA
STMicroelectronics
10%
Doc ID 022454 Rev 1
100 V
7/19
Bill of material
Table 2.
AN4006
Bill of material (continued)
Reference
Description
Value
D11
TVS diode
SMAJ58A
D28,D30,D31,
D33,D37,D38,
D39,D40,D41,
D43
Schottky diode
BAT46J
D20,D26,D44
LED
Green LED SMD
J1,J2
Power jack
SA, SP
Std
J3
RJ45 connector
DATA & POWER INPUT
Std
J4
RJ45 connector
DATA OUTPUT
Std
J5
Terminal block 2
way
MOR-10X10.5-P5-2PIN
L2
SMT inductor
1 mH
LPS4018105ML
Coilcraft
L3
SMT inductor
2 µH
SER2011202MBL
Coilcraft
L5
SMT inductor
10 µH
MSS7341103ML
Coilcraft
Q6
Transistor, PNP
MMBTA92
300 V
SOT23
STMicroelectronics
Q10, Q14
Transistor, NPN
MMBT3904LT1
40 V
SOT23
Std
Q11,Q12,Q13,
Q15
MOSFET,
N-channel
IRF8707
30 V
SO8
IR
Q17
MOSFET,
N-channel
Si4848DY
150 V
SO8
VISHAY
Q21
MOSFET,
P-channel
IRF6216PbF
150 V
SO8
NM
Q22
MOSFET,
N-channel
STS4NF100
100 V
SO8
STMicroelectronics
R1,R2,R5,R7
Chip resistor
0Ω
603
Std
R9
Chip resistor
1 kΩ
603
Std
R10,R11,R12,
R13
Chip resistor
75 Ω
603
Std
R17,R26,R37,
R43,R49,R54
Ferrite Bead
MPZ012101A
805
TDK
R26,R37,R49,
R54
Chip resistor
0Ω
805
Std
R19,R20,R22,
R25R39,R40,
R41,R42
Chip resistor
NM
603
NM
R58,R72
Chip resistor
124 kΩ
603
Std
R32,R51
Chip resistor
47 kΩ
805
Std
R38,R65
Chip resistor
4.75 kΩ
603
Std
8/19
Tol
Voltage
Body
Vendor
SMA
STMicroelectronics
100 V
SOD323
STMicroelectronics
2.2 V
PLCC-2
Std
Std
1%
100 Ω,
4A
1%
1%
Doc ID 022454 Rev 1
AN4006
Table 2.
Bill of material
Bill of material (continued)
Reference
Description
Value
R44,R52,R119
Chip resistor
R45
Tol
Body
Vendor
1 kΩ
603
Std
Chip resistor
47 kΩ
603
Std
R53,R59
Chip resistor
10 Ω
805
Std
R57
Chip resistor
5.6 Ω
805
Std
R60
Chip resistor
2.2 Ω
805
Std
R62
Chip resistor
200 Ω
603
Std
R64,R98
Chip resistor
0Ω
603
Std
R70
Chip resistor
39 kΩ
603
Std
R106,R117
Chip resistor
10 kΩ
603
Std
R88
Chip resistor
270 kΩ
805
Std
R89
Chip resistor
2.7 kΩ
603
Std
R90
Chip resistor
499 Ω
603
Std
R91
Chip resistor
10 Ω
603
Std
R92
Chip resistor
1Ω
603
Std
R93
Chip resistor
820 Ω
1%
603
Std
R94
Chip resistor
21 kΩ
1%
603
Std
R95
Chip resistor
24.9 kΩ
1%
603
Std
R96
Chip resistor
0Ω
603
Std
R97
Chip resistor
30.9 Ω
1%
603
Std
R102
Chip resistor
35.7 Ω
1%
805
Std
R103
Chip resistor
510 Ω
603
Std
R104
Chip resistor
4.75 kΩ
603
Std
R107,R115,R1
21
Chip resistor
100 kΩ
603
Std
R108,R109
Chip resistor
0.10 Ω
1206
Std low value
R111
Chip resistor
12.4 kΩ
603
Std
R120
Chip resistor
1 MΩ
603
Std
T1,T2
POE+ Magnetics
ETH1-230LD
Coilcraft
T6
Power transformer
MA5509-AL
Coilcraft
U1
POE+ controller
PM8803
U2,U3,U7
SMT optocoupler
Fairchild FOD817AS
4PDIP
Fairchild
U4
Shunt regulator
TS431AILT
SOT23-5
STMicroelectronics
1%
1%
1%
1%
Doc ID 022454 Rev 1
Voltage
HTSSOP20 STMicroelectronics
9/19
Test results
AN4006
4
Test results
4.1
Efficiency measurements with synchronous rectification
Figure 3.
Efficiency measurements at 48 V input
%FFICIENCY
#URRENT!
DCDCHS
OVERALLPAIRSWITHDIODEBRIDGES
DIODEINPUTSTAGE
ACTIVEBRIDGEINPUTSTAGEESTIM
OVERALLPAIRSWITHACTIVEBRIDGEESTIM
!-V
The difference between dc-dc and overall measurements is about 3-4% from 10 A to 18 A.
Figure 4.
Efficiency of the different circuits on the converter input stage
%FFICIENCY
#URRENT!
2*DATATX
ACTIVEBRIDGE
DIODEBRIDGE
EFFSUMDIODEBRIDGE
"OOSTERALONE
EFFSUMACTIVEBRIDGE
!-V
10/19
Doc ID 022454 Rev 1
AN4006
Test results
Figure 4 shows the various contributions to the total losses of the PD interface section of the
converter:
●
RJ45 and data transformer value is small but not negligible at high input current/power
●
Booster value is negligible
●
Major contribution comes from the rectification bridge; an active bridge with MOSFETs
for a total value of about 150 mΩ per leg ( about 100 mΩ for the P-channel MOSFET
and 50 mΩ for the N-channel ) will assure a gain of about 1.6% on the total efficency
over the full input current range.
I out [ A ]
Figure 5.
Booster current characteristics
1.5
1.4
1.3
1.2
1.1
1.0
0.9
0.8
0.7
0.6
05
0.5
0.4
0.3
0.2
0.1
0.0
I tot
I booster
I hot swap
Estimated Ibooster
Estimated Ihotswap
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5
I in [ A ]
AM11001v1
The external MOSFET carries about 85% of the whole input current. The current ratio is
inversely proportional to the Ron of the MOSFET used, in this case 65 mΩ for the external
MOSFET while for the PM8803 internal hot-swap MOSFET 400 mΩ can be used, as
confirmed by the estimations done.
Figure 6.
Booster power dissipation vs. input current
0DISSIPATED;7=
0DISSTOT
0DISSBOOSTER
0DISSHOTSWAP
)IN;!=
!-V
The power dissipation of the MOSFET booster is about 6 times higher than the internal hotswap MOSFET.
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Test results
AN4006
4.2
Converter waveforms
4.2.1
Startup sequence using PowerDsine 9501G injector
Figure 7.
Startup with 0 A load
Figure 8.
Startup with 10 A load
The current unbalance in the Ethernet cable at steady state (between Tx, Rx and spare
pairs) is minimum even at high load: see pink and blue traces.
For details on the injector please visit www.microsemi.com.
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4.2.2
Test results
Primary-side MOSFET
Figure 9.
Primary-side power MOSFET waveforms at 0 A load
Figure 10. Primary-side power MOSFET waveforms at 16 A load
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Test results
4.2.3
AN4006
Secondary-side MOSFET
Figure 11. Secondary-side power MOSFET waveforms at 0 A load
Figure 12. Secondary-side power MOSFET waveforms at 16 A load
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4.2.4
Test results
Output ripple
Figure 13. Output ripple measurement at 0 A
Figure 14. Output ripple measurement at 16 A
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Test results
AN4006
Figure 15. Output ripple measurement at 16 A with infinite persistance
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4.2.5
Test results
Gloop measurement and load transient response
Figure 16. Control loop of the converter at 48 V input and 18 A output
Figure 17. Response of the converter to a 8 A - 16 A load transient
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Revision history
5
AN4006
Revision history
Table 3.
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Document revision history
Date
Revision
22-Feb-2012
1
Changes
Initial release
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