DN425 - High Power PoE PD Interface with Integrated Flyback Controller

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High Power PoE PD Interface with Integrated Flyback Controller
Design Note 425
Dilian Reyes
Introduction
To this day, Power over Ethernet (PoE) continues to gain
popularity in today’s networking world. The 12.95W
delivered to the Powered Device (PD) input supplied by
the Power Sourcing Equipment (PSE) is a universal supply. Each PD provides its own DC/DC conversion from a
nominal 48V supply, thus eliminating the need for a correct voltage wall adapter. However, higher power devices
can not take advantage of standard PoE because of its
power limitations, and must rely on a large wall adapter
as their primary supply. The new LTC4268-1 breaks this
power barrier by allowing for power of up to 35W for such
power-hungry 2-pair PoE applications. The LTC4268-1
provides a complete solution by integrating a high power
PD interface control with an isolated flyback controller.
PSE that looks for such a class. Once a PSE detects
and classifies the PD, it fully powers on the device. The
LTC4268-1 provides a low inrush current limit, allowing load capacitance to ramp up to the line voltage in
a controlled manner without interference from the PSE
current limit. After the load capacitance is charged up, the
LTC4268-1 switches to the high input current limit and
provides a power good signal to its switching regulator
indicating that it can start its operation. During this time,
the LTC4268-1 remains in its high current limit state allowing for up to 35W delivered to the load.
Synchronous Flyback Controller
Once power is switched over to the synchronous flyback
controller, the LTC4268-1 regulates the output voltages
by sensing the average of all the output voltages via a
transformer winding during the flyback time. This allows for tight output regulation without the use of an
PD Interface Controller
The PD interface controller provides the same 25k signature detection resistance defined in the standard PoE.
An extended optional class can be read by a customized
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
L1
0.33μH
T1
PA1558NL
•
•
L2
4.7μH
C8
0.1μF
100V
+
D1
SMAJ58A
C1A
12μF
100V
C1B
2.2μF
100V
R9
20k
1/4W
PoE+
+
C18
22μF
16V
C19
0.1μF
R13
29.4k
1%
R20
301k
1%
D11
BAS21
R10
91Ω
PoE–
•
R2
10Ω
C7
1000pF
100V
C11
220pF
PWRGD
PWRGD
UVLO
ILIM_EN
VCC
FB
•
L3
0.33μH
•
SENSE+
SG
SHDN
R18
100k
RCLASS
VPORTN VPORTN VPORTN VNEG VNEG VNEG PGDLY
R21
20k
RCLASS
RPGDLY
15k
AUX–
D12
B2100
tON
SYNC RCMP
RtON
100k
ENDLY
RCMP
2.1k
OSC
RENDLY
150k
SFST
VCMP
CCMP GND
COSC CCMP
33pF 0.1μF
Q4
Si4362DY
RSENSE
0.015Ω
1/8W
1%
SENSE–
LTC4268-1
C33
3300pF
R27
10k
C26
680pF
R13
B0540W
Q7
Q6
FMMT718 FMMT618
R17
330Ω
C27
0.1μF
T2
PA0184
•
C28
2200pF
•
0.033μF
CSFST
Figure 1. High Efficiency, Triple Output, High Power PD
09/07/425
R8
10Ω
1/4W
+
PG
R15
47Ω
C24
1μF
R22
15Ω
D14
BAT54
dn425 F01
C23
4700pF
250VAC
5V
2.4A
100μF
R28
10k
11.8V
0.4A
22μF
×2
Q3
Si4488DY
VPORTP
+ C6
+ C10
100k
R14
4.7k
C5
47μF
R3
10Ω
1/4W
Q1
Si4470EY
Q2
Si4488DY
Q5
FMMT723
AUX+
+
C4
1500pF
C21
47μF
×2
+ C22
3.3V
4A
100μF
optoisolator, providing improved dynamic response
and reliability. Synchronous rectification increases the
conversion efficiency and cross-regulation effectiveness
above a conventional flyback topology. No external driver
ICs or delay circuits are needed to achieve synchronous
rectification; a single resistor is all that is needed to
program the synchronous rectifier’s timing.
High Efficiency, Triple Output, High Power PD
Figure 1 shows a design using the LTC4268-1 in a high
power, triple output PD. A high power PSE connects
through an Ethernet cable to the RJ45 connector. PSE
detection and power is passed through the data pairs’
high power Ethernet transformer or directly to the spare
pairs in this 2-pair 10/100BaseT PoE system. The PSE
power is then controlled by the LTC4268-1 PD interface
and forwarded on to its switching regulator. An auxiliary
supply option can also be connected to bypass and disable the PD interface which gives the auxiliary priority in
power supply over PoE. Power conversion is then from
the auxiliary supply down to the output voltages.
The small supply of the LTC4268-1 utilizes an isolated
flyback topology with synchronous rectification that
requires no optoisolator, lowering the parts count. This
circuit gives efficiencies at full load of 83% when powered from a PSE and over 85% power sourced from an
auxiliary supply.
PSE and Auxiliary Supplies
Standard PSEs are capable of providing as low as 15.4W
at the port output. This would not be sufficient power for
a high power PD operating at full load. Here, a customized
PSE capable of delivering higher power must be used,
or a PSE controller designed for high power such as an
LTC4263-1 single port PSE controller. In cases where a
high power PSE is not available, an auxiliary supply can
be used.
2-Pair vs 4-Pair PD
2-pair power is used today in IEEE 802.3af systems. One
pair of conductors is used to deliver the current and a
second pair is used for the return while two conductor
pairs are not powered. This architecture offers the simplest
implementation method but suffers from higher cable
loss than an equivalent 4-pair system.
4-pair power delivers current to the PD via two conductor
pairs in parallel allowing for an even higher level of power.
This lowers the cable resistance but raises the issue of
current balance between each conductor pair. Differences
in resistance of the transformer, cable and connectors
along with differences in diode bridge forward voltage
in the PD can cause an imbalance in the currents flowing
through each pair. Using two independent LTC4268-1s
(Figure 2) allows for interfacing and power from two
independent PSEs, and independent DC/DC converters
resolve the current imbalance.
PD
RJ45
CAT5
1
Rx1
2
0.1μF
SMAJ58A
DF1501S
LTC4268-1
TYP APP
3
Tx1
6
–
+ +
+
– –
VOUT
0.1μF
SMAJ58A
4
Rx2
LTC4268-1
TYP APP
5
DF1501S
7
Tx2
8
dn4DR F02
Figure 2. 4-Pair, High Power PD Diagram
Conclusion
The LTC4268-1 is a highly integrated solution for the next
generation of PD products. It offers PoE PD functionality with control for efficient high power delivery to the
output load.
Data Sheet Download
For applications help,
call (408) 432-1900, Ext. 2360
www.linear.com
Linear Technology Corporation
dn425f LT/TP 0907 451K • PRINTED IN THE USA
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2007
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
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