DN338 - Power over Ethernet Isolated Power Supply Delivers 11.5W at 90% Efficiency

Power over Ethernet Isolated Power Supply
Delivers 11.5W at 90% Efficiency – Design Note 338
Jesus Rosales
fewer switches to turn on and off the better. A push-pull
converter could be used, but the additional complexity
is not justified at this power level. A single transistor
forward converter is another option, but requires
an additional output inductor and rectifier. A flyback
converter is the simplest choice. Flyback converters
are thought to be less efficient than forward and pushpull converters, but that changes when the output is
synchronously rectified (a MOSFET is used instead of
a diode to rectify the output).
When powering IP telephones, wireless access points,
PDA charging stations and other PDs (Powered Devices)
from an Ethernet cable, designers have at most 12.95W
of available power per the IEEE 802.3af standard. Increased demands for power mandate a very efficient
power converter especially for class 3 devices (consuming between 6.49W and 12.95W). The more power lost
in the converter, the less power available for the PD.
The voltage available from the PSE (Power Sourcing
Equipment) ranges from 44V to 57V, but PDs need to
operate with as much as 20Ω of series wire resistance.
A PD can never draw more than 350mA or 12.95W
continuously. With the maximum input current of
350mARMS, the input voltage can droop as much as
7V bringing the lower side of the input range to 37V.
To avoid interfering with the classification signature
impedance measurement, a PD must not draw significant current below 30V.
The LT®1725 switching regulator controller greatly
simplifies the design of PoE supplies. The LT1725 is
specifically designed for the isolated flyback topology
and includes features that make it a good match for PoE
supplies, including programmable input undervoltage
lockout, hysteretic start-up and a patented feedback
circuit that eliminates the need for an optocoupler while
providing excellent output regulation1.
There are many topologies to choose from when designing an isolated DC/DC converter, but for PoE (Power
over Ethernet) applications, the choices are few. When
trying to maximize efficiency every milliwatt counts.
MOSFET gate driving losses become significant so the
In order to maximize power to the load, the converter
chosen must have synchronous rectification. Diodes
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their
respective owners.
1U.S. Patent No. 05438499, 05305192, 0584163.
T1
PULSE PB2134
PULSE ENG.
20Ω
PSE
22μF
100V
100MV12AX
0.82μF
100V
LTC4257-1
1
2
3
4
NC
GND
RCLASS SIGDISA
NC
PWRGD
VIN
VOUT
8
100k
10
9
6
0.047μF
2N7002
5
2N7002
–VIN
RCLASS
1%
R1
47Ω
Q11
MMBT2907ALT
220pF
3.01k
1%
Q12
UVLO
ISENSE
LT1725
3VOUT
VC
14
12
4
13
100k
62k
4.7k
62k
RCMPC
SGND PGND SFST
6
11
1
3
5
2
VG
7
330
3.3nF
0.1μF
47pF
0.22μF
100Ω
0.1μF
10k
680pF
0.22Ω
1/2W
MM3Z6V2ST1
Figure 1. 36V-72V Input to 3.3V at 3.5A Output, Isolated Synchronous Flyback Converter
05/04/338_conv
1μF
Q13
Q3
Si7892DR
GATE
tON MINENAB ROCMP ENDLY OSCAP
10k
47Ω
Q1
Si4490
16
VCC
FB
B0540W
10
D6
BAT54
47μF
20V
15
8
VG
100k
7
VG
10k
D1
SMAJ58A
–VOUT
9
3
Q9
MMBT3904
+
+VOUT
R28 3.3V
1k 3.5A
BAT54
t
26.7k
1%
0.1μF
+
t
BAS21LT1
MMBT3904
0.1μF
100V
11
1
10k
1/4W
150μF
6.3V
w3
6TPC150M
1
1nF
8
10k
t
200k
8
t
+VIN
37V TO 54V
4
5
t
t
4.7μF
DO1608C-472C
T2
PA0184
DN338 F01
will simply dissipate too much power. Figure 1 shows a
schematic for an isolated synchronous flyback converter
using the LT1725 that achieves efficiencies as high as
90% at 11.5W out. Figure 2 shows an efficiency curve
for this synchronous converter and a second curve for
the same converter with a 6CWQ06FN diode used in
place of Q3 (making the converter nonsynchronous).
While the synchronous converter needs a few extra
components to control the rectifying MOSFET Q3, the
resulting efficiency gain (approximately 10%) is significant. An additional benefit of the lower power dissipation
with a MOSFET versus a diode is that a heat sink is no
longer necessary, allowing for a dramatic reduction in
board space. The other 5% efficiency gain comes from
the elimination of preload. A synchronous converter
does not need any preload to keep the output voltage
in regulation while the nonsynchronous converter does.
The output of a nonsynchronous converter can float
up, uncontrolled, without a preload.
Another advantage of a synchronous converter is tighter
load regulation as shown by Figure 3. The main reason
is that the forward voltage in a rectifier diode changes
with load current while the voltage drop in a MOSFET
remains consistent and low.
This circuit was designed primarily to provide 3.3V at
3.5A from an input of 37V to 54V. Nevertheless, converter operation is seamless over the full 36V to 72V
input range (remove D1 if operated at 72VIN). Operation
of this circuit is straightforward. MOSFET Q1 turns on
and energy is stored in the transformer T1. Energy is
then delivered to the output during the time MOSFET
Q1 is off. MOSFET Q3 turns on whenever MOSFET Q1
turns off, providing output rectification. Secondary
MOSFET Q3 is driven by transistor drivers Q12 and
Q13. T2 inverts the LT1725 gate signal and drives the
common bases of Q12 and Q13. R1, D6, Q9 and Q11
buffer the primary side gate signal and provide a small
delay so that Q1 and Q3 are never on at the same time.
Complementing this converter is the LTC ®4257-1, which
provides complete signature and interface functions for
a PD operating in an IEEE 802.3af PoE system.
Conclusion
To get the most out of the power available for a PoE supply, a converter needs to have synchronous rectification.
The flyback topology offers the simplest solution and
can operate synchronously with small incremental cost.
The power consumed by the Q3 MOSFET in this design
is roughly one tenth of that of a rectifying diode. The
1.4W of total converter dissipation is evenly distributed
among switching MOSFETs, power transformer and
controller IC, allowing the designer to compact board
layout. The LT1725 greatly simplifies circuit design because of its patented feedback circuit which eliminates
the need for an optocoupler and a secondary reference
without sacrificing load regulation.
90
3.40
85
SYNCHRONOUS
3.35
NONSYNCHRONOUS
VOUT (V)
EFFICIENCY (%)
80
75
70
3.30
SYNCHRONOUS
NONSYNCHRONOUS
65
3.25
60
3.20
55
0.5
1
2.5
2
1.5
LOAD CURRENT (A)
3
3.5
0.5
1
1.5
2
2.5
LOAD CURRENT (A)
Data Sheet Download
www.linear.com
Linear Technology Corporation
3.5
DN338 F03
DN338 F01
Figure 2. Efficiency Comparison Between a Synchronous
and Nonsynchronous Converter Using the LT1725
3
Figure 3. Load Regulation Comparison Between a
Synchronous and Nonsynchronous Converter Using
the LT1725
For applications help,
call (408) 432-1900
dn338f_conv LT/TP 0504 344K • PRINTED IN THE USA
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