high-efficiency, low cost flyback converter uses no ... · top view of the reference design. bottom...
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Maxim > App Notes > Amplifier and Comparator Circuits Power-Supply Circuits
Keywords: powered device, PD, power over Ethernet, PoE, IEEE 802.3af, IEEE 802.3at, Class 2 PD, Class 3 PD, IP phones, IP cameras, security cameras, WAP, wireless access point, pointof sales, POS, thin client, Ethernet repeater, active clamped forward, flyback
Feb 10, 2011
APPLICATION NOTE 4984High-Efficiency, Low Cost Flyback Converter Uses No Optocoupler Feedback
Abstract: This reference design is for a highly efficient, flyback, 12V, Class 3 powered device (PD). The design features the MAX5969B as its controller. The design alsouses the MAX5974A, which controls current-mode PWM converters and provides frequency foldback for PoE applications. Using these devices, this reference design isIEEE® 802.3af/at compliant and is a high-performance, compact, and cost efficient solution for a Class 3 PD. The design can also be customized to 12V/2A Class 4 PD.
General Description
This reference design is for a highly efficient, flyback, 12V/1A Class 3 powered device (PD) that is IEEE 802.3af/3at complaint and cost effective. The reference design canbe customized to a 12V/2A Class 4 PD.
The design features the MAX5969B and MAX5974A. The MAX5969B controller is fully compliant with the IEEE 802.3af/at standard in a power-over-Ethernet (PoE) system.The device can also get power from a wall adapter (WAD). The MAX5974A controls wide-input-voltage, active-clamped, inductive feedback, current-mode PWM convertersand provides frequency foldback for PoE applications. Using these devices, this reference design is IEEE 802.3af/at compliant. It is also a high-performance, compact, andeconomical solution for a Class 3 PD or a Class 4 PD.
Specifications
The 12V/1A PD meets the following specifications:
Input voltage: 36V to 57VWAD input voltage: 36V to 57V (10V to 57V with a different transformer)VOUT1: 12V/1A (up to 2A by optimizing the switching frequency)Output ripple: ±2.5% (can be reduced by adding more output ceramic capacitors or disabling the frequency foldback, FFB)Load transient VP-P: ±3% (25% step-load)Line and load regulation: ±2%Switching frequency: 300kHz (150kHz in FFB mode)Total efficiency with a load of 2A at 3V and 48V input: 90.3%, including input LAN transformer and diode bridge (93.3%, not including input LAN transformer anddiode bridge)
Design Features
Inductive feedback: no optocoupler or shunt-regulator, so costs are loweredSelf-driving input voltage: achieves synchronous rectifier, provides high efficiency and good load regulationFrequency foldback: 150kHz switching frequency when load is about 1W, provides high efficiency at light loads
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AVAILABLE
Top view of the reference design. Bottom view of the reference design.
Performance Data
Transient Response
VIN = 36VVOUT = 12V/0.6A–0.9A–0.6ACH1: 200mV/div, 12V output voltageCH4: 500mA/div, output currentTime base: 200µs/div
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LINE REGULATION vs. INPUT VOLTAGE
11.850
11.900
11.950
12.000
12.050
12.100
12.150
12.200
35.00 40.00 45.00 50.00 55.00 60.00
INPUT VOLTAGE (V)
OU
TPU
T V
OLT
AG
E (V
)
POUT = 1.2W_12V
POUT = 12W_12V
EFFICIENCY vs. OUTPUT CURRENT
0.00
10.00
20.00
30.00
40.00
50.00
60.00
70.00
80.00
90.00
100.00
0.000 0.200 0.400 0.600 0.800 1.000
12V OUTPUT CURRENT (A)
EFFI
CIE
NC
Y (%
)
48V_12V AT 0.1A-1A
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1 2 3 4
A
B
C
D
4321
D
C
B
ATitle
Number RevisionSize
Letter
fo teehS :etaDDrawn By:
DT1
DITHER/SYNC2
RT3
FFB4
5 6 87
PGND 10
CS 9
NDRV 11
AUXDRV 12
13141516
SS DCLMP EN VC
COMP FB SGND CSSC
U2
C2
R1VDD1
DET2 2EC 9
N.C.4
VSS5 RTN 6
WAD 7
PG 8N.C.3
CLS 10
EPEP
U1
R4
D3
VDD
VSS
C6 + C7
GND
12V
RTN
VC
C3
R6
C14
PG
R15
R16
R10
C15C16 R18
R19
R21
RTN
RTN
R7
R8
C4
R14
C10 C12C5
RTN
D10
FB
VDD
RTN
C24
R26
R17
R9PG
R25
RTN
RTN
VDD
VSS
12345 6 7
8
N3
1234 5
67
8
N2
R12R2
VSS
VDD
2
3
1
4
AC
V+
AC
V-
D1
2
3
1
4
AC
V+
AC
V-
D2
TC1 12
TD1- 10
TD1+ 11TX1+P1
TX1-P2
Y2
20
G2
18
Y119
G1
17
21 22
VC1 13
TC2 6
TD2- 5
TD2+ 4TX2+P3
TX2-P6
VC2 14
TC3 1
TD3- 2
TD3+ 3TX3+P4
TX3-P5
VC3 15
CT 7
TD4- 9
TD4+ 8TX4+P7
TX4-P8
VC4 16
J1
DA
TA &
PO
WER
IN
DA
TA O
UT
VSS
FB
VDD
C18
R38
R11
R3
C1
R5 D4
D11
3
8
104
2
5
7
9
T1
D6 D7
RTN
CLS
CLS
MAX5974A
MAX5969B
MAX5969B MAX5974A FLYBACK 12V CLASS3 PD
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Related PartsMAX5969B IEEE 802.3af/at-Compliant, Powered Device Interface Controllers with Integrated Power MOSFET MAX5974A Active-Clamped, Spread-Spectrum, Current-Mode PWM Controllers
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Application note 4984: www.maxim-ic.com/an4984More informationFor technical support: www.maxim-ic.com/supportFor samples: www.maxim-ic.com/samplesOther questions and comments: www.maxim-ic.com/contact
AN4984, AN 4984, APP4984, Appnote4984, Appnote 4984Copyright © by Maxim Integrated ProductsAdditional legal notices: www.maxim-ic.com/legal
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