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

Page 1 of 4

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

Page 2 of 4

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

Page 2 of 4Page 2 of 4

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

Page 3 of 4

IEEE is a registered service mark of the Institute of Electrical and Electronics Engineers, Inc.

Related PartsMAX5969B IEEE 802.3af/at-Compliant, Powered Device Interface Controllers with Integrated Power MOSFET MAX5974A Active-Clamped, Spread-Spectrum, Current-Mode PWM Controllers

Automatic UpdatesWould you like to be automatically notified when new application notes are published in your areas of interest? Sign up for EE-Mail™.

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

Page 4 of 4