Design Idea DI-88

Classification (Class 0-3) Interface Circuit

Design Highlights

• Simple interface for Power Over Ethernet (PoE) Powered

Devices (PDs)

• Includes PoE detection and classification circuits for all

classes (0, 1, 2 and 3)

• Compliance to IEEE802.3af PoE standards verified by

University of New Hampshire Interopability Consortium

(UNH-IOC)*

• Includes under voltage lockout (42 VDC on, 34 VDC off)

PoE Detection and Classification

PoE is becoming widely adopted for networking and VoIP

telecom applications. A typical PD solution is shown in

Figure 1 and has a PoE interface circuit and a DPA-Switch

DC-DC converter block (see EPR-68 for full details of

operation of the DC-DC converter).

The PoE specification requires the PD to implement

three functions: detection, classification and pass-switch

connection.

Detection occurs as the input voltages rises from 2.5 to

10 VDC. Resistor R31 within the PD presents the detection

impedance to the PSE.

Classification occurs as the input voltages rises from 14.5 to

20.5 VDC. The sending device determines the class of the PD

by monitoring the amount of current drawn by the PD during

this phase. For classes 1, 2 and 3, the classification current (I

is programmed by resistor R34, as per the table.

Components R32, R33, R34, R35, Q31, Q32, Q33, Q34, VR31

and U31 are not installed for Class 0 applications, and the

classification current is programmed by R31 (24.9 k).

Operation

ResistorR31providesthedetectionimpedance.Inordertoallow

correct operation over the detection voltage range, Zener diode

www.powerint.com

Figure 1. PoE Interface Circuit – Class 2.

DI-88

®

November 2005

Application

Device

Power Output

Input Voltage

Output Voltage

Topology

PoE/VoIP

DPA423G

6.49 W

36-57 VDC

3.3 V

Flyback

Class

P

OUT

(min)

P

OUT

(max)

I

CL

(min)

I

CL

(max)

R34

W

W

mA

mA

Ohms

0

0.44

12.95

0.5

4

-

1

0.44

3.84

9

12

133

2

3.84

6.49

17

20

69.8

3

6.49

12.95

26

30

45.3

+

D

S

C

L

F

X

CONTROL

DPA-Switch

U1

DPA423G

U2

PC357

N1TA

Q20

MMBTS

3906

U3

CAT431L

1

2

3

9,10

6,7

4

5

PI-4042-111005

Q35

100 V

0.250 m

Ω

Q34

Q33

Q31

Q32

U31

1%

VR33

15 V

VR3

SMAJ

150A

VR32

27 V

R31

24.9 k

Ω

1%

J2-1

3.3 V, 2 A

J2-2

RTN

R36

51 k

Ω

R6

8.66 k

Ω

1%

R9

51

Ω

C5

0.1

µF

C12

0.33

µF

C13

0.1

µF

C6

22

µF

10 V

R4

1 M

Ω

1%

R5

649 k

Ω

1%

R10

75

Ω

R23

174 k

Ω

1%

R21

10 k

Ω

R22

10 k

Ω

R11

1 k

Ω

R12

34 k

Ω

1%

R13

20.0 k

Ω

1%

R7

10 k

Ω

D1

SL42

L2

1

µH, 2 A

C7

330

µF

6 V

C8

330

µF

6 V

C9

330

µF

6 V

C10

1

µF

10 V

D2

BAV21

C4

1000 pF

1500 V

C1

1.5

µF

100 V

L1

10

µH

1 A

C2

1.5

µF

100 V

R37

220 k

Ω

R35

100 k

Ω

R33

2 k

Ω

1%

VR31

11 V

R34

69.8

Ω

1%

C11

1

µF

50 V

PoE Interface

(1,2)

(4,5)

(3,6)

(7,8)

Ethernet

(RJ-45)

Connector

D101

DL4002

DL4002

D102

D103

DL4002

DL4002

D104

D105

DL4002

DL4002

D106

D107

DL4002

DL4002

D108

R32

100 k

Ω

*UNH-IOC test reports are available on the PI website www.powerint.com/poe