PoE/VoIP

DPA424P

12.94 W

Forward

Figure 1. PoE Interface Circuit–Using a Bipolar Transistor Pass-Switch and DPA424P.

www.powerint.com

DI-70

November 2005

Power Output

Input Voltage

Output Voltage

Topology

Device

Design Idea DI-70

Classification (Class 0) Interface Circuit

Application

Design Highlights

• Simple, low-cost, discrete PoE interface circuit

• Low cost bipolar pass-switch: 87% efficient (Figure 5)

• MOSFET pass-switch: 97% efficient (Figures 2 and 5)

• Fully compliant with IEEE 802.3af

Operation

Power over Ethernet (PoE) is becoming widely adopted for

networking and (VoIP) telecom applications. A typical PD

solution is shown in Figure 1, having a PoE interface circuit

and a DPA-Switch DC-DC converter block (see DI-69 for full

details of operation of the DC-DC converter).

ThePoEspecificationrequiresthePDtoprovidethreefunctions:

detection, classification, pass-switch connection.

When an input voltage is first applied to the PD, it must present

the correct detection impedance in the voltage range of 2.5VDC

to 10 VDC. This impedance is provided by R51.

The second (classification) phase occurs at input voltages

14.5 VDC to 20.5 VDC. The PD must draw a specified current

5 V / 7.5 V / 20 V

34-57 VDC

to identify the device class (Class 0 specifies 0 mA to

4 mA). This is again accomplished by resistor R51.

Bipolar Transistor Pass-Switch

In the third phase, the bipolar pass-switch (Q51 in Figure 1)

connects the input voltage to the power supply at voltages

above approximately 30 VDC (28 V + V

VR51 conducts, driving current through resistor R52 to

the base of Q51. Resistor R53 prevents turn-on under other

conditions. Once the power supply has started, components

D51, D52, C51 and R54 enhance the base-current drive by

coupling power from the power supply bias winding.

MOSFET Pass-Switch

An alternative MOSFET-based third phase solution

(Figure 2) connects the input to the power supply at input

voltages above approximately 30 VDC (28 V + V

Resistor R53 prevents turn-on under other conditions and

ZenerdiodeVR52limitstheQ51gate-sourcevoltagewhenthe

input voltage is high (>42 VDC). In the last phase of start up,

®

U1

DPA424P

D6

BAV

19WS

Q22

Si4804

D21

SL13

15 V

Q21

Si4804

C2

1

µF

100 V

VR1

SMAJ

150

C5

47

µF

10 V

D41

BAV19WS

D31

20CJQ060

R21

10

Ω

R22

10

Ω

R23

10 k

Ω

VR21

C21

2.2 nF

U2

T1

R1

649 k

Ω

1%

R3

1.0

Ω

C22-C24

100

µF 5 V

VR41

6.8 V

D42

IN4148

C31

100

µF

10 V

C41

4.7

µF, 35 V

C25

1

µF

10 V

L1

1

µH 2.5 A

PoE Interface

L2

16

µH 4 A

C6

4.7

µF

20 V

D11

BAV19WS

U3

LM431AIM3

C11

2.2

µF

10 V

R14

1 k

Ω

R15

10.0 k

Ω

1%

R16

10.0 k

Ω

1%

R4

160

Ω

R12

150

Ω

R13

11

Ω

C1

1

µF

100 V

U2

PC357

N1T

R11

10 k

Ω

PI-3824-111005

DPA-Switch

5 V, 2.4 A

J2-2

RTN

J2-1

7.5 V, 0.4 A

J2-3

20 V, 10 mA

J2-4

C13

68 nF

4

5

3

6

7

1

8

7

2

7

8

6

5

4

3

Ethernet

(RJ-45)

Connector

R2

13.3 k

Ω

1%

R52

20 k

Ω

D101

DL4002

DL4002

D102

D103

DL4002

DL4002

D104

D105

DL4002

DL4002

D106

D107

DL4002

DL4002

D108

R22

10 k

Ω

R23

174 k

1%

R21

10 k

R51

24.9 k

Ω

1% 1/4 W

D51

BAV19

D52

BAV19

C51

1 nF

50 V

R54

20

Ω

VR51

28 V

(1,2)

(4,5)

(3,6)

(7,8)

D

S

C

L

F

X

CONTROL

C12

100 nF

R53

20 k

Ω

Q51

TIP29C (100 V/1 A)

or MMBTA06

C4

220 nF

Q20

MMBTS3906