Design Idea DI-70
®
DPA-Switch
PoE Detection and
Application
PoE/VoIP
Device
DPA424P
Power Output
12.94 W
Input Voltage
34-57 VDC
Classification (Class 0) Interface Circuit
Output Voltage
5 V / 7.5 V / 20 V
Topology
Forward
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
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
R52
). Zener diode
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.
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).
The PoE specification requires the PD to provide three functions:
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 2.5 VDC
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
Ethernet
(RJ-45)
D101
Connector
DL4002
(1,2)
PoE Interface
L1
1
µH
2.5 A
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
G(Q51)
).
Resistor R53 prevents turn-on under other conditions and
Zener diode VR52 limits the Q51 gate-source voltage when the
input voltage is high (>42 VDC). In the last phase of start up,
D41
BAV19WS
C41
4.7
µF,
35 V
VR41
D42
6.8 V IN4148
20 V, 10 mA
J2-4
1
8
Q22
Si4804
C21
2.2 nF
R21
10
Ω
R22
10
Ω
D31
20CJQ060
4
3
7.5 V, 0.4 A
7
6
DL4002
D102
VR51
28 V
R1
649 kΩ
1%
3
7
6
L2
16
µH
4 A
C31
100
µF
10 V
C22-C24
100
µF
5 V
J2-3
C25
R4 1
µF
160
Ω
10 V
5 V, 2.4 A
J2-2
D103
DL4002
(4,5)
R52
20 kΩ
4
5
8
7
D6
BAV
19WS
D21
SL13
15 V
5
2
RTN
J2-1
DL4002
D104
R51
24.9 kΩ
1% 1/4 W
C1
1
µF
100 V
C2
1
µF
100 V
T1
R23
174 k
1%
Q21 VR21
Si4804
R23
10 kΩ
C6
4.7
µF
20 V
U2
Q20
MMBTS3906
D105
DL4002
(3,6)
D51
BAV19
C51
1 nF
50 V
DPA-Switch
U1
L
DPA424P
CONTROL
C
R21
10 k
R22
10 kΩ
R16
10.0 kΩ
1%
U2
PC357
N1T
R11
10 kΩ
R12
150
Ω
D11
BAV19WS
DL4002
D106
D
C12
100 nF
R13
11
Ω
D52
BAV19
D107
DL4002
(7,8)
R53
20 kΩ
R54
20
Ω
VR1
SMAJ
150
S
X
F
DL4002
D108
Q51
TIP29C (100 V/1 A)
or MMBTA06
C4
220 nF
R2
13.3 kΩ
1%
R3
1.0
Ω
C5
47
µF
10 V
C11
2.2
µF
10 V
U3
LM431AIM3
C13
68 nF
R14
1 kΩ
R15
10.0 kΩ
1%
PI-3824-111005
Figure 1. PoE Interface Circuit–Using a Bipolar Transistor Pass-Switch and DPA424P.
DI-70
www.powerint.com
November 2005
POWERINT [ POWER INTEGRATIONS, INC. ]
