Design Idea DI-70
DPA-Switch® PoE Detection and
Classification (Class 0) Interface Circuit
Topology
Device
Power Output
Input Voltage
Output Voltage
Application
Forward
PoE/VoIP
12.94 W
34-57 VDC
DPA424P
5 V / 7.5 V / 20 V
to identify the device class (Class 0 specifies 0 mA to
4 mA). This is again accomplished by resistor R51.
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
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 + VR52). 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).
MOSFET Pass-Switch
ThePoEspecificationrequiresthePDtoprovidethreefunctions:
detection, classification, pass-switch connection.
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 + VG(Q51)).
Resistor R53 prevents turn-on under other conditions and
ZenerdiodeVR52limitstheQ51gate-sourcevoltagewhenthe
input voltage is high (>42 VDC). In the last phase of start up,
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
D41
BAV19WS
20 V, 10 mA
J2-4
C41
4.7 µF, 35 V
VR41
D42
Ethernet
4
3
6
6.8 V IN4148
(RJ-45)
D101
D31
L1
Connector
DL4002
PoE Interface
20CJQ060
1 µH 2.5 A
7.5 V, 0.4 A
J2-3
1
3
8
(1,2)
7
C31
100 µF
10 V
Q22
Si4804
7
6
C25
VR51
28 V
L2
16 µH 4 A
DL4002
D102
C22-C24
100 µF 5 V
R4 1 µF
C21
2.2 nF
5 V, 2.4 A
J2-2
160 Ω
10 V
R1
649 kΩ
1%
5
2
8
R21
10 Ω
R22
10 Ω
7
4
R52
20 kΩ
D6
BAV
D103
DL4002
RTN
J2-1
5
19WS
Q21
Si4804
VR21
D21
SL13
15 V
T1
(4,5)
C1
1 µF
100 V
C2
1 µF
100 V
R23
174 k
1%
C6
4.7 µF
20 V
R23
10 kΩ
R51
24.9 kΩ
1% 1/4 W
U2
R16
10.0 kΩ
DL4002
D104
1%
R21
10 k
Q20
MMBTS3906
R22
10 kΩ
D105
DL4002
U2
PC357
N1T
(3,6)
R11
10 kΩ
DPA-Switch
C51
C12
100 nF
R12
150 Ω
U1
1 nF
D51
BAV19
DPA424P
D
L
50 V
DL4002
D106
R13
CONTROL
11 Ω
C
D11
BAV19WS
D52
BAV19
R3
S
X
F
R14
C11
2.2 µF
10 V
C13
68 nF
VR1
SMAJ
150
1.0 Ω
1 kΩ
D107
DL4002
C4
220 nF
R2
R53
R54
20 Ω
C5
47 µF
10 V
20 kΩ
U3
R15
10.0 kΩ
1%
(7,8)
LM431AIM3
13.3 kΩ
1%
Q51
PI-3824-111005
TIP29C (100 V/1 A)
or MMBTA06
DL4002
D108
Figure 1. PoE Interface Circuit–Using a Bipolar Transistor Pass-Switch and DPA424P.
November 2005
DI-70
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