Design Idea DI-88
DPA-Switch® PoE Detection and
Classification (Class 0-3) Interface Circuit
Application
Device
Power Output
Input Voltage
Output Voltage
Topology
PoE/VoIP
DPA423G
6.49 W
36-57 VDC
3.3 V
Flyback
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 (ICL),
is programmed by resistor R34, as per the table.
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)*
POUT
POUT
ICL
ICL
Class
R34
(min)
(max)
(min)
(max)
W
W
mA
0.5
9
mA
4
Ohms
-
•
Includes under voltage lockout (42 VDC on, 34 VDC off)
0
1
2
3
0.44
0.44
3.84
6.49
12.95
3.84
6.49
12.95
PoE Detection and Classification
12
20
30
133
69.8
45.3
17
26
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).
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).
The PoE specification requires the PD to implement
three functions: detection, classification and pass-switch
connection.
Operation
ResistorR31providesthedetectionimpedance.Inordertoallow
correct operation over the detection voltage range, Zener diode
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.
C4
1000 pF R7
1500 V 10 kΩ
Ethernet
(RJ-45)
L1
10 µH
1 A
C10
1 µF
10 V
C8
330 µF
6 V
C7
330 µF
6 V
D1
D101
L2
PoE Interface
J2-1
SL42
Connector
DL4002
1 µH, 2 A
3.3 V, 2 A
1
2
3
9,10
(1,2)
+
C9
330 µF
6 V
R5
649 kΩ
1%
DL4002
D102
6,7
4
R31
24.9 kΩ
1%
VR31
11 V
R36
R4
1 MΩ
1%
J2-2
RTN
51 kΩ
VR3
SMAJ
150A
D103
DL4002
D2
BAV21
C11
1 µF
50 V
R32
100 kΩ
R11
1 kΩ
(4,5)
5
C1
1.5 µF
100 V
C2
1.5 µF
100 V
Q32
R10
R12
34 kΩ
1%
75 Ω
DL4002
D104
R21
10 kΩ
R23
174 kΩ
1%
DPA-Switch
U1
DPA423G
U2
VR32
27 V
PC357
N1TA
D105
DL4002
Q20
MMBTS
3906
R33
2 kΩ
1%
Q31
D
S
L
R22
10 kΩ
CONTROL
(3,6)
(7,8)
C
R37
220 kΩ
C13
0.1 µF
X
F
DL4002
D106
Q33
R9
R35
100 kΩ
51 Ω
C5
U31
1%
0.1 µF
U3
CAT431L
D107
DL4002
Q34
R34
69.8 Ω
1%
R6
C6
R13
C12
0.33 µF
VR33
15 V
8.66 kΩ
22 µF
10 V
20.0 kΩ
1%
1%
Q35
PI-4042-111005
100 V
DL4002
D108
0.250 mΩ
Figure 1. PoE Interface Circuit – Class 2.
*UNH-IOC test reports are available on the PI website www.powerint.com/poe
November 2005
DI-88
www.powerint.com