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 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) • • • 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. 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. Ethernet (RJ-45) D101 Connector DL4002 POUT (min) POUT (max) ICL (min) ICL (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 Class 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 Resistor R31 provides the detection impedance. In order to allow correct operation over the detection voltage range, Zener diode C4 1000 pF R7 1500 V 10 kΩ L1 10 µH 1A PoE Interface (1,2) R32 100 kΩ D105 DL4002 (3,6) DL4002 D106 D107 DL4002 (7,8) Q31 R33 2 kΩ 1% Q33 LMV431A1M DL4002 D104 MMBTAO6L Q32 R36 51 kΩ U31 1% R34 69.8 Ω 1% R4 1 MΩ 1% C1 1.5 µF 100 V VR32 27 V MMBTAO6L (4,5) VR31 11 V MMST3904 D103 DL4002 R31 24.9 kΩ 1% MMBTAO6L DL4002 D102 9,10 1 R5 649 kΩ 1% C8 330 µF 6V C7 330 µF 6V C10 1 µF J2-1 10 V 3.3 V, 2 A L2 1 µH, 2 A + C9 330 µF 6V 6,7 2 VR3 SMAJ 150A D1 SL42 J2-2 RTN 4 3 D2 BAV21 5 C2 1.5 µF 100 V D C11 1 µF 50 V R11 1 kΩ R10 75 Ω R21 R23 DPA-Switch 174 kΩ 10 kΩ 1% U1 DPA423G Q20 L MMBTS R22 3906 10 kΩ CONTROL R12 34 kΩ 1% U2 PC357 N1TA C R37 220 kΩ R35 100 kΩ S X F C5 0.1 µF C13 0.1 µF R9 51 Ω U3 CAT431L Q34 R6 8.66 kΩ 1% VR33 15 V DL4002 D108 C6 22 µF 10 V Q35 100 V 0.250 mΩ C12 0.33 µF R13 20.0 kΩ 1% PI-4042-111005 Figure 1. PoE Interface Circuit – Class 2. *UNH-IOC test reports are available on the PI website www.powerint.com/poe DI-88 www.powerint.com November 2005 DI-88 of the classification circuit to approximately 20 mW at high input line (57 VDC) plus detection resistor (R31) dissipation of 130 mW. • • • • For Class 0, remove components VR31, R32, R33, R34, R35, Q31, Q32, Q33, Q34 and U31. R34 values: Class 1, R34 = 133 Ω; Class 2, R34 = 68.9 Ω; Class 3, R34 = 45.3 Ω. It is possible to use either bipolar transistor or MOSFET pass-switches (Q35). A bipolar transistor is less expensive, but a MOSFET gives higher pass-switch efficiency. See design idea (DI-70) for details. The bias current source (Q31) is used to allow the classification current source to be turned-off for minimal power loss once input voltage exceeds 28 VDC. This limits the dissipation Current (mA) 0.60 10 12 14 PI-4044-111005 16 20 18 22 Voltage (V) Figure 3. Classification Current (Class 1: R34 = 133 Ω). Class 3 Max 30 23.75 kΩ 0.20 10 Class 2 Class 3 Detection Voltage Range 0.40 Classification Voltage Range 8 Current (mA) Class 1 Class 2 Class 3 Detect Min Detect Max Class 1 Max 12 Class 1 Min PI-4043-080905 0.80 Class 1 Classification Voltage Range 26 PI-4045-111005 Key Design Points 14 Current (mA) VR31 inhibits the classification circuit at input voltages below 11 V. Components Q32, Q31 and R32 form a 350 µA bias current source programmed by resistor R33 working in conjunction with the base-emitter voltage of Q31. Transistor Q33 forms the classification current source programmed by resistor R34 working in conjunction with the 1.24 V voltage reference U31. Transistor Q34 disables the classification current source when Zener diode VR32 conducts (when the input voltage exceeds approximately 28 V). Class 3 Min 22 Class 2 Max 18 26.5 kΩ Class 2 Min 14 0.00 0 2 4 6 8 10 10 12 12 14 16 18 20 22 Voltage (V) Voltage (V) Figure 4. Classification Current (Classes 2 & 3: R34 = 69.8 Ω and R34 = 45.3 Ω). Figure 2. Detection Impedance V-I Curve. For the latest updates, visit www.powerint.com Power Integrations reserves the right to make changes to its products at any time to improve reliability or manufacturability. Power Integrations does not assume any liability arising from the use of any device or circuit described herein. POWER INTEGRATIONS MAKES NO WARRANTY HEREIN AND SPECIFICALLY DISCLAIMS ALL WARRANTIES INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE, AND NON-INFRINGEMENT OF THIRD PARTY RIGHTS. The products and applications illustrated herein (transformer construction and circuits external to the products) may be covered by one or more U.S. and foreign patents or potentially by pending U.S. and foreign patent applications assigned to Power Integrations. A complete list of Power Integrations' patents may be found at www.powerint.com. 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