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| Design Idea DI-70 (R) 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 + 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). 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 H 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 + VG(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 F, 35 V VR41 D42 6.8 V IN4148 20 V, 10 mA J2-4 1 8 Q22 Si4804 C21 2.2 nF R21 10 R22 10 D31 20CJQ060 4 3 7.5 V, 0.4 A 7 6 DL4002 D102 VR51 28 V R1 649 k 1% 3 7 6 L2 16 H 4 A C31 100 F 10 V C22-C24 100 F 5 V J2-3 C25 R4 1 F 160 10 V 5 V, 2.4 A J2-2 D103 DL4002 (4,5) R52 20 k 4 5 8 7 D6 BAV 19WS D21 SL13 15 V 5 2 RTN J2-1 DL4002 D104 R51 24.9 k 1% 1/4 W C1 1 F 100 V C2 1 F 100 V T1 R23 174 k 1% Q21 VR21 Si4804 R23 10 k C6 4.7 F 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 k R16 10.0 k 1% U2 PC357 N1T R11 10 k R12 150 D11 BAV19WS DL4002 D106 D C12 100 nF R13 11 D52 BAV19 D107 DL4002 (7,8) R53 20 k R54 20 VR1 SMAJ 150 S X F DL4002 D108 Q51 TIP29C (100 V/1 A) or MMBTA06 C4 220 nF R2 13.3 k 1% R3 1.0 C5 47 F 10 V C11 2.2 F 10 V U3 LM431AIM3 C13 68 nF R14 1 k R15 10.0 k 1% PI-3824-111005 Figure 1. PoE Interface Circuit-Using a Bipolar Transistor Pass-Switch and DPA424P. DI-70 www.powerint.com November 2005 DI-70 the under-voltage threshold for the DPA-Switch is programed by components R1, R21, R22, R23 and Q20, setting the turn-on voltage to 42 VDC and turn-off voltage to 34 VDC. MOSFET Pass-Switch: * Choose R52 to limit power dissipation in VR51 and VR52. * Choose R53 to ensure MOSFET turn-off below 28 VDC. * Choose VR51 to block Q51 turn-on below <28 VDC. * Higher values of R52, R53 and VR51 will limit dissipation. * Choose VR52 to limit the maximum gate to source voltage on Q51 (typically 15 VDC is a good choice). PI-4239-110805 Key Design Points Bipolar Transistor Pass-Switch: * Choose Q51 bipolar with sufficient current and voltage capability and highest available DC-current gain. * Choose R52 to give sufficient base-drive at turn-on, to allow the DC-DC converter to start. * Choose R54 (typically 10-20 ) to limit the capacitively coupled current spike during switching. VIN 0 V to 57 V R51 24.9 k/1% 1/4 W VR52 15 V 200 mW VR51 28 V 500 mW DC-DC 6 Class 0 Current (mA) 4 Classification Voltage Range Class 0 Max + R52 51 k 1/8 W Q51 IRF530N (100 V / 0.11 ) 2 400 A @ 10 V 880 A @ 22 V RTN 0V R53 249 k 1/8 W DC-DC RTN 0 10 12 14 16 Class 0 Min PI-3825-030304 18 20 22 Figure 2. PoE Interface Circuit-Using a MOSFET Pass-Switch. Class 0 Detect Min Detect Max PI-4238-110905 Voltage (V) Figure 4. Classification Current (Class 0). PI-3828-081905 0.80 0.60 Current (mA) 23.75 k 100 Efficiency (%) 0.40 Detection Voltage Range 90 80 IRF530N TIP29 Enhanced 0.20 26.5 k 70 0.00 0 2 4 6 8 10 12 60 30 35 40 45 50 55 60 Voltage (V) Figure 3. Detection Impedance V-I Curve. For the latest updates, visit www.powerint.com Input Voltage (V) Figure 5. Pass-Switch Efficiency. 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. Power Integrations grants its customers a license under certain patent rights as set forth at http://www.powerint.com/ip.htm. The PI logo, TOPSwitch, TinySwitch, LinkSwitch, DPA-Switch, EcoSmart, Clampless, E-Shield, Filterfuse, PI Expert and PI FACTS are trademarks of Power Integrations, Inc. Other trademarks are property of their respective companies. (c)Copyright 2005, Power Integrations, Inc. Power Integrations MAIN PHONE NUMBER +1 408-414-9200 F 11/05 5245 Hellyer Avenue APPLICATIONS FAX +1 408-414-9760 San Jose, California 95138 For a complete listing of worldwide sales offices, please visit www.powerint.com APPLICATIONS HOTLINE +1 408-414-9660 www.powerint.com |
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