Design Idea DI-70 ® DPA-Switch PoE Detection and Classification (Class 0) Interface Circuit Application Device Power Output Input Voltage Output Voltage Topology PoE/VoIP DPA424P 12.94 W 34-57 VDC 5 V / 7.5 V / 20 V Forward Design Highlights • • • • to identify the device class (Class 0 specifies 0 mA to 4 mA). This is again accomplished by resistor R51. 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 The PoE specification requires the PD to provide three functions: 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 Zener diode VR52 limits the Q51 gate-source voltage when the 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 2.5 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 PoE Interface (1,2) DL4002 D102 D103 DL4002 1 R52 20 kΩ 4 5 C1 1 µF 100 V R51 24.9 kΩ 1% 1/4 W Q22 Si4804 7 6 R1 649 kΩ 1% D31 20CJQ060 8 3 VR51 28 V (4,5) DL4002 D104 L1 1 µH 2.5 A D41 BAV19WS C2 1 µF 100 V 7 6 D21 SL13 15 V R23 10 kΩ (3,6) D51 BAV19 CONTROL D52 BAV19 D107 DL4002 (7,8) DL4002 D108 R53 20 kΩ DPA-Switch U1 L DPA424P D R54 20 Ω Q51 TIP29C (100 V/1 A) or MMBTA06 VR1 SMAJ 150 S X C4 220 nF R2 13.3 kΩ 1% 5 V, 2.4 A 5 2 J2-2 RTN J2-1 C6 4.7 µF 20 V U2 U2 PC357 N1T R16 10.0 kΩ 1% R11 10 kΩ R12 150 Ω C F J2-3 C25 R4 1 µF 160 Ω 10 V R21 10 k R22 10 kΩ D105 DL4002 DL4002 D106 C31 100 µF 10 V C22-C24 100 µF 5 V D6 BAV 19WS Q20 MMBTS3906 C51 1 nF 50 V J2-4 7.5 V, 0.4 A 8 7 Q21 VR21 Si4804 R23 174 k 1% 3 L2 16 µH 4 A C21 2.2 nF R21 10 Ω R22 10 Ω T1 4 20 V, 10 mA C41 4.7 µF, 35 V VR41 D42 6.8 V IN4148 D11 BAV19WS R3 1.0 Ω C5 47 µF 10 V C11 2.2 µF 10 V U3 LM431AIM3 C12 100 nF R13 11 Ω 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 DI-70 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 Ω) to limit the capacitively coupled current spike during switching. VIN DC-DC VR51 28 V 500 mW R51 24.9 k/1% 1/4 W R52 51 k 1/8 W R53 249 k 1/8 W VR52 15 V 200 mW RTN + Class 0 Classification Voltage Range 2 Q51 IRF530N (100 V / 0.11 Ω) DC-DC Figure 2. PoE Interface Circuit–Using a MOSFET Pass-Switch. 10 12 14 16 18 20 Class 0 Detect Min Detect Max 0.60 100 Efficiency (%) 23.75 kΩ Detection Voltage Range 0.20 26.5 kΩ 22 Voltage (V) Figure 4. Classification Current (Class 0). PI-4238-110905 0.80 Class 0 Min 0 RTN PI-3825-030304 Current (mA) 880 µA @ 22 V 400 µA @ 10 V 0V 0.40 Class 0 Max 4 PI-3828-081905 0 V to 57 V 6 PI-4239-110805 Key Design Points 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). Current (mA) 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. 90 80 IRF530N TIP29 Enhanced 70 0.00 0 2 4 6 8 10 60 12 30 Voltage (V) 35 40 45 50 55 60 Input Voltage (V) Figure 3. Detection Impedance V-I Curve. Figure 5. Pass-Switch Efficiency. 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