advertisement High Power PoE PD Interface with Integrated Flyback Controller Design Note 425 Dilian Reyes Introduction To this day, Power over Ethernet (PoE) continues to gain popularity in today’s networking world. The 12.95W delivered to the Powered Device (PD) input supplied by the Power Sourcing Equipment (PSE) is a universal supply. Each PD provides its own DC/DC conversion from a nominal 48V supply, thus eliminating the need for a correct voltage wall adapter. However, higher power devices can not take advantage of standard PoE because of its power limitations, and must rely on a large wall adapter as their primary supply. The new LTC4268-1 breaks this power barrier by allowing for power of up to 35W for such power-hungry 2-pair PoE applications. The LTC4268-1 provides a complete solution by integrating a high power PD interface control with an isolated flyback controller. PSE that looks for such a class. Once a PSE detects and classifies the PD, it fully powers on the device. The LTC4268-1 provides a low inrush current limit, allowing load capacitance to ramp up to the line voltage in a controlled manner without interference from the PSE current limit. After the load capacitance is charged up, the LTC4268-1 switches to the high input current limit and provides a power good signal to its switching regulator indicating that it can start its operation. During this time, the LTC4268-1 remains in its high current limit state allowing for up to 35W delivered to the load. Synchronous Flyback Controller Once power is switched over to the synchronous flyback controller, the LTC4268-1 regulates the output voltages by sensing the average of all the output voltages via a transformer winding during the flyback time. This allows for tight output regulation without the use of an PD Interface Controller The PD interface controller provides the same 25k signature detection resistance defined in the standard PoE. An extended optional class can be read by a customized , LT, LTC and LTM are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. L1 0.33μH T1 PA1558NL • • L2 4.7μH C8 0.1μF 100V + D1 SMAJ58A C1A 12μF 100V C1B 2.2μF 100V R9 20k 1/4W PoE+ + C18 22μF 16V C19 0.1μF R13 29.4k 1% R20 301k 1% D11 BAS21 R10 91Ω PoE– • R2 10Ω C7 1000pF 100V C11 220pF PWRGD PWRGD UVLO ILIM_EN VCC FB • L3 0.33μH • SENSE+ SG SHDN R18 100k RCLASS VPORTN VPORTN VPORTN VNEG VNEG VNEG PGDLY R21 20k RCLASS RPGDLY 15k AUX– D12 B2100 tON SYNC RCMP RtON 100k ENDLY RCMP 2.1k OSC RENDLY 150k SFST VCMP CCMP GND COSC CCMP 33pF 0.1μF Q4 Si4362DY RSENSE 0.015Ω 1/8W 1% SENSE– LTC4268-1 C33 3300pF R27 10k C26 680pF R13 B0540W Q7 Q6 FMMT718 FMMT618 R17 330Ω C27 0.1μF T2 PA0184 • C28 2200pF • 0.033μF CSFST Figure 1. High Efficiency, Triple Output, High Power PD 09/07/425 R8 10Ω 1/4W + PG R15 47Ω C24 1μF R22 15Ω D14 BAT54 dn425 F01 C23 4700pF 250VAC 5V 2.4A 100μF R28 10k 11.8V 0.4A 22μF ×2 Q3 Si4488DY VPORTP + C6 + C10 100k R14 4.7k C5 47μF R3 10Ω 1/4W Q1 Si4470EY Q2 Si4488DY Q5 FMMT723 AUX+ + C4 1500pF C21 47μF ×2 + C22 3.3V 4A 100μF optoisolator, providing improved dynamic response and reliability. Synchronous rectification increases the conversion efficiency and cross-regulation effectiveness above a conventional flyback topology. No external driver ICs or delay circuits are needed to achieve synchronous rectification; a single resistor is all that is needed to program the synchronous rectifier’s timing. High Efficiency, Triple Output, High Power PD Figure 1 shows a design using the LTC4268-1 in a high power, triple output PD. A high power PSE connects through an Ethernet cable to the RJ45 connector. PSE detection and power is passed through the data pairs’ high power Ethernet transformer or directly to the spare pairs in this 2-pair 10/100BaseT PoE system. The PSE power is then controlled by the LTC4268-1 PD interface and forwarded on to its switching regulator. An auxiliary supply option can also be connected to bypass and disable the PD interface which gives the auxiliary priority in power supply over PoE. Power conversion is then from the auxiliary supply down to the output voltages. The small supply of the LTC4268-1 utilizes an isolated flyback topology with synchronous rectification that requires no optoisolator, lowering the parts count. This circuit gives efficiencies at full load of 83% when powered from a PSE and over 85% power sourced from an auxiliary supply. PSE and Auxiliary Supplies Standard PSEs are capable of providing as low as 15.4W at the port output. This would not be sufficient power for a high power PD operating at full load. Here, a customized PSE capable of delivering higher power must be used, or a PSE controller designed for high power such as an LTC4263-1 single port PSE controller. In cases where a high power PSE is not available, an auxiliary supply can be used. 2-Pair vs 4-Pair PD 2-pair power is used today in IEEE 802.3af systems. One pair of conductors is used to deliver the current and a second pair is used for the return while two conductor pairs are not powered. This architecture offers the simplest implementation method but suffers from higher cable loss than an equivalent 4-pair system. 4-pair power delivers current to the PD via two conductor pairs in parallel allowing for an even higher level of power. This lowers the cable resistance but raises the issue of current balance between each conductor pair. Differences in resistance of the transformer, cable and connectors along with differences in diode bridge forward voltage in the PD can cause an imbalance in the currents flowing through each pair. Using two independent LTC4268-1s (Figure 2) allows for interfacing and power from two independent PSEs, and independent DC/DC converters resolve the current imbalance. PD RJ45 CAT5 1 Rx1 2 0.1μF SMAJ58A DF1501S LTC4268-1 TYP APP 3 Tx1 6 – + + + – – VOUT 0.1μF SMAJ58A 4 Rx2 LTC4268-1 TYP APP 5 DF1501S 7 Tx2 8 dn4DR F02 Figure 2. 4-Pair, High Power PD Diagram Conclusion The LTC4268-1 is a highly integrated solution for the next generation of PD products. It offers PoE PD functionality with control for efficient high power delivery to the output load. 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