LINEAR dn425f handbook
Contents
1. DESIGN NOTES 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 sup ply Each PD provides its own DC DC conversion from a nominal 48V supply thus eliminating the need for a cor rect 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 acomplete solution by integrating a high power PD interface control with an isolated flyback controller PD Interface Controller The PD interface controller provides the same 25k sig nature detection resistance defined in the standard PoE An extended optional class can be read by a customized cB 12uF 2 2uF Twv LD ow PWRGD PWRGD UVLO ILM EN Vec LTC4268 1 SHDN ReLass VPoRTN VPoRTN VpoRrTN VneG VneG Vneg PGDLY ton AUX AUX 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 allow2. A LY tinier LINEAR TECHNOLOGY CORPORATION 2007
3. 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 dis able 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 pow ered 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 ahigh power PD operating at full load Here acustomized 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 3a
4. f systems One pair of conductors is used to deliver the current and a Data Sheet Download www linear com 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 currentto 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 0 1uF DF1501S X00 X00 LTC4268 1 TYP APP DF1501S Tx2 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 functional ity with control for efficient high power delivery to the output load For applications help call 408 432 1900 Ext 2360 Linear Technology Corporation 1630 McCarthy Blvd Milpitas CA 95035 7417 408 432 1900 FAX 408 434 0507 www linear com dn425f LT TP 0907 451K PRINTED IN THE US
5. ing 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 al lowing 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 al lows for tight output regulation without the use of an 47 LT LTC and LTM are registered trademarks of Linear Technology Corporation All other trademarks are the property of their respective owners 11 u PA1558NL 0 33uH D Q D14 BAT54 0 033uF Csest 4700pF 250VAC Figure 1 High Efficiency Triple Output High Power PD 09 07 425 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
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LINEAR dn425f handbook