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Voltages should be measured throughout the circuit to determine if any components are subject to breakdown. The primary concern, however, is how much surge energy passes through to the Ethernet physical application-specific integrated circuit (PHY ASIC), on the left side of the schematic. The transient-voltage-suppression (TVS) diodes across the signal lines limit the voltage to protect the transient current suppressors (TCS), a current limiting device. In this design, the TCS provides most of protection for the PHY, although some PHYs may be robust enough as not to require a TCS. The primary improvement comes from the addition of MOVs in the center taps of the LAN transformers. These limit the voltage seen by the RJ45 jack, LAN transformer, and the remainder of the circuitry. The 680V MOVs were chosen as a compromise between minimizing leakage current during the 500V insulation breakdown test (part of test 2.1.10) and minimizing voltages during the 6kV surge test, clamping around 1.1kV. For lower level surges, smaller MOVs can be used according to the table below: The 1.1kV clamp is why the LAN transformer (Hipot rated at 1.5kV) and standard RJ45 connector both survive the surge event. In addition, the surge voltage at the 1000pF Bob Smith termination could be reduced to a 1kV rating, saving cost. Lastly, Ethernet circuits with PoE typically have the following three grounds, each isolated from the other: Earth (cable shield, RJ45 housing, Bob Smith termination return path), PHY ASIC ground (circuit ground), and PoE power supply ground. Care must be taken to ensure the isolation between the grounds can withstand the 1.1kV surge voltage that appears throughout the circuit. | 25 | Surge Voltage 1kV 2.5kV 4kV 6kV MOV size MOVs not needed 7mm 10mm 14mm Part # Quantity Function Notes Not critical 1 RJ45 Connector No special requirement 14D681K 4 Surge clamp 10D681K for 4Kv 07D681K for 2.5Kv Not required for 1kV 75Ω resistor 4 Bob Smith Termination Minimize cable reflec- tions, improves EMC .01uF Capacitor 4 Bob Smith DC block for PoE 1000pF, 2kV 1 Bob Smith Termination 1kV would probably work PT61020EL 1 LAN transformer Other part numbers listed in schematic to handle other PoE needs 0.1uF capacitor 4 AC bypass Improves transformer balance and com- mon-mode rejection CDSOD323-T05 4 TCS protection CDSOD323-T05LC may be required for > 1GbE systems TCS-DL004- 250-WHC 4 Current limit protection for PHY May not be required for some PHY SMLJ58A 1 TVS PoE protection Working voltage must be > 57V CD214A-F1100 1 Schottky PoE protection Working voltage must be > 57V 0.1uF capacitor 1 If required by PoE controller Figure 2: PoE Circuitry (Source: Bourns, Inc.) SUMMARY A GbE/PoE protection design that passes ITU-T K.21 enhanced 6kV surges was presented. The solution outlined in this article provided what the various components do and what options are available to further optimize the design for specific alternative requirements. BOM for 6kV Surge Protection

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