**Network Transformer Selection and PCB Design Issues in Lightning Protection Circuits for Network Ports**
There are two primary approaches to protect network ports from lightning. One method is to direct the lightning current to discharge the high voltage before reaching the network transformer, minimizing the transformer’s impact while also reducing the potential for common-mode overvoltage. The other approach involves utilizing the network transformer’s insulation withstand voltage to isolate high voltage on the primary side through careful device selection and PCB design, thereby achieving isolation and interface protection.
The following considerations are crucial during the selection of network transformers and network port devices, as well as in the PCB design for network port lightning protection circuits:
1. To ensure sufficient common-mode isolation withstand voltage, the transformer must have an AC insulation withstand voltage between the primary and secondary of no less than AC 1500V.
2. Prefer RJ45 connectors without indicator lights. If indicators are necessary, use light guide technology to direct the indicator light to the panel on the chip side, avoiding the indicator control signal from passing through high-voltage signal lines and sensitive circuitry.
3. Position the current-limiting resistor of the indicator light control circuit near the control chip to prevent overvoltage from directly affecting the control chip.
4. The Ethernet signal lines adhere to differential line routing rules to ensure impedance matching, with each pair of differential lines being as long as possible.
5. If the network transformer’s front stage (on the RJ45 connector side) includes a middle tap and uses a Bob-Smith circuit—comprising a 75Ω resistor and a 1000pF capacitor connected to PGND—it is advisable for the capacitor’s voltage rating to exceed DC2000V. The resistor should be a single 1/10W type, and using multiple resistors in place of a single one is not recommended.
6. A Bob-Smith circuit should be implemented for each network transformer and Ethernet interface to prevent the need for multiplexing Bob-Smith circuits across multiple Ethernet interfaces.
7. For PCBs with more than 6 layers, where the insulating material between adjacent layers is less than 12 mil, high-voltage and low-voltage lines should be separated onto different layers and should not cross or run in close proximity.
8. Since common mode protection is provided by the isolation characteristics of the network transformer, it is important to maintain sufficient spacing between high-voltage signal lines (including differential lines and Bob-Smith circuit wiring) and other signal lines (such as indicator control lines), power lines, and ground lines to avoid unintentional discharge paths.
Finally, effective isolation between high-voltage and low-voltage areas requires careful PCB layout design. High-voltage areas may include connector pins, wiring, vias, resistor pads, and capacitor pads, while low-voltage areas may include wiring, vias, resistor pads, and screws. For the same insulation distance, voltage withstand capability typically ranks as follows: ground screw capacitance and resistor pads > trace vias > surface traces > inner traces. This ranking is due to the larger exposed metal area of screws, which can act as discharge paths. Capacitor and resistor pads, being metal and having sharp edges, are prone to sharp discharge points. Vias, with their bright tin surface, are susceptible to breakdown discharge but have a smaller metal area compared to pads. Surface traces on the PCB are coated with insulating green oil, and inner layer traces are surrounded by dielectric material, providing higher voltage withstand capability compared to the other components.
There are two primary approaches to protect network ports from lightning. One method is to direct the lightning current to discharge the high voltage before reaching the network transformer, minimizing the transformer’s impact while also reducing the potential for common-mode overvoltage. The other approach involves utilizing the network transformer’s insulation withstand voltage to isolate high voltage on the primary side through careful device selection and PCB design, thereby achieving isolation and interface protection.
The following considerations are crucial during the selection of network transformers and network port devices, as well as in the PCB design for network port lightning protection circuits:
1. To ensure sufficient common-mode isolation withstand voltage, the transformer must have an AC insulation withstand voltage between the primary and secondary of no less than AC 1500V.
2. Prefer RJ45 connectors without indicator lights. If indicators are necessary, use light guide technology to direct the indicator light to the panel on the chip side, avoiding the indicator control signal from passing through high-voltage signal lines and sensitive circuitry.
3. Position the current-limiting resistor of the indicator light control circuit near the control chip to prevent overvoltage from directly affecting the control chip.
4. The Ethernet signal lines adhere to differential line routing rules to ensure impedance matching, with each pair of differential lines being as long as possible.
5. If the network transformer’s front stage (on the RJ45 connector side) includes a middle tap and uses a Bob-Smith circuit—comprising a 75Ω resistor and a 1000pF capacitor connected to PGND—it is advisable for the capacitor’s voltage rating to exceed DC2000V. The resistor should be a single 1/10W type, and using multiple resistors in place of a single one is not recommended.
6. A Bob-Smith circuit should be implemented for each network transformer and Ethernet interface to prevent the need for multiplexing Bob-Smith circuits across multiple Ethernet interfaces.
7. For PCBs with more than 6 layers, where the insulating material between adjacent layers is less than 12 mil, high-voltage and low-voltage lines should be separated onto different layers and should not cross or run in close proximity.
8. Since common mode protection is provided by the isolation characteristics of the network transformer, it is important to maintain sufficient spacing between high-voltage signal lines (including differential lines and Bob-Smith circuit wiring) and other signal lines (such as indicator control lines), power lines, and ground lines to avoid unintentional discharge paths.
Finally, effective isolation between high-voltage and low-voltage areas requires careful PCB layout design. High-voltage areas may include connector pins, wiring, vias, resistor pads, and capacitor pads, while low-voltage areas may include wiring, vias, resistor pads, and screws. For the same insulation distance, voltage withstand capability typically ranks as follows: ground screw capacitance and resistor pads > trace vias > surface traces > inner traces. This ranking is due to the larger exposed metal area of screws, which can act as discharge paths. Capacitor and resistor pads, being metal and having sharp edges, are prone to sharp discharge points. Vias, with their bright tin surface, are susceptible to breakdown discharge but have a smaller metal area compared to pads. Surface traces on the PCB are coated with insulating green oil, and inner layer traces are surrounded by dielectric material, providing higher voltage withstand capability compared to the other components.
