PCB Tech - The PCB in the lightning protection circuit of the network port

Network transformer selection and PCB design issues in the lightning protection circuit of the network port

There are two ways to protect the network port from lightning. One way of thinking is to give the lightning current a way to discharge the high voltage before the network transformer, so as to minimize the impact on the transformer, and at the same time, pay attention to reducing the common-mode overvoltage. Possibility of differential mode overvoltage. Another idea is to use the insulation withstand voltage of the network transformer to isolate the high voltage at the primary of the network transformer through good device selection and PCB design, so as to achieve isolation and protection of the interface.

The following points should be paid attention to in the process of network transformer and network port device selection and PCB design of the network port lightning protection circuit:

1. In order to ensure the withstand voltage of the common-mode isolation, the transformer needs to meet the AC insulation withstand voltage between the primary and secondary not less than AC1500V.

2. The RJ45 without lights is preferred. If you want to lead the lights, it is recommended to use the light guide technology to lead the light of the indicator to the panel on the chip side to avoid the indicator control signal from passing through the high-voltage signal line and the Bob-Smitch circuit. area.

3. The current-limiting resistor of the indicator light control circuit should be placed on the side of the control chip, close to the control chip, to prevent overvoltage from directly impacting the control chip.

4. The Ethernet signal lines follow the routing rules of differential lines to ensure impedance matching, and the length of a pair of differential lines is as long as possible.

5. If the front stage of the network transformer (on the side of the RJ45 connector) has a middle tap and adopts a Bob-Smith circuit, that is, a 75Ω resistor and a 1000pF capacitor connected to PGND. It is recommended that the withstand voltage of the capacitor is greater than DC2000V, and the resistance power is recommended to choose a single resistor of 1/10W, and it is not suitable to use exclusion.

6. A Bob-Smith circuit is used for a network transformer and an Ethernet interface to avoid multiplexing the Bob-Smith circuits of multiple Ethernet interfaces.

7. For single boards with PCB layers greater than 6 layers, since the insulating material of adjacent layers is less than 12mil, high-voltage and low-voltage lines should not be laid on adjacent layers, and should not be crossed or run in close distance.

8. Since the common mode protection is completed by the isolation characteristics of the network transformer, there should be enough space between the high-voltage signal line (differential line and Bob-Smith circuit wiring) and other signal lines (indicator control line), power line, and ground line. The insulation, there is no accidental discharge path.

Finally, to achieve effective isolation between the high-voltage area and the low-voltage area, it is necessary to pay attention to the PCB layout design between the two. In the high voltage area, there may be high voltages: connector pins, wiring, vias, resistance pads, and capacitor pads. Possibly with low voltage: wiring, vias, resistance pads, screws. For the same insulation distance, the withstand voltage capability is in the order of ground screw capacitance and resistance pads> trace vias> surface traces> inner traces. This is because the screw is entirely a metal body, and the exposed area is relatively large, which makes it easy to become a discharge path. The surfaces at both ends of the capacitor and resistance welding are metal. At the same time, because the shape is a rectangular parallelepiped, it has edges and corners, and it is easy to form a sharp discharge. There are many vias in the mesh part, the surface is bright tin, and it is easy to produce breakdown discharge, but compared with the resistance and capacitor welding ends, the metal area is relatively small. The traces on the surface of the PCB board are coated with insulating green oil, and the traces on the inner layer are surrounded by dielectric. Compared with the above, the voltage withstand capability should be higher.