Pins Design of LAN Transformer

The function of center tap of LAN Transformer

The center tap of network transformer is also called LAN Transformer CT pin

There are two main functions of middle tap CT foot

1. Provide low impedance path for common mode current and reduce common mode current / voltage (same as choke)

2. Provide DC bias for Rx / TX signal in transceiver

Center tap, if PHY is voltage driven, connect bias voltage, CT connect capacitance!

PHY is divided into two types, one is voltage driven, the other is current driven.

The voltage driven needs to be connected to the power supply; the current driven needs to be directly connected to the ground with a capacitor! The voltage connected generally plays a role of bias. When you do not use the isolator to directly interconnect PHY, you need the appropriate bias level. Phy of different manufacturers is different.

So for different chips, the connection of the center tap is closely related to PHY. For details, please refer to the datasheet and reference design of the chip.

As for why the tap in the middle of the LAN Transformer is connected with different voltages, this is also determined by the UTP port level specified in the PHY chip data used.

What level is determined must be connected to the corresponding voltage. That is, if it is 2.5V, pull it up to 2.5V, if it is 3.3V, pull it up to 3.3V.

3. For different problem frequency points, we can choose different capacitance values to provide a low impedance return path. For different chips and PCBs, how much of this capacity can achieve the effect needs to be tried practically, but it can be determined that this capacitance has a great influence on the radiation emission of the network port

Second, we began to understand the pin diagram of network transformer chip and bob-smith circuit of LAN Transformer

In the pin diagram design of the network transformer chip, in order to suppress the common mode interference brought out by RJ45 interface through the cable, we changed the conventional LAN Transformer into the LAN Transformer with the common mode suppression effect of the interface in the design process, and the schematic diagram of this kind of transformer is shown in the figure above

Bob Smith circuit: the NC spare pin of RJ45 interface (outside the LAN Transformer ) must be designed with Bob Smith circuit to achieve signal impedance matching and suppress external interference. After testing, Bob Smith circuit can suppress interference by about 10 dB.

Secondary coil capacitance design: Although the LAN Transformer has isolation function, there are several pf distributed capacitors between the primary coils of the transformer; in order to improve the isolation function of the transformer, it is recommended to increase the filter capacitance to the ground on the secondary circuit of the transformer, such as C4-C7 on the circuit diagram, which is 5pf ~ 10PF.

LC filter design: in the drive power circuit of the transformer, add LC filter to suppress the interference brought by the power system, such as L1, C1, C2, C3 on the circuit diagram, L1 uses magnetic beads, the typical value is 600 Ω / 100MHz, and the capacitance value is 0.01 μ f ~ 0.1 μ F.

Series resistance design: in the design of 100M Ethernet, if the network drive voltage level is properly reduced without affecting the communication quality, it will be helpful for EMC interference suppression; it can also be applied to the differential line of the transmitter and receiver of the transformer secondary by series resistance of 10 Ω to suppress interference.

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