The role of Ethernet transformer filters and center taps

In Ethernet equipment, when connecting RJ45 through PHY, an Ethernet transformer filter will be added in the middle. Some transformer center taps are connected to the power supply, and some are connected to the capacitor to the ground. And when the power supply is connected, the power supply value can be different, 3.3V, 2.5V, and 1.8V are available. What is the purpose of this transformer?

1. Why are some of the middle taps connected to the power supply? Some ground? This is mainly determined by the driving type of the UTP port of the PHY chip used. There are two types of driving, voltage driving and current driving. The voltage-driven one should be connected to the power supply; the current-driven one should be directly connected to a capacitor to the ground! Therefore, for different chips, the connection method of the center tap is closely related to the PHY. For details, please refer to the datasheet and reference design of the chip.

2. Why do you connect different voltages when connecting to the power supply? This is also determined by the UTP port level specified in the PHY chip data used. What level is decided, you have to connect the corresponding voltage. That is, if it is 2.5v, it will be pulled up to 2.5v, and if it is 3.3v, it will be pulled up to 3.3v.

3. What is the function of this transformer, can it not be connected? In theory, it is possible to connect it directly to the RJ45 without the need for a transformer, and it can work normally. However, the transmission distance is very limited, and when it is connected to a different level network port, it will also have an impact. And the external interference to the chip is also very large. When connected to the network transformer, it is mainly used for signal level coupling. First, it can enhance the signal and make the transmission distance longer; second, the chip end is isolated from the outside, the anti-interference ability is greatly enhanced, and a great protection effect (such as lightning strike) is added to the chip; When it reaches the network ports of different levels (for example, some PHY chips are 2.5V, and some PHY chips are 3.3V), it will not affect each other‘s devices.

In general, the network transformer mainly has the functions of signal transmission, impedance matching, waveform repair, signal clutter suppression and high voltage isolation.

Center tap action:

1. Reduce common-mode current and common-mode voltage on the cable by providing a low-impedance return path for common-mode noise on the differential line;

2. For some transceivers a DC bias voltage or power source is provided.

The integrated RJ45 common mode suppression can be done better, and the influence of parasitic parameters is relatively small;

One advantage of choosing independent devices is that the ground under the isolation transformer can be separated, that is, GND and PGND. Not only will the internal common mode interference not go out, even if the external network cable is coupled with noise, it will go down to the chassis through the distributed capacitance of the network cable to PGND. superior