The role of network transformers in Ethernet

In an Ethernet device, when the RJ45 is connected through the PHY, a network transformer is added in the middle. Some transformers are connected to the power supply at the center of the transformer, and some are connected to the ground. And when connected to the power supply, the power supply value can be different, 3.3V, 2.5V, 1.8V. The role of this network transformer is analyzed as follows:

1. Why is the middle tap connected to the power supply? Some grounding? This is mainly determined by the type of UTP port drive used by the PHY chip. There are two types of drive, voltage drive and current drive. The voltage drive should be connected to the power supply; if the current is driven, it can be connected directly to the ground! Therefore, for different chips, the center tap connection is closely related to the PHY. For details, please refer to the chip's datasheet and reference design.

2. Why do you connect different voltages when you connect the power supply? This is also determined by the UTP port level specified in the PHY chip data used. To determine what level, you have to pick up 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 role of this network transformer? Can you not pick it up? In theory, it can be connected to the RJ45 without a transformer, and it can work normally. However, the transmission distance is very limited, and it will also have an impact when connected to different level network ports. Moreover, external interference with the chip is also large. When connected to a network transformer, it is primarily used for signal level coupling. First, the signal can be enhanced to make the transmission distance farther; secondly, the chip end is isolated from the outside, the anti-interference ability is greatly enhanced, and the chip is greatly protected (such as lightning strike); When the network ports of different levels (if the PHY chip is 2.5V and some PHY chips are 3.3V), it will not affect each other's devices.

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