Why are some of the center taps connected to the power supply? Some are grounded? This is mainly determined by the UTP port drive type of the PHY chip used.

The center tap serves two main purposes:
❶ Provide a low-impedance path for common-mode current, reducing common-mode current/voltage (same as Choke)
❷ Provide DC bias for Rx/Tx signal in Transceiver
So for different problem frequency points, we can choose different capacitance values to provide a low impedance return path. For different chips and different PCBs, the selection of this capacitance value can achieve the effect, and it needs to be tried in practice, but one thing is certain, this capacitance has a great influence on the radiated emission of the network port.

Regarding the connection method of the middle tap (some are grounded, some are connected to the power supply, and the level value may be different), there is the following Q&A on the Internet: Q: In the Ethernet device, when connecting the RJ45 through the PHY, there will be one in the middle. Network transformer. Some transformers have a center tap connected to the power supply, and some are connected to the capacitor to ground. And when connected to the power supply, the power supply value can be different, 3.3V, 2.5V, 1.8V are available. What is the function of this transformer?
A: The following is a general answer:
▶1. Why are some of the center taps connected to the power supply? Some are grounded? This is mainly determined by the UTP port drive type of the PHY chip used. There are two types of drive, voltage drive and current drive. If it is driven by voltage, it must be connected to the power supply; if it is driven by current, it can 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 are different voltages connected to the power supply? This is also determined by the UTP port level specified in the PHY chip data used. What level is determined, you have to connect the corresponding voltage. That is, if it is 2.5v, pull it up to 2.5v, and if it is 3.3v, pull it up to 3.3v.
▶3. What is the function of this transformer? Can it not be connected? Theoretically speaking, it can be connected directly to the RJ45 without a transformer, and it can work normally. However, the transmission distance is very limited, and it will also be affected when connected to different level network ports.
Moreover, 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, the signal can be enhanced to make the transmission distance longer; second, the chip end is isolated from the outside, the anti-interference ability is greatly enhanced, and the chip is greatly protected (such as lightning); third, when connected When connecting to network ports with different levels (for example, some PHY chips are 2.5V, and some PHY chips are 3.3V), it will not affect each other's equipment.