In Ethernet devices, a lan transformer is added in the middle when the RJ45 is connected through the PHY. Some transformers have a center tap to ground. And when connected to the power supply, the power value can be different, 3.3V, 2.5V and 1.8V are available. The role of this transformer analysis is as follows:

1.The center tap why are some connected to the power supply or ground? This is mainly with the use of PHY chip UTP port drive type decision, there are two types of drive, voltage drive and current drive. Voltage driven, the need to connect the tap to the power supply; current driven through the capacitor grounding. So for different chips, the center tap connection, and PHY is closely related to the specific reference to the chip's data sheet and reference design.

2.Why connect the power supply, and connect a different voltage? This is also the PHY chip used in the data specified in the UTP port-level decision. What level is decided, you have to connect the corresponding voltage.

3.What exactly is the role of this transformer? Can it not be connected? Theoretically, it is possible not to connect the transformer, directly to the RJ45 which is also able to work properly. But then, the transmission distance is very limited, and when connected to different levels of network ports, there will be an impact. And the external interference with the chip is also very large. When connected to the network transformer, it is mainly used for signal-level coupling. One, it can enhance the signal, so that its transmission distance is farther; second, so that the chip end and external isolation, anti-interference ability is greatly enhanced, and the chip increases a great deal of protection (such as lightning strikes); third, when received at different levels (such as PHY chip is 2.5V, some PHY chip is 3.3V) of the network port, there will be no impact on each other's equipment.