Network transformer basics

Transformer function: In the Ethernet device, when the RJ45 is connected through the PHY, a network transformer is added in the middle. Some transformer center taps 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 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. For current drive, the tap needs to be connected to the power supply; the voltage drive is grounded through the capacitor. 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 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.

Transformer function

First, electrical isolation

   The signal level generated by any CMOS process chip is always greater than 0V (depending on the chip's process and design requirements). When the PHY output signal is sent to 100 meters or more, there will be a large loss of DC component. . Moreover, if the external network cable is directly connected to the chip, electromagnetic induction (thunder) and static electricity can easily cause damage to the chip.

    Then the equipment grounding method is different. The grid environment will cause the 0V levels of the two parties to be inconsistent, so the signal will be transmitted from A to B. Since the 0V level of the A device is different from the 0V level of the B point, this may cause a large Current flows from a device with a high potential to a device with a low potential.

The network transformer couples the differential signal sent by the PHY with a differential mode coupled coil to enhance the signal and is coupled to the other end of the connected network line by electromagnetic field conversion. This not only makes the physical connection between the network cable and the PHY but also transmits the signal, blocks the DC component in the signal, and can also transmit data in devices with different 0V levels.

     The network transformer itself is designed to withstand voltages from 2KV to 3KV. It also plays a role in lightning protection. Some friends' network equipment is easily burnt out during thunderstorms. Most of them are caused by unreasonable PCB design, and most of them burned the interface of the equipment. Few chips were burnt, that is, the transformer played a protective role.

    The isolation transformer meets the insulation requirements of IEEE802.3, but does not suppress EMI.

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