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Network transformer basics

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2019-12-02 08:21

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【Outline Description】Products are mainly used in: high-performance digital switches; SDH / ATM transmission equipment; ISDN, ADSL, VDSL, POE power equipment integrated services digital equipment; FILT fiber loop equipment; Ethernet switches, etc., such as Yutai Electronics YL18-2050S , YL18-3002S and so on are more common!

Transformer introduction

Products are mainly used in: high-performance digital switches; SDH / ATM transmission equipment; ISDN, ADSL, VDSL, POE power equipment integrated services digital equipment; FILT fiber loop equipment; Ethernet switches, etc., such as Yutai Electronics YL18-2050S , YL18-3002S and so on are more common! The data pump is a device that is available on a consumer PCI network card. The data pump is also called a network transformer or can be called a network isolation transformer. It plays two main roles on a network card. One is to transmit data. The differential signal sent by the PHY is coupled and filtered by a differential-mode coupled coil to enhance the signal, and is coupled to different levels by electromagnetic field conversion. Connect the other end of the network cable; one is to isolate different levels between different network devices connected to the network cable to prevent different voltages from being transmitted through the network cable to damage the device. In addition, data mercury can also provide some lightning protection for the device.

Transformer function

In an 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.

Second, common mode suppression

Each of the wires in the twisted pair is intertwined with a double spiral structure. The magnetic field generated by the current flowing through each wire is constrained by the spiral shape. The direction of current flowing through each of the wires in the twisted pair determines the degree to which each pair of wires emits noise. The degree of emission caused by the differential mode and common mode current flowing on each pair of wires is different, and the noise emission caused by the differential mode current is small, so the noise is mainly determined by the common mode current.

1. Differential mode signal in twisted pair

For a differential mode signal, its current on each conductor is transmitted in a reverse direction over a pair of conductors. If the pair of wires are evenly wound, these opposite currents produce an equal, reverse-polarized magnetic field that causes their outputs to cancel each other out.

2. Common mode signal in twisted pair

The common mode current flows in the same direction on the two wires and returns to the ground via the parasitic capacitance Cp. In this case, the current produces magnetic fields of equal magnitude and polarity, and their outputs do not cancel each other out. The common mode current creates an electromagnetic field on the surface of the twisted pair, which acts just like an antenna.

Third, common mode, differential mode noise and its EMC

There are two types of noise on the cable: radiated noise and conducted noise generated from the power cable and signal cable. These two categories are divided into common mode noise and differential mode noise. Differential mode conducted noise is the noise current generated by the noise voltage inside the electronic device and the same path as the signal current or the power supply current, as shown in Figure 4. The way to reduce this noise is to combine a differential mode choke, a shunt capacitor, or a low-pass filter with a capacitor and inductor on the signal and power lines to reduce high-frequency noise.

The intensity of the electric field generated by this noise is inversely proportional to the distance from the cable to the observation point, proportional to the square of the frequency, and proportional to the area of the current and current loop. Therefore, the method of reducing this radiation is to add an LC low-pass filter at the signal input to prevent noise current from flowing into the cable; use a shielded cable or a flat cable to transmit return current and signal current in adjacent wires to make the loop The area is reduced.

Common mode conducted noise is generated by the noise current flowing between the earth and the cable, driven by the noise voltage inside the device, through the parasitic capacitance between the earth and the device.

The method of reducing the common mode conduction noise is to connect a common mode choke coil in a signal line or a power line, connect a capacitor between the ground and the wire, form an LC filter for filtering, and filter common mode conducted noise.

Ethernet network transformer

Network transformers are also known as network isolation transformers, Ethernet transformers, and network filters. The product types are divided into single-port, dual-port, multi-port, 10/100BASE, 1000BASE-TX and RJ45 interface integrated network isolation transformers. Products are mainly used in: RJ45 network card, Ethernet switch, network router, ADSL, VDSL digital equipment, EOC terminal, EPON/GPON three network convergence equipment, network set-top box, smart TV, network camera, SDH/ATMSDH/ATM, PC motherboard, Industrial motherboards and other equipment.

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E529589-KUDA UL certificate

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E529589-KUDA UL certificate