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Common Network Transformer Usage Problems

In Ethernet devices, when connecting RJ45 through PHY, a network transformer is added in the middle. Some transformers connect the center tap to the power supply, while others connect the capacitor to the ground. Moreover, when connecting the power supply, the power supply value can be different, 3.3V, 2.5V, and 1.8V are all available. So:

Why do some center taps connect to the power supply and some connect to the ground?

• This is mainly determined by the UTP port driver type of the PHY chip used. There are two types of drivers, voltage drive and current drive. Voltage drive must be connected to the power supply; current drive 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 PHY. For details, please refer to the chip datasheet and reference design.

Why do different voltages connect when connecting the power supply?

• This is also determined by the UTP port level specified in the PHY chip data used. What level is determined, the corresponding voltage must be connected. That is, if it is 2.5v, pull it up to 2.5v, if it is 3.3v, pull it up to 3.3v.
• The selection of the two tap voltage connection methods of VDD_RX and VDD_TX should be divided into two situations:
• a If the chip has the automatic identification function of MDI/MDIX function, VDD_RX=VDD_TX;
• b If the chip does not have the automatic identification function of MDI/MDIX function, VDD_RX should be directly connected to the signal ground, and VDD_TX should be connected to the power supply end;
• Note: Many chips now require the transformer turns ratio to be 1:1. If it is a 1:1 transformer, RX and TX can be interchanged. If some chips require a special transformer turns, then TX and RX must not be interchanged.

What is the function of this transformer? Can it be left unconnected?

• In general, the network transformer mainly has the functions of signal transmission, impedance matching, waveform repair, signal noise suppression and high voltage isolation.
• In theory, it is possible to connect directly to RJ45 without connecting a transformer, and it can also work normally. However, the transmission distance is very limited, and when connected to different level network ports, it will also be affected. And the external interference to the chip is also very large. When the network transformer is connected, it is mainly used for signal level coupling to enhance the signal and make it transmit 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); thirdly, when connected to network ports of different levels (such as some PHY chips are 2.5V, some PHY chips are 3.3V), it will not affect each other’s equipment.

What is the role of the center tap of the network transformer?

1.  By providing a low impedance return path for common mode noise on the differential line, the common mode current and common mode voltage on the cable are reduced;
2.  Provide a DC bias voltage or power source for some transceivers.

Is it better to choose an integrated or separate network transformer and network port?

• The integrated RJ45 common mode suppression can be better, and the parasitic parameters are less affected;
• One advantage of using independent devices is that the ground under the isolation transformer can be separated, that is, GND and PGND. Not only will the internal common mode interference not go out, but even if the external network cable couples the noise, it will go down to the chassis through the distributed capacitance of the network cable to PGND
• Relatively speaking, the matching of separate network transformers and network ports is freer, more flexible, and less expensive.

Definition of Each Magnetic ring of network transformer coil

The network transformer is composed of corresponding groups of coils. Each group of coils is basically composed of T1+K1+A1, and some are composed of T1+K1. There are also many forms of coil composition. This article will not go into details one by one. A special article will be written in the future. This article will discuss the relevant functions and definitions of each magnetic ring in the T1+K1+A1 coil composition:

• Transformer of network transformer T1 An electronic component that uses the principle of electromagnetic induction to transfer electric energy or transmit signals from one circuit to another. The transformer is an important component for electric energy transmission or signal transmission.

Definition: A combination of two or more stationary coils with mutual inductive coupling is called a transformer. The usual use of a transformer is to connect one coil to an alternating power supply and the other coil to a load, and transmit the energy output by the power supply to the load through an alternating magnetic field. The circuits connected to the power supply are called the primary circuit (primary side) and the secondary circuit (secondary side). The voltages (effective values) of the primary and secondary coils are generally different, so the transformer is named after this.

Transformers can be divided into two types: iron core transformers and air core transformers. The core transformer is a transformer that winds the primary and secondary coils on an iron core (soft magnetic material), and uses the high U value of the iron core to strengthen the mutual inductance coupling. It is widely used in power transmission and distribution, electrical measurement, welding and electronic circuits.

• Common mode choke (CMC, COMMON MODE CHOKE) of network transformer T2 As the name suggests, the common mode choke is a component used to resist common mode noise signals (useless signals, interference signals). It forms a high impedance to common mode noise signals, but has little effect on differential mode signals (useful signals).

Common mode signals refer to signals with the same polarity input at both input ends, which will cause electromagnetic interference. Electromagnetic interference is divided into radiated interference and conducted interference (entering the power line). When the signal transmission is asymmetric and the impedance is not matched, the conversion of differential mode signals will generate common mode signals for digital terminal equipment. CMC has no effect on differential mode signals. Differential mode signals refer to signals with equal magnitude and opposite polarity input at both input ends, which will not generate radiation.