Detailed explanation of the principle and function of network transformer

1. Traditional network transformer
The traditional network transformer generally adopts the two loop design of main transformer+common mode inductance. According to the CCMR requirements of common mode inductance, whether to support POE, port integration, OCL of main transformer and other parameters, the traditional network transformer has a variety of models. However, due to its small magnetic ring, it is difficult to achieve automatic processing. In the actual production process, it is produced by manual winding, resulting in low production efficiency, unstable quality, large volume, limited high-frequency performance, and poor consistency. So, the separated network transformer emerged.

2. RJ45 integrated network transformer
Integrate RJ45 sockets, transformers, common mode chokes, and terminal resistors (sometimes including necessary capacitors) into a single, compact RJ45 interface housing (usually an SMD patch package). Integrated network transformers have the advantages of high space utilization, reduced design complexity, and optimized production and consistency. However, they also have disadvantages such as a unit price that is 30% to 50% higher than separate solutions, weak high-voltage isolation and lightning protection capabilities, fixed parameters, and low flexibility.

3. Separated network transformer
Separated network transformers are divided into inductive and capacitive types. Capacitive network transformers consist of capacitors, common mode inductors, and autotransformers. Capacitive network transformers do not have magnetic saturation problems, have good lightning resistance, low magnetization loss, and good RE characteristics. The structure of an inductive network transformer is similar to that of a traditional network transformer, except that the common mode inductor and the main transformer are directly mounted on the PCB as separate components without the need for traditional encapsulation. The volume is reduced by more than 60%, and the annular magnetic core is changed to an I-shaped magnetic core, which supports fully automatic winding and welding, greatly improving production efficiency. And it has the advantages of good symmetry, flexible parameters, support for irregular structures, and superior EMC performance.

4. Capacitive network port transformer
The capacitive network transformer adopts high-voltage ceramic capacitors (withstand voltage 1-2kV) instead of the main transformer to achieve electrical isolation requirements. At the same time, an autotransformer is added, which, like the secondary winding of the original main transformer, provides a channel for the discharge of common mode interference energy. The autotransformer is directly grounded, eliminating the Bob Smith circuit, reducing costs and improving its lightning protection ability.
The setting of the capacitance value of the coupling capacitor will have certain limitations on signal transmission, and its applicability is relatively narrow, only supporting ≤ 2.5Gbps networks. Because a large capacitance value can cause a slower response and is not suitable for high-frequency transmission requirements. When the capacitance value is too small, it is easy to experience overshoot. Although canceling the Bob Smith circuit saves costs and improves lightning protection capabilities, its ability to suppress impedance matching and common mode interference between network cables has decreased. The capacitive network transformer eliminates the main transformer and is not suitable for current type PHY chips. In addition, although the autotransformer has a center tap, the center tap is directly grounded and does not support POE power supply.