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Detailed explanation of common network transformer classification and usage
2026-05-19
1、 Network Transformer
Network transformers are based on the principle of electromagnetic induction and achieve signal transmission through magnetic coupling between primary and secondary coils. It generally consists of three parts: common mode inductor, center tap, and transformer. When the differential signal output by the PHY chip passes through the primary coil, the changing magnetic field induces a corresponding signal in the secondary coil, thereby achieving electrical isolation and signal coupling. The number of turns in a network transformer is 1:1, which means that the network transformer transmits AC signals in a 1:1 ratio.
2、 The function of network transformer:
- Filtering out common mode interference: Because network transformers have common mode inductors, they can effectively filter out common mode interference, enhance signal quality, and improve transmission distance.
- Isolation: Due to the function of a transformer, signals propagate through magnetism, effectively isolating the DC voltage between the PHY and RJ45 terminals, preventing ground loop interference, lightning surges, and electrostatic discharge (ESD) damage to internal circuits. At different external levels, the network transformer only couples AC signals to maintain the same level as the PHY terminal and protect the PHY chip. The isolation voltage is usually 1500V~6000V (AC/1min), which meets safety standards such as IEEE 802.3, UL, IEC.
- Anti interference: The center tap of the PHY end can provide DC bias for the signal; Or provide a low impedance path for common mode current.
- Optimizing waveform: By suppressing high-frequency interference (such as electromagnetic interference EMI) introduced by the network cable through common mode inductance, it can effectively filter out clutter in the signal, optimize the transmission waveform, and improve signal integrity.
3、 Classification of common network transformers
- 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.
- 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.
- 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.
- Capacitive network 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.
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.
- Inductive network transformer
The structure of inductive network transformers is similar to that of traditional network transformers, and can directly replace them. Inductive network transformers have the advantages of consistent and reliable performance, strong environmental adaptability, signal integrity, and excellent automation production.
High isolation strength: 2500VAC isolation voltage (industrial grade), resistant to lightning surge (10/700 μ s waveform).
Wide band support: covering speeds from 10Mbps to 10Gbps. 10/100M two-way signal transmission, 1000M/2.5G/5G/10G four-way signal transmission.
PoE compatibility: The magnetic flux design is compatible with 802.3AF/AT/BT, and the DC component is directly bypassed through the magnetic core.
Wide temperature operation: -40 ℃~+105 ℃ (industrial grade model), without capacitor aging issues.
Anti saturation characteristics: high saturation magnetic flux density, able to withstand high PoE current.
Low insertion loss: 100MHz frequency point insertion loss<1.5dB.
Impedance matching: precise 100 Ω differential impedance to reduce signal reflection.
High isolation strength: 2500VAC isolation voltage (industrial grade), resistant to lightning surge (10/700 μ s waveform).
Wide band support: covering speeds from 10Mbps to 10Gbps. 10/100M two-way signal transmission, 1000M/2.5G/5G/10G four-way signal transmission.
PoE compatibility: The magnetic flux design is compatible with 802.3AF/AT/BT, and the DC component is directly bypassed through the magnetic core.
Wide temperature operation: -40 ℃~+105 ℃ (industrial grade model), without capacitor aging issues.
Anti saturation characteristics: high saturation magnetic flux density, able to withstand high PoE current.
Low insertion loss: 100MHz frequency point insertion loss<1.5dB.
Impedance matching: precise 100 Ω differential impedance to reduce signal reflection.
- POE
A technology that synchronously transmits data and DC power through standard Ethernet cables (such as Cat5e/Cat6) has completely changed the way network devices are powered. POE is DC power supply. In Ethernet networks with speeds of 100Mbps and below, as only two pairs of Ethernet cables are used, the remaining two pairs can be used for POE power supply. In Ethernet networks with speeds of gigabit and above, all four pairs of Ethernet cables are used for data transmission. To support POE, it is necessary to transmit power and data simultaneously on the Ethernet cables. This requires the network transformer and common mode inductor to have a certain degree of anti saturation ability. At the same time, the wire diameter of the coil also needs to be thickened to support current flow, which leads to an increase in the volume of the transformer and common mode inductor.
- Bob Smith circuit
The Bob Smith circuit is a patent circuit applied by Bob Smith in 1994, which aims to achieve impedance matching, reduce interference, and achieve better EMC performance. In addition, the Bob Smith circuit also has the function of suppressing electromagnetic interference (providing about 10dB EMI attenuation) and providing a common mode discharge circuit in common mode interference (such as lightning strikes) scenarios. Its circuit is generally connected to a 75 Ω resistor at the center tap of each transformer, and then grounded through a 1nf/2KV capacitor (lightning protection function).
4、 Application scenarios
- Ethernet network devices (core application scenarios):
Switch: All ports require network transformers to isolate the ground circuit between devices, prevent noise interference, and protect port chips.
Router: Both WAN and LAN ports require network transformers for isolation and signal coupling.
Network interface card: installed on the network card inside the computer, server, and workstation, connecting the motherboard and RJ45 interface.
Wireless access point/router: provides a port for wired Ethernet connection.
Modems: Ethernet output port for DSL/Cable modems.
Network Attached Storage: Provides ports for network connectivity.
IP Phone: A port that connects to the network and often requires support for PoE.
Industrial Ethernet switches/devices: provide stronger isolation and anti-interference capabilities in harsh environments.
Router: Both WAN and LAN ports require network transformers for isolation and signal coupling.
Network interface card: installed on the network card inside the computer, server, and workstation, connecting the motherboard and RJ45 interface.
Wireless access point/router: provides a port for wired Ethernet connection.
Modems: Ethernet output port for DSL/Cable modems.
Network Attached Storage: Provides ports for network connectivity.
IP Phone: A port that connects to the network and often requires support for PoE.
Industrial Ethernet switches/devices: provide stronger isolation and anti-interference capabilities in harsh environments.
- Devices that support Power over Ethernet:
PoE powered devices: The Ethernet port of the powered device end (such as IP cameras, wireless APs, VoIP phones, LED lighting, sensors) requires a special network transformer, whose center tap is used to separate DC power supply and AC data signals.
PoE powered devices: Ports on the power supply device end (such as PoE switches, PoE injectors) also need to support PoE network transformers to inject DC power into data cable pairs.
PoE powered devices: Ports on the power supply device end (such as PoE switches, PoE injectors) also need to support PoE network transformers to inject DC power into data cable pairs.
- Industrial automation and control systems: PLC: Ethernet port connected to the control network; Industrial HMI: Network interface for human-machine interface; Industrial sensors/actuators: More and more industrial field devices are connected through Ethernet.
- Motor driver/frequency converter: Provides network monitoring and control interfaces. Scenarios that require high reliability, strong electromagnetic interference resistance, wide temperature range, and lightning/surge protection. Industrial grade network transformers have higher requirements in these aspects.
- Telecommunications and base station equipment: used for high-speed Ethernet interconnection between internal boards of the equipment; Network management interface for base station equipment; Need to meet strict communication industry standards and reliability requirements.
- Consumer electronics: devices with wired network interfaces such as smart TVs, game consoles, set-top boxes, etc; Smart home gateway and hub; Office peripherals such as printers and scanners.
- Automotive electronics: With the development of in vehicle Ethernet (such as 100BASE-T1, 1000BASE-T1), it is used for high-speed data transmission networks in vehicles (such as information entertainment systems, ADAS cameras, radar sensor interconnection). Need to meet the requirements of vehicle regulations (such as AEC-Q100).
- Medical electronic devices: medical imaging equipment, monitors, diagnostic equipment, and other devices that require networked data transmission. The requirements for safety isolation are extremely high (such as enhanced isolation) and must comply with medical equipment safety standards.
- Testing and measuring equipment: Network interfaces for testing equipment such as network analyzers and oscilloscopes.
Why does Ethernet interface require a network transformer? Why connect a network transformer?
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