How Network Transformers Work

Network transformers are also known as "data mercury", or network isolation transformers.

Plays two main roles in the network interface: one is to enhance the data transmission of the differential signal transmitted by the PHY through a filter that combines differential mode coupling and coil coupling, and Convert the electromagnetic field to a different level connecting the other end of the wire. The second is that the isolation line is connected to different levels between different network devices, in order to prevent different voltages from being transmitted through the network lines, thereby damaging the equipment. In addition, data mercury can also play a role in equipment lightning protection. It is mainly used in network switches, routers, network cards, and hubs for signal communication, high-voltage insulation, grid transformer resistance matching, and electromagnetic interference suppression.

Network transformers usually have two drive modes: voltage drive and current drive.

01

Current drive mode

The equivalent circuit diagram is as follows:

This is an older driver, only Suitable for 10M and 100M networks.

By adjusting the constant current source and the size of the current, the carrier frequency can be changed. When the current drive method is connected to a transformer, the transformer middle plug must be connected to the strain voltage (creating bias and current sense). The pull-up voltage is determined by the PHY chip. Generally there are 1.8V and 2.5V. Just read the datasheet of the PHY chip.

02

Voltage drive method

This is a commonly used control method at present, not only can be applied For 10M and 100M networks, it can also be applied to Gigabit networks. The power supply of the Gigabit PoE network is also based on this principle, just like the voltage source adjusts the voltage to realize the carrier change. When the voltage drive method is connected to the transformer, the center pin of the transformer does not need to be connected to the voltage, but can be directly connected to the ground of the capacitor. A typical voltage control mode is as follows:

Its equivalent circuit is as follows:

Why do I need to connect a network transformer?

In fact, the transformer theoretically does not work, but the risk is too great. The advantages of the network transformer are as follows:

1. Increase the transmission distance. The power of the PHY chip driver is limited. When the network line is long, the signal reaches the receiving end, and the signal may be attenuated to the point where it no longer works. But after adding a network transformer, the drive capability is significantly improved through the transformer output, allowing further transmission of the signal.

2. Reduce interference when receiving PHY chips. The receiver and transmitter and the network transformer are equivalent to isolating the PHY from the network lines. The network lines are exposed outdoors and are susceptible to various interferences. Without insulation, the digital output of the PHY chip is prone to instability.

3. Improve the compatibility of the receiving and transmitting terminals of the PHY chip. If the PHY receiver uses 3.3V and the PHY transmitter uses 5V, the level signals between the two are not compatible without a network transformer. Network transformers ensure proper signal transmission regardless of the voltage used for reception and transmission.

From this analysis:

It can be noticed that the conformal inductors are located at different locations: one at the end of the cable and another at the end of the PHY.

Note:

The PHY side is more expensive than the cable side, probably because it is less commonly used.

Current-mode PHYs are not recommended to use network transformers at the end of the PHY, which may cause the network to shut down. Voltage type PHY does not matter.

@100kHz, 0.1V, 8mA DC bias 350uhmin.

The network transformer manufacturer's product plug will use an open circuit OCL inductor: a sinusoidal signal voltage with a frequency of 100kHz and an amplitude of 0.1V will detect a network transformer with an inductance greater than 350uH, coil power supply plus 8mA DC offset of the open circuit OCL inductor.

Why is this so? There are two reasons:

1. The reason why the manufacturer must add the 8mA DC offset detection condition to the coil is that during the operation of the LAN, the network transformer, due to the positive and negative polarity of the rectangular data The number of pulses is different, a DC or slow offset of no more than 8mA is automatically generated in the network transformer coil, the DC or slow offset in the coil reduces the OCL of the coil, and the OCL drop makes the flat roof tilt the rectangular data pulse, while when the flat roof Errors occur when the height of the roof is inclined, resulting in bit errors.

2. On the other hand, recently, while the network transformer is transmitting data signals, it must also be used to transmit DC voltage (POE power supply system) to electronic equipment tens of meters away. The PoE current is relatively large and can reach the ampere level. PoE current is also a continuous or slow movement of the laminator inside the network transformer. Slow current excursions can cause a decrease in inductor inductance, which alters the network transformer's ability to cancel EMI.