How network transformers work:
Network transformers are also known as "data mercury", also known as network insulation transformers. It 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 the other end of the different level connecting line ; 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.
1. Current drive mode:
figure 1
The equivalent circuit diagram is as follows:
figure 2
This is an older driver that only works on 10M and 100M networks.
By adjusting the size of the constant current source and 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.
2. Voltage drive mode:
This is a commonly used control method at present, and it can be applied not only to 10M and 100M networks, but also 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:
figure 3
His equivalent circuit is as follows:
Figure 4
Why connect a network transformer?
In fact, the transformer theoretically does not work, but the risk is too great. The advantages of grid transformers 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 the network transformer, the driving capability is significantly improved through the transformer output, allowing the signal to be further transmitted;
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.
Figure 5
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.
Notice:
The PHY side is more expensive than the cable side, probably because it is not commonly used.
Current-mode PHYs are not recommended to use a network transformer 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: Indicates that a sinusoidal signal voltage at a frequency of 100 kHz and an amplitude of 0.1 V will detect the open circuit OCL inductor of a network transformer with an inductance greater than 350 uH, coil supply plus 8 mA DC offset .
So why? 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 automatically generates no more than 8mA in the network transformer coil due to the difference in the number of positive and negative rectangular data pulses. DC or slow offset, DC or slow offset inside the coil reduces the OCL of the coil, the OCL drop makes the flat roof tilt the rectangular data pulse, and errors occur when the flat roof is highly tilted, resulting in bit errors.
2. On the other hand, recently, while the network transformer transmits data signals, it is also 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. Current or slow excursions can cause the inductor to decrease in inductance, changing the network transformer's ability to cancel EMI.
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