When all CMOS chips are working, the signal level always exceeds 0V (according to the requirements of chip manufacturing and design scheme). When the PHY output signal is sent to 100m or even longer area, there will be a large DC component damage. And if the external network line is connected to the chip immediately, electromagnetic (lightning) and electrostatic induction are very easy to cause chip damage.
Moreover, different equipment grounding devices and different natural environment of power grid will cause different 0V levels of each other. In this way, the signal is transmitted from a to B. because the 0V level of the selected network signal transformer a is different from that of point B, which may cause a large current flow from the equipment with high potential difference to the equipment with low potential difference.
The differential signal sent out by PHY by the network transformer is improved by using the differential mode coupling electromagnetic coil coupling filter, and is coupled to the other end of the connecting network line according to the transformation of the magnetic field. In that way, the price of network signal transformer not only makes the network line and PHY have no physical connection, but also changes the transmission signal. The decoration cuts off the DC component in the signal and can transmit data in different 0V level equipment.
The network transformer itself is designed to withstand the working voltage of 2KV ~ 3KV. It also has the function of lightning protection and maintenance. The Internet equipment of some potters who choose Ethernet RJ45 connector is very easy to be burned in the storm temperature. Most of them are caused by unscientific PCB design scheme, and most of them damage the socket of the equipment, very few chips are damaged, that is to say, the transformer has the maintenance effect.
The insulation requirements of IEEE802.3 can be considered for isolation transformer, but EMI cannot be suppressed.
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