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Transformer introduction
Mainly used for: high-performance digital switches; SDH/ATM transmission equipment; ISDN.ADSL.VDSL.POE integrated service data equipment; FILT optical fiber loop equipment; Ethernet switches, etc.! Data pumps are devices that are available on consumer-grade PCI network cards. Data pumps are also known as network transformers or network isolation transformers. It has two main functions on the network card, one is to transmit data, it uses differential mode coupling coil to filter the PHY differential signal to enhance the signal, and converts the coupling to different levels through the magnetic field to connect the other end of the network cable; one is to protect the network cable connection Different levels between different network devices to prevent different voltages from damaging devices according to network cable transmission. In addition, data mercury can also play a certain role in lightning protection for equipment.
Transformer Efficacy:
In the Ethernet equipment, according to the Ethernet equipment, the PHY is connected to the RJ45 point, and a network transformer will be added in the middle. Some transformers center tap to ground. And when the power supply is connected, the power supply value can be different, 3.3V, 2.5V, and 1.8V.
Transformer role:
1. Electrical isolation
The signal level generated by any CMOS chip is always greater than 0V (depending on the manufacturing and design requirements of the chip), and the PHY will have a large DC component loss when the output signal is sent to an area of ​​100 meters or more. If the external network cable is directly connected to the chip, electromagnetic induction (lightning) and static electricity can easily damage the chip.
Then there are different grounding methods of equipment. Different power grid environments will lead to inconsistent 0V levels on both sides, and the signal is transmitted from A to AB. Because the 0V level of device A and the 0V level of point B are different, it may cause a large current to flow from a strong potential. Equipment flows into equipment with a low potential.
The network transformer uses the differential mode coupling coil to filter the PHY differential signal to enhance the signal, and convert the coupling to the other end of the connection network cable through the magnetic field. This not only makes the network cable and PHY have no physical connection between them, the signal is replaced and transmitted, the DC component in the signal is cut off, but also the data can be transmitted in different 0V level devices.
The network transformer was originally designed to withstand 2KV~3KV voltage. It also acts as lightning protection. Some friends’ network equipment is easily burned in thunderstorms, most of which are thunderstorms. Due to the unscientific design of the PCB, and the large equipment interface is burned, few chips are burned, and the transformer plays a protective role.
The protective transformer can meet the insulation requirements of IEEE802.3, but cannot suppress EMI.
2. Common mode rejection
Each wire in a twisted pair should be wrapped around each other in a double helix. The magnetic field created by the current flowing through each wire is bound by the spiral. The direction of current flowing through each wire of a twisted pair determines the level of noise emitted by each wire. The transmission levels caused by the differential mode and common mode currents of each conductor are different. The noise transmission caused by the differential mode current is small, and the noise is mainly determined by the common mode current.
1. Differential mode signal in twisted pair
For differential mode signals, its current in each wire travels in opposite directions on a pair of wires. If the pair of wires were uniformly coiled, these opposing currents would produce oppositely polarized magnetic fields of the same size, making their derivations against each other.
2. Common mode signal in twisted pair
Common mode current flows in the same direction on both wires and returns to ground through parasitic capacitor Cp. In this case, the currents generate magnetic fields of the same size and polarity, the derivations of which cannot resist each other. Common mode currents create a magnetic field on the twisted surface, which functions the same as an antenna.
3. Common mode, differential mode noise and its EMC
There are two types of noise on cables: radiated noise and transmission noise from power and signal cables. These two categories are divided into common mode noise and differential mode noise. Differential-mode transmission noise is the noise current generated by noise voltages inside an electronic device that follows the same path as the signal current or supply current, as shown in Figure 4. The way to reduce this noise is to place differential mode choke coils in series on the power line and power line. A low pass filter consists of a capacitor or capacitor and inductor in parallel to reduce high frequency noise.
The field strength generated by this noise is inversely proportional to the distance from the cable to the observation point, positively related to the square of the frequency, and related to the current and the area of ​​the current loop. Therefore, the way to reduce this radiation is to add an LC low-pass filter at the signal input to prevent noise current from flowing into the cable; shielded or flat cables should be used to carry the return current and signal current to reduce the loop area.
Common mode conducted noise is generated by the noise current flowing between the ground and the cable through the parasitic capacitance between the ground and the equipment, driven by the noise voltage in the equipment.
The method to reduce common mode transmission noise is to connect a common mode choke coil in series in the power line or the power supply line. Parallel capacitors. Form an LC filter for filtering to filter out common mode transmission noise.