Concept
A transformer with two or more independent separated coils can be called an isolation transformer, because the primary and secondary are completely separated (isolated), and energy is transferred through the magnetic field.
Network isolation transformer circuit diagram
Application in RS-232/485
RS-232/485 is the most widely used serial interface in the PC and communication industry. It is defined as a single-ended standard that increases the communication distance in low-speed serial communication. But in actual circuit applications, RS-232/485 communication is often affected by interference and surges. There are many reasons for interference. To solve the problem of interference, you must first find out the cause of the interference, and then address the problem. Solve the problem. Generally, an isolation transformer is used to solve the problem of interference. This method is mainly for the conduction interference from the power supply, which can block most of the conduction interference before the isolation transformer, and also has the function of power supply voltage conversion. isolation transformer may be used to protect against high frequency transient interference serial interface damage, this technology is being widely used in the respective fields, as a safety isolation interface circuitry and the rear stage circuit, the protective effect.
However, In many cases, isolation converters alone cannot achieve the effect of isolation protection. In fact, as long as there is a possibility of surge pulses at the input of the circuit, or the power supply at the input is unstable (such as car batteries, etc.), the circuit is still There will be high-frequency transient interference, which will cause great damage to the circuit. Generally, switching high-power inductive loads, such as motors, transformers, relays, etc., or lightning processes, will produce high-amplitude transient interference. Without proper protection, the RS-422 or RS-485 communication interface will be damaged, and even the subsequent circuit will be damaged. For this kind of transient interference, isolation or bypass methods can be used to protect. The most commonly used and most effective method is In addition to connecting a TVS tube or voltage regulator tube (low dropout) to the input of the isolation converter, the withstand voltage of the TVS tube, the clamping voltage and other parameters should also be selected appropriately. If the withstand voltage and the clamping voltage are too high, It will not provide protection, and it will affect the operation of the circuit if it is too low.
Common application problems
in Ethernet equipment, when connected to RJ45 through PHY, a network transformer will be added in the middle. Some transformers are connected to the center tap of the power supply, and some are connected to the capacitor to the ground. And when connected to the power supply, the power value can be different. 3.3V, 2.5V, 1.8V.
What exactly does this transformer do?
Here is a general answer:
1. Why are some of the center taps connected to power? Some are grounded? This is mainly driven by the UTP port of the PHY chip used. Depending on the type, there are two types of this drive, voltage drive and current drive. The voltage drive must be connected to the power supply; the current drive is directly connected to a capacitor to the ground! So for different chips, the center tap connection method , Is closely related to PHY, please refer to the chip's datasheet and reference design for details.
2, Why is it connected to a different voltage when power is connected? This is also determined by the UTP port level specified in the PHY chip data used What level is determined, you have to connect the corresponding voltage. That is, if it is 2.5v, it will be pulled up to 2.5v, if it is 3.3v, it will be pulled up to 3.3v.
3. What is the function of this transformer? , Can you not connect it? In theory, it is possible to connect directly to the RJ45 without a transformer, and it can work normally. However, the transmission distance is very limited, and when connected to a different level network port It will also have an impact. And the external interference to the chip is also great. When the network transformer is connected, it is mainly used for signal level coupling. First, it can enhance the signal to make the transmission distance longer; Second, The chip side is isolated from the outside, the anti-interference ability is greatly enhanced, and the chip has a great protection effect (such as lightning strike); third, when it is connected to different levels (if some PHY chips are 2.5V, some PHY When the chip is a 3.3V) network port, it will not affect each other's equipment. In general, the network transformer mainly has the functions of signal transmission, impedance matching, waveform repair, signal clutter suppression and high voltage isolation.
Another: Data mercury is also called a network transformer or a network isolation transformer. It has two main functions on a network card, one is to transmit data, it uses differential mode coupling coils for the differential signal sent by the PHY Coupling filtering is used to enhance the signal, and is coupled to the other end of the connected network cable of different levels through the conversion of the electromagnetic field; one is to isolate the different levels between the different network devices connected by the network cable to prevent the transmission of different voltages through the network cable from damaging the equipment. In addition, data mercury can also play a certain role in lightning protection for equipment.