Solution to the emc problem of communication switching power supply
Communication switching power supply is widely used in program controlled switching, optical data transmission, wireless base stations, cable television systems, and IP networks due to its advantages of small size, light weight, high efficiency, reliable operation, and remote monitoring. It is the driving force for the normal operation of information technology equipment.
With the development of information technology, information technology equipment is spread throughout the country, from developed central cities to remote mountainous areas, providing great convenience for communication and information transmission between people. Due to the differences between urban and rural areas, the power supply network of communication equipment includes both stable large power grid power supply methods and independent small hydropower power supply methods. Under the power supply mode of small hydropower stations, due to changes in water volume, significant changes in user electricity consumption, and unstable operation of power generation equipment, the waveform distortion of the power grid is severe and voltage fluctuations are large. At the same time, the non-standard wiring of the distribution system poses a severe challenge to the communication switching power supply.
Railway communication and power communication are developing and growing. Due to the strong induced voltage generated by electric locomotives, the ground voltage fluctuates greatly, resulting in significant fluctuations in the grid voltage. A strong electric field can easily cause transient instability in the operation of switching power supply equipment. The communication switching power supply operating near the high-voltage power grid, although the grid voltage is stable, is easily affected by strong electromagnetic field interference caused by changes in grid load.
Therefore, the communication switching power supply should have strong electromagnetic interference resistance, especially the adaptability to lightning strikes, surges, and grid voltage fluctuations. It should also have sufficient anti-interference ability to static interference, electric field, magnetic field, and electromagnetic waves, ensuring its normal operation and stability in power supply to communication equipment.
On the other hand, due to the power switch transistor, rectifier or freewheeling diode, and main power transformer inside the communication switching power supply working in the mode of high-voltage, high current, and high-frequency switching, the voltage and current waveform is mostly square wave. During the square wave switching process of high voltage and high current, severe harmonic voltage and current will be generated. These harmonic voltages and currents are transmitted through the input line of the power supply or the output line of the switching power supply, causing interference to other devices and the power grid that are powered by the communication power supply on the same power grid. At the same time, they also cause interference to devices powered by the communication power supply, such as program controlled switching equipment, wireless base stations, optical transmission equipment, and cable television equipment, making them unable to work properly; On the other hand, severe harmonic voltage and current generate electromagnetic interference inside the switching power supply, which causes instability in the internal operation of the switching power supply and reduces its performance. Some electromagnetic fields radiate into the surrounding space through the gaps in the switch power supply casing, and together with the radiated electromagnetic fields generated through power lines and DC output lines, they propagate through space, causing interference to other high-frequency equipment and equipment sensitive to electromagnetic fields, leading to abnormal operation of other equipment.
Electromagnetic compatibility issues of switching power supplies
The electromagnetic compatibility issues caused by communication switching power supply operating in high voltage and high current switching states are quite complex. In terms of electromagnetic compatibility of the entire machine, there are mainly several types: common impedance coupling, line to line coupling, electric field coupling, magnetic field coupling, and electromagnetic wave coupling. The three elements of electromagnetic compatibility are: interference source, propagation path, and interference object. Common impedance coupling mainly refers to the common impedance between the interference source and the interfered object electrically, through which the interference signal enters the interfered object. Line to line coupling mainly refers to the mutual coupling between wires or PCB wires that generate interference voltage and interference current due to parallel wiring. Electric field coupling is mainly due to the presence of potential differences, resulting in the coupling of the induced electric field to the interfered body. Magnetic field coupling mainly refers to the coupling of low-frequency magnetic fields generated near high current pulse power lines to interference objects. Electromagnetic wave coupling is mainly caused by high-frequency electromagnetic waves generated by pulsating voltage or current, which radiate outward through space and couple with the corresponding interfering body. In fact, each coupling method cannot be strictly distinguished, only with different focuses.
In a switching power supply, the main power switch operates in a high-frequency switching mode at a high voltage. The switching voltage and current are both square waves, and the spectrum of higher-order harmonics contained in this square wave can reach more than 1000 times the frequency of the square wave. At the same time, due to the leakage inductance and distributed capacitance of the power transformer, as well as the unsatisfactory working state of the main power switching device, high-frequency and high-voltage peak harmonic oscillations are often generated when high-frequency is turned on or off. The higher-order harmonics generated by this harmonic oscillation are transmitted to the internal circuit through the distributed capacitance between the switch tube and the heat sink or radiated into space through the heat sink and transformer. Switching diodes used for rectification and continuation are also an important cause of high-frequency interference. Due to the high-frequency switching state of the rectifier and freewheeling diodes, the presence of parasitic inductance and junction capacitance in the diode leads, as well as the influence of reverse recovery current, causes them to operate at high voltage and current change rates, resulting in high-frequency oscillations. Due to the fact that rectifier and freewheeling diodes are generally close to the power output line, the high-frequency interference generated by them is most likely to be transmitted through the DC output line.
In order to improve power factor, active power factor correction circuits are used in communication switching power supplies. At the same time, in order to improve the efficiency and reliability of the circuit and reduce the electrical stress of power devices, a large number of soft switching technologies have been adopted. Among them, zero voltage, zero current, or zero voltage zero current switching technology is the most widely used. This technology greatly reduces the electromagnetic interference generated by switching devices. However, soft switching lossless absorption circuits often utilize l and c for energy transfer, utilizing the unidirectional conductivity of diodes to achieve unidirectional energy conversion. Therefore, the diodes in this resonant circuit become a major source of electromagnetic interference.
