Solutions to EMC problems in communication switching power supplies
From the three elements of electromagnetic compatibility, to solve the electromagnetic compatibility of switching power supply, we can start from three aspects.
1) Reduce the interference signal generated by interference sources;
2) Cut off the propagation path of interference signals;
3) Enhance the anti-interference ability of the interfered object.
When solving the electromagnetic compatibility within the switching power supply, the above three methods can be used comprehensively, based on the cost-effectiveness ratio and the ease of implementation. External interference caused by switching power supplies, such as harmonic currents in power lines, conduction interference in power lines, electromagnetic field radiation interference, etc., can only be solved by reducing interference sources. On the one hand, it can enhance the design of input and output filter circuits, improve the performance of active power factor correction (apfc) circuits, reduce the voltage and current change rate of switching tubes and rectifier freewheeling diodes, and adopt various soft switching circuit topologies and control methods. wait. On the other hand, strengthen the shielding effect of the casing, improve the gap leakage of the casing, and conduct good grounding treatment. As for external anti-interference capabilities, such as surges and lightning strikes, the lightning protection capabilities of the AC input and DC output ports should be optimized. Usually, the combined lightning strike waveform of 1.2/50μs open circuit voltage and 8/20μs short circuit current can be solved by a combination of zinc oxide varistor and gas discharge tube due to its small energy. For electrostatic discharge, usually in the small signal circuits of the communication port and control port, TVS tubes and corresponding grounding protection are used, the electrical distance between the small signal circuit and the chassis is increased, or devices with anti-static interference are selected to solve the problem. Fast transient signals contain a wide spectrum and are easily transmitted into the control circuit in the form of common mode. Use the same anti-static methods to reduce the distributed capacitance of the common mode inductor and strengthen the common mode signal filtering of the input circuit (such as adding Common mode capacitors or insertion loss ferrite magnetic rings, etc.) to improve the anti-interference performance of the system.
To reduce the internal interference of the switching power supply, achieve its own electromagnetic compatibility, and improve the stability and reliability of the switching power supply, we should start from the following aspects: pay attention to the correct partitioning of digital circuits and analog circuits PCB wiring, digital circuits and analog circuits Correct decoupling of the power supply; pay attention to single-point grounding of digital circuits and analog circuits, single-point grounding of high-current circuits and small currents, especially current and voltage sampling circuits, to reduce common resistance interference and the influence of ground loops; pay attention to phase differences when wiring. The spacing between adjacent lines and the nature of the signals are to avoid crosstalk; reduce the impedance of the ground wire; reduce the area surrounded by high-voltage and high-current circuits, especially the primary side of the transformer and the switch tube and power supply filter capacitor circuit; reduce the output rectifier circuit and continued The area surrounded by the current diode circuit and the DC filter circuit; reduce the leakage inductance of the transformer and the distributed capacitance of the filter inductor; use filter capacitors with high resonant frequency, etc.
The data lines and address lines of MCUs and LCDs work at high frequencies and are the main sources of interference that produce radiation; small signal circuits are the weakest link against external interference. Appropriately add TVs with high anti-interference capabilities and high-frequency capacitors and iron. Oxygen magnetic beads and other components are used to improve the anti-interference ability of small signal circuits; small signal circuits that are close to the chassis should be properly insulated and withstand voltage treatment. The radiator of the power device and the electromagnetic shielding layer of the main transformer must be properly grounded. Comprehensive consideration of various grounding measures will help improve the electromagnetic compatibility of the entire machine. The large-area grounding between each control unit is shielded with a grounding plate, which can improve the stability of the internal working of the switching power supply.
On the rectifier rack, it is necessary to consider the electromagnetic coupling between each rectifier, the layout of the whole machine ground wire, the correct relationship between the AC input neutral wire, ground wire and DC ground wire, lightning protection ground wire, and the correct distribution of electromagnetic compatibility levels, etc. .
In the internal and external interference and anti-interference of the switching power supply, the common-mode signal has a very complicated relationship with the working mode of the switching device, the installation of the radiator, and the connection between the PCB board and the chassis of the whole machine. The common-mode signal must be controlled under certain conditions. Then it can be converted into a differential mode signal. The simplest way to solve common mode interference is to solve the problems between each circuit unit, the port of the whole machine, and the chassis. Whole-machine shielding is difficult to implement and costly, so this measure is only used when there is no alternative.
