Three elements and solutions for electromagnetic interference of switching power supply.ac power

　　The three elements that form the electromagnetic interference of the switching power supply are the interference source, the propagation path and the disturbed equipment

　　First of all, the interference source of the switching power supply should be suppressed, and the cause of the interference should be directly eliminated;

　　The second is to eliminate the coupling and radiation between the interference source and the victim device, and cut off the propagation path of electromagnetic interference;

　　The third is to improve the anti-interference ability of the disturbed device and reduce its sensitivity to noise.

　　At present, several measures to suppress interference are basically to cut off the coupling channel between the electromagnetic interference source and the disturbed device, and they are indeed effective methods. The commonly used methods are shielding, grounding and filtering.

　　The use of shielding technology can effectively suppress the electromagnetic radiation interference of the switching power supply. For example, power switch tubes and output diodes usually have a large power loss. In order to dissipate heat, it is often necessary to install a radiator or directly install it on the power baseboard. When the device is installed, an insulating sheet with good thermal conductivity is required for insulation, which creates a distributed capacitance between the device and the bottom plate and the heat sink. The bottom plate of the switching power supply is the ground wire of the AC power supply, so through the distributed capacitance between the device and the bottom plate The electromagnetic interference is coupled to the AC input terminal to generate common mode interference. The solution to this problem is to sandwich a shielding sheet between two insulating sheets, and connect the shielding sheet to the DC ground, cutting off the propagation of radio frequency interference to the input power grid. Way. In order to suppress the radiation generated by the switching power supply and the influence of electromagnetic interference on other electronic equipment, the shielding cover can be processed in full according to the method of shielding the magnetic field, and then the whole shielding cover is connected with the system casing and the ground as a whole, and the electromagnetic field can be protected. Perform effective shielding. Some parts of the power supply connected to the ground can play a role in suppressing interference. For example, the grounding of the electrostatic shielding layer can suppress the interference of the changing electric field; the conductor used for electromagnetic shielding can not be grounded in principle, but the shielded conductors that are not grounded often enhance electrostatic coupling and produce the so-called "negative electrostatic shielding" effect, so grounding is still better , So that electromagnetic shielding can play the role of electrostatic shielding at the same time. The common reference point of the circuit is connected to the ground, which can provide a stable reference potential for the signal loop. Therefore, after the safety ground wire, the shield ground wire and the common reference ground wire in the system form a ground bus bar, they are all connected to the earth in the end.

　　In the circuit system design, the principle of "one point grounding" should be followed. If multiple points of grounding are formed, a closed ground loop will appear. When the magnetic field lines pass through the loop, magnetic induction noise will be generated. In fact, it is difficult to achieve "one point grounding". Therefore, in order to reduce the ground impedance and eliminate the influence of distributed capacitance, flat or multi-point grounding is adopted, and a conductive plane (the conductive plane layer of the bottom board or multilayer printed circuit board, etc.) is used as a reference ground. All parts that need to be grounded are Close to the reference ground. In order to further reduce the voltage drop of the ground loop, bypass capacitors can be used to reduce the amplitude of the return current. In a circuit system where low-frequency and high-frequency coexist, the ground wires of the low-frequency circuit, high-frequency circuit, and power circuit should be separately connected, and then connected to a common reference point.

　　Filtering is a good way to suppress conducted interference. For example, a filter connected to the input end of the power supply can suppress the interference generated by the switching power supply and fed back to the power grid, and it can also suppress the noise from the power grid from harming the power supply itself. In the filter circuit, many special filter components are also used, such as through-core capacitors, three-terminal capacitors, and ferrite magnetic rings, which can improve the filter characteristics of the circuit. Appropriate design or selection of filters, and correct installation and use of filters are an important part of anti-interference technology.

　　EMI filtering technology is an effective measure to suppress spike interference, which can filter out conduction interference caused by various reasons, and connect it to the input end of the switching power supply. In the circuit, C1 and C5 are high-frequency bypass capacitors, used to filter the differential mode interference between the two input power lines; L1 and C2, C4; L2 and C3, C4 form a common mode interference filter link, used to filter out the power supply Asymmetric common mode interference between line and ground; the primary and secondary turns of L3 and L4 are equal and opposite in polarity, and the flux generated by the alternating current in the magnetic core is opposite, which can effectively suppress common mode interference. The test shows that as long as the parameters of the components are appropriately selected, the conduction interference generated by the switching power supply can be better suppressed.

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