In the EMI rectification of switching power supply, the causes of interference in different frequency bands and the suppression methods: Within 1MHZ----mainly differential mode interference 1. Increase the X capacitance; 2. Add differential mode inductance; 3. The low-power power supply can be processed by a PI filter (it is recommended to use a larger electrolytic capacitor near the transformer). 1MHZ---5MHZ---Differential mode common mode mixing A series of X capacitors are connected in parallel with the input terminal to filter out the differential interference and analyze which interference is exceeded and solve it. 1. For differential mode interference exceeding the standard, the X capacitance can be adjusted, the differential mode inductor is added, and the differential mode inductance is adjusted; 2. Common mode inductance can be added for common mode interference exceeding the standard, and reasonable inductance is used to suppress; 3. The rectifier diode characteristics can also be changed to handle a pair of fast diodes such as FR107 pair of common rectifier diodes 1N4007. 5M---The above is mainly based on common touch interference, and the method of suppressing common touch is adopted. For the grounding of the casing, 2-3 turns of a magnetic ring on the ground wire will have a greater attenuation effect on the interference above 10MHZ; You can choose the core-bonded copper foil that is close to the transformer, and the copper foil is closed. The size of the absorption circuit of the back-end output rectifier and the parallel capacitance of the primary large circuit are processed. For 20--30MHZ. 1. For a class of products, you can adjust the Y2 capacitance to the ground or change the Y2 capacitor position; 2. Adjust the Y1 capacitor position and parameter value between the secondary side; 3. Bread copper foil outside the transformer; the innermost layer of the transformer is shielded; adjust the arrangement of the windings of the transformer. 4. Change the PCB LAYOUT; 5. The output line is connected to a small common-mode inductor surrounded by a double line; 6. Connect the RC filter at both ends of the output rectifier and adjust the reasonable parameters; 7. Add BEAD CORE between the transformer and the MOSFET; 8. Add a small capacitor to the input voltage of the transformer. 9. You can increase the MOS drive resistance. 30---50MHZ is generally caused by high-speed turn-off of MOS tube 1. Can increase the MOS drive resistance; 2. The RCD buffer circuit uses a 1N4007 slow tube; 3. VCC supply voltage is solved by 1N4007 slow tube; 4. Or the front end of the output line is connected in series with a small common-mode inductor wound by two wires; 5. Connect a small snubber circuit in parallel with the DS pin of the MOSFET; 6. Add BEAD CORE between the transformer and the MOSFET; 7. Add a small capacitor to the input voltage of the transformer; 8. When the PCB core LAYOUT, the circuit ring composed of large electrolytic capacitor, transformer and MOS is as small as possible; 9. The circuit loop formed by the transformer, output diode, and output smoothing electrolytic capacitor is as small as possible. 50---100MHZ is generally caused by the output rectifier reverse recovery current 1. It is possible to string magnetic beads on the rectifier tube; 2. Adjust the absorption circuit parameters of the output rectifier; 3. It can change the impedance of the secondary capacitor across the Y capacitor branch, such as adding BEAD CORE at the PIN pin or connecting the appropriate resistor in series; 4. It is also possible to change the MOSFET, the output of the rectifier diode body to the space radiation (such as iron clip card MOSFET; iron clip card DIODE, change the ground point of the heat sink). 5. Increase the shielding copper foil to suppress radiation to the space. The switching power supply above 200MHZ has a basic radiation amount, which can generally exceed the EMI standard. In EMI, I only know that copper foil with high-frequency current flows is placed on the inside of the circuit board. The copper foil flowing through the direct current or the copper foil with low-frequency current is placed on the outside. The high-frequency and high-current copper foil is as short as possible. There is no problem with EMC instrument testing. Of course, low-end Pai-type filtering can be used. Above 20W, transformer screen + common mode inductor is required. When Y capacitor is used, the magnitude of post-stage ripple has a greater impact on EMI. . In addition, the copper foil screen of the transformer is much better than the winding effect. .. the cost is also low. When the feedback circuit has resonance, EMI is also very difficult. In particular, a series of high-frequency spike voltages coming out of the transformer will increase the MEI value of the frequency band by 10 or more, which is more common in constant current switching power supplies. How to minimize the loop area is a difficult problem. How to layout the common mode filter part with the primary part, how to place the large electrolytic capacitor, how to do the heat sink, how to connect the grounding, all need to be considered, X capacitor The layout of the Y capacitor is very particular. For example, under the common mode inductor, do not go to any primary copper wire, but the filter circuit can pass. If there is a PFC inductor, this inductor is far away from the common mode inductor, and it is better not to place the control IC under this inductor, especially in CRM mode. 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