What are the measures to prevent EMI in switching power supply design?
Within 1MHZ----mainly differential mode interference, which can be solved by increasing the X capacitance
1MHZ---5MHZ---differential mode and common mode mixed, using the input terminal and a series of X capacitors to filter out differential mode interference and analyze which interference exceeds the standard and solve it; 5M---and above are mainly common mode interference , adopt the method of suppressing co-touching. For the shell that is grounded, using a magnet to surround the ground wire for 2 turns will greatly attenuate the interference above 10MHZ (diudiu2006); for 25--30MHZ, you can increase the Y capacitance to the ground and wrap copper around the transformer. , change PCBLAYOUT, connect a small magnetic ring with double wires in front of the output line, wind it for at least 10 turns, and connect RC filters at both ends of the output rectifier tube.
30---50MHZ is generally caused by the high-speed opening and closing of the MOS tube. It can be solved by increasing the MOS driving resistance, using 1N4007 slow tube for the RCD buffer circuit, and using 1N4007 slow tube for the VCC supply voltage.
100---200MHZ is generally caused by the reverse recovery current of the output rectifier. You can string magnetic beads on the rectifier.
Most of the problems between 100MHz and 200MHz are caused by PFCMOSFET and PFC diodes. Now MOSFET and PFC diode string beads are effective. The horizontal direction can basically solve the problem, but the vertical direction is very helpless.
The radiation of switching power supply generally only affects the frequency band below 100M. Corresponding absorption loops can also be added to MOS and diodes, but the efficiency will be reduced.
Measures to prevent EMI when designing switching power supplies
1. Minimize the PCB copper foil area of noise circuit nodes; such as the drain and collector of the switch tube, the nodes of the primary and secondary windings, etc.
2. Keep the input and output terminals away from noisy components, such as transformer wire packs, transformer cores, heat sinks of switch tubes, etc.
3. Keep noisy components (such as unshielded transformer wire packs, unshielded transformer cores, and switch tubes, etc.) away from the edge of the casing, because under normal operation the edge of the casing is likely to be close to the outside ground wire.
4. If the transformer does not use electric field shielding, keep the shield and heat sink away from the transformer.
5. Minimize the area of the following current loops: secondary (output) rectifier, primary switching power device, gate (base) drive circuit, and auxiliary rectifier.
6. Do not mix the gate (base) drive feedback loop with the primary switching circuit or auxiliary rectifier circuit.
7. Adjust and optimize the damping resistor value so that it does not produce ringing sounds during the dead time of the switch.
8. Prevent EMI filter inductor saturation.
9. Keep the turning node and secondary circuit components away from the shield of the primary circuit or the heat sink of the switch tube.
10. Keep primary circuit swing nodes and component bodies away from shields or heat sinks.
11. Place the high-frequency input EMI filter close to the input cable or connector end.
12. Keep the high frequency output EMI filter close to the output wire terminals.
13. Keep a certain distance between the copper foil of the PCB opposite the EMI filter and the component body.
14. Put some resistors on the lines of the rectifier of the auxiliary coil.
15. Connect the damping resistor in parallel to the magnetic rod coil.
16. Connect damping resistors in parallel across the output RF filter.
17. When designing the PCB, it is allowed to place a 1nF/500V ceramic capacitor or a series of resistors across the static end of the primary of the transformer and the auxiliary winding.
18. Keep EMI filters away from power transformers; especially avoid positioning them at the ends of the wrap.
19. If the PCB area is sufficient, the pins for the shield winding and the position for the RC damper can be left on the PCB. The RC damper can be connected across both ends of the shield winding.
20. If space permits, place a small radial lead capacitor (Miller capacitor, 10 pF/1 kV capacitance) between the drain and gate of the switching power FET.
21. If space permits, place a small RC damper at the DC output.
22. Do not place the AC socket and the heat sink of the primary switch tube close together.
