Measures to prevent EMI when designing switching power supplies
1. Minimize the PCB copper foil area of noisy circuit nodes to the greatest extent possible; Such as the drain and collector of the switch tube, the nodes of the primary winding, etc.
2. Keep the input and output terminals away from noisy components such as transformer wire bundles, transformer cores, heat dissipation fins of switch tubes, etc.
3. Keep noise components (such as unshielded transformer wire packages, unshielded transformer cores, and switch tubes, etc.) away from the edge of the enclosure, as the edge of the enclosure is likely to be close to the external grounding wire under normal operation.
4. If the transformer does not use electric field shielding, keep the shielding body and heat dissipation fins away from the transformer.
5. Minimize the area of the following current loops as much as possible: secondary (output) rectifiers, primary switching power devices, gate (base) drive circuits, and auxiliary rectifiers.
6. Do not mix the drive feedback loop of the gate (base) with the primary switch circuit or auxiliary rectifier circuit.
7. Adjust and optimize the damping resistance value so that it does not produce a ringing sound during the dead time of the switch.
8. Prevent EMI filtering inductance saturation.
9. Keep the bending nodes and components of the secondary circuit away from the shielding body of the primary circuit or the heat sink of the switch tube.
10. Keep the swinging nodes and component bodies of the primary circuit away from shielding or heat dissipation fins.
11. Place the EMI filter for high-frequency input close to the input cable or connector end.
12. Keep the EMI filter with high-frequency output close to the output wire terminal.
13. Maintain a certain distance between the copper foil on the PCB board opposite the EMI filter and the component body.
14. Place some resistors on the rectifier circuit of the auxiliary coil.
15. Connect damping resistors in parallel on the magnetic rod coil.
16. Connect damping resistors in parallel at both ends of the output RF filter.
17. In PCB design, it is allowed to place 1nF/500V ceramic capacitors or a series of resistors, which can be connected between the primary static end of the transformer and the auxiliary winding.
18. Keep the EMI filter away from the power transformer; Especially avoid positioning at the end of the wrapping.
19. When the PCB area is sufficient, a foot position for placing the shielding winding and a position for placing the RC damper can be left on the PCB. The RC damper can be connected across both ends of the shielding winding.
20. If space permits, place a small radial lead capacitor (Miller capacitor, 10 pF/1kV capacitor) between the drain and gate of the switching power field-effect transistor.
21. If space allows, place a small RC damper at the DC output end.
22. Do not lean the AC socket against the heat sink of the primary switch tube.
