What measures can be taken to prevent EMI in the design of switching power supplies?
1. to minimise the noise circuit nodes of the PCB copper foil area; such as the drain and collector of the switching tube, the node of the primary and secondary windings, and so on.
2. Keep the input and output terminals away from noisy components, such as transformer wire wraps, transformer cores, switching tube heat sinks, and so on.
3. Keep noisy components (e.g., unshielded transformer wire wraps, unshielded transformer cores, switching tubes, etc.) away from the edge of the enclosure, which is likely to be close to the outside earth wire under normal operation.
4. If the transformer is not shielded from electric fields, keep the shield and heat sink away from the transformer.
5. Minimise the area of the following current loops: secondary (output) rectifiers, primary switching power devices, gate (base) drive lines, auxiliary rectifiers.
6. Do not mix the gate (base) drive return loop with the primary switching circuit or the auxiliary rectifier circuit.
7. Adjust and optimise the value of the damping resistor so that it does not produce a ringing noise during the dead time of the switch.
8. Prevent saturation of the EMI filter inductor.
9. Keep corner nodes and secondary circuit components away from primary circuit shields or switching tube heat sinks.
10. Keep the swing nodes and components of the primary circuit body away from the shield or heat sink.
11. Keep high frequency input EMI filters close to the input cable or connector terminals.
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 board opposite to the EMI filter and the component body.
14. Put some resistors on the line of the rectifier of the auxiliary coil.
15. Connect damping resistors in parallel with the magnet bar coil.
16. Place damping resistors in parallel with the output RF filter terminals.
17. Allow for a 1nF/500V ceramic capacitor or also a series of resistors across the static end of the transformer primary and the auxiliary windings in the PCB design.
18. Keep the EMI filter away from the power transformer; in particular, avoid positioning it at the end of the winding.
19. If the PCB area is sufficient, leave a foot space on the PCB for the shielding winding and a place for the RC damper, which can be connected across the shielding 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. Place a small RC damper on the DC output if space permits.
22. Do not place the AC socket against the heat sink of the primary switching tube.
