Measures to prevent EMI when designing switching power supply
1. Minimize the PCB copper foil area of the noisy circuit nodes; such as the drain and collector of the switch tube, and the nodes of the primary and secondary windings.
2. Keep the input and output terminals away from noise components, such as transformer wire packs, transformer cores, and heat sinks of switching tubes.
3. Keep noisy components (such as unshielded transformer wire wraps, unshielded transformer cores, and switching tubes, etc.) away from the edge of the case, because the edge of the case is likely to be close to the outside ground wire under normal operation.
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 line, auxiliary rectifier.
6. Do not mix the gate (base) drive feedback loop with the primary switching circuit or auxiliary rectification circuit.
7. Adjust the optimal damping resistor value so that it does not produce ringing sound during the dead time of the switch.
8. Prevent EMI filter inductor saturation.
9. Keep the turning node and the components of the secondary circuit away from the shield of the primary circuit or the heat sink of the switch tube.
10. Keep swing nodes and component bodies of the primary circuit away from shields or heat sinks.
11. Make the EMI filter for high-frequency input close to the input cable or connector end.
12. Keep the EMI filter for high frequency output 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 in the line of the rectifier for the auxiliary coil.
15. Connect the damping resistor in parallel on the coil of the magnetic rod.
16. Connect damping resistors in parallel across the output RF filter.
17. It is allowed to put 1nF/500V ceramic capacitors or a series of resistors in the PCB design, and connect them 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 winding.
19. When the PCB area is sufficient, the pins for the shield winding and the position for the RC damper can be left on the PCB, and the RC damper can be connected across the two ends of the shield winding.
20. Space permitting place a small radial lead capacitor (Miller, 10 pF/1 kV cap) 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 put the AC socket close to the heat sink of the primary switching tube.
