EMI prevention measures when designing switching power supplies

1. Minimize the PCB copper foil area of the noise circuit node to the maximum extent; Such as the switch pipe drain, collector, primary winding node and so on.

2. Keep the input and output ends away from noise elements, such as transformer coil, transformer magnetic core, heat sink of switch tube, etc.

3. Keep the noise elements (such as the unshielded transformer coil, the unshielded transformer core, and the switch tube, etc.) away from the enclosure edge because the enclosure edge is likely to be close to the outer ground wire under normal operation.

4. If the transformer does not use electric field shielding, keep the shield and radiator away from the transformer.

5. Minimize the area of the following current loop: secondary (output) rectifier, primary switching power device, grid (base) drive line, auxiliary rectifier.

6. Do not mix the gate (base) drive feedback loop with the primary switch circuit or the auxiliary rectifier circuit.

7. Adjust and optimize the damping resistance value so that it does not produce ringing in the dead zone time of the switch.

8. Prevent EMI filtering inductance saturation.

9. Keep turning nodes and secondary circuit elements away from primary shield or heat sink of switching tube.

10. Keep the oscillating nodes and components of the primary circuit away from the shield or radiator.

11. Keep the EMI filter of high frequency input close to the input cable or connector end.

12. Keep the EMI filter of high frequency output close to the output wire terminal.

13. Keep the distance between the copper foil of the PCB plate opposite the EMI filter and the component body.

Put some resistance on the line of the rectifier for the auxiliary coil.

15. Parallel damping resistance on magnetic rod coil.

16. Parallel damping resistance at both ends of the output RF filter.

17. 1nF/ 500v ceramic capacitors are allowed in the PCB design or may be a series of resistances, which are bonded between the primary static end of the transformer and the auxiliary windings.

18. Keep EMI filter away from power transformer; In particular, avoid positioning at the end of the wrap.

19. In the case of sufficient area of PCB, the pin position for the shielding winding and the position of the RC damper can be left on the PCB, and the RC damper can be bonded at both ends of the shielding winding.

20. If space permits, place a footpath leading capacitor (miller capacitance, 10 pico / 1 kv capacitance) between the drain and gate of the switching power field effect tube.

Place a small RC damper at dc output if space permits.

Do not place the AC socket against the heat sink of the primary switch tube.