Inventory of measures to prevent EMI when designing switch mode power supplies
1. Minimize the PCB copper foil area of noise circuit nodes to the greatest extent possible; Such as the drain and collector of the switching transistor, the nodes of the primary winding, etc.
2. Keep the input and output terminals away from noisy components such as transformer wire packages, transformer cores, heat sinks for switch tubes, and so on.
3. Keep noise components (such as unshielded transformer wire packages, unshielded transformer cores, and switch tubes, etc.) away from the edge of the casing, as the casing edge 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 sink away from the transformer.
5. Try to minimize the area of the following current loops: secondary (output) rectifier, primary switching power device, gate (base) drive circuit, auxiliary rectifier.
6. Do not drive the feedback loop of the gate (base) and the primary switch circuit or auxiliary rectifier circuit
The roads are mixed together.
7. Adjust and optimize the damping resistor value to prevent ringing during the dead time of the switch.
8. Prevent EMI filter inductance saturation.
9. Keep the turning 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 shields or heat sinks.
11. Place the high-frequency input EMI filter close to the input cable or connector end.
12. Keep the EMI filter with high-frequency output close to the output wire terminal.
13. Keep 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. During PCB design, it is allowed to accommodate 1nf/500V ceramic capacitors or a series of resistors, which can be connected across the primary static end and auxiliary winding of the transformer.
18. Keep the EMI filter away from the power transformer; Especially avoid positioning at the end of the package.
19. If the PCB area is sufficient, the pins for placing the shielding winding and the position for placing the RC damper can be left on the PCB, and the RC damper can be connected across the two ends of the shielding winding.
If space permits, place a small radial lead capacitor (Miller capacitor, 10 picofarads/1 kV capacitor) between the drain and gate of the switching power field-effect transistor.
If space permits, place a small RC damper at the DC output end.
22. Do not place the AC socket against the heat sink of the primary switch tube.
