Switching power supplies, as energy conversion devices operating in a switching state, exhibit high voltage and current change rates, resulting in significant interference. The primary sources of this interference are concentrated around the power switch, heat sink, and related high-level transformer. The locations of these interference sources are clearly defined within the circuit. With a relatively low switching frequency (ranging from tens of kilohertz to several megahertz), the main forms of interference are conducted and near-field interference. Additionally, printed circuit board (PCB) traces are often manually routed, introducing randomness that complicates the extraction of PCB distribution parameters and the estimation of near-field interference.
Within 1 MHz, the dominant interference is differential mode, which can be effectively mitigated by increasing X capacitors. Between 1 MHz and 5 MHz, both differential and common-mode interference coexist. At this stage, input filtering using X capacitors helps reduce differential interference, while identifying and addressing the specific type of interference exceeding the standard becomes essential. Above 5 MHz, common-mode interference dominates. Grounding techniques such as wrapping the ground with two turns of magnetic material can significantly suppress interference above 10 MHz. For frequencies between 25–30 MHz, larger Y capacitors, copper shielding around the transformer, optimized PCB layout, and small magnetic rings with at least 10 turns placed before the output line, along with RC filters at the output rectifier, can help reduce noise.
Between 30–50 MHz, high-speed turn-off of MOS transistors typically causes interference. Increasing the MOS drive resistance or using an RCD snubber circuit with a slow 1N4007 diode can help mitigate this issue. For 100–200 MHz, interference is often due to the reverse recovery current of the output rectifier, and placing magnetic beads on the rectifier can be effective. Most PFC MOSFETs and PFC diodes operate in this frequency range, and stringing them appropriately can solve most issues, though vertical direction problems remain challenging.
Radiation from switching power supplies usually affects frequencies below 100 MHz. Adding an absorption loop to the MOS and diode may help, but it could reduce efficiency.
**EMI Prevention Measures During Switching Power Supply Design:**
1. Minimize the copper area on the PCB for noise-sensitive nodes, such as the drain and collector of the switch transistor, and the primary and secondary winding nodes.
2. Keep input and output terminals away from noise components like transformers, cores, and heat sinks.
3. Ensure that unshielded components like transformers and switches are kept away from the case edge, as it is close to the ground during normal operation.
4. If the transformer lacks electric field shielding, keep the shield and heat sink away from the transformer.
5. Reduce the area of current loops, including the secondary rectifier, primary switching device, gate drive lines, and auxiliary rectifier.
6. Avoid mixing gate drive feedback loops with primary or auxiliary rectifier circuits.
7. Optimize damping resistance to prevent ringing during the dead time of the switch.
8. Prevent saturation of EMI filter inductors.
9. Keep corner node components and secondary circuits away from the primary shield or switch heat sink.
10. Keep the swing node of the primary circuit and component bodies away from shields or heat sinks.
11. Place the high-frequency EMI filter close to the input cable or connector.
12. Position the high-frequency EMI filter near the output wire terminal.
13. Maintain a distance between the PCB copper foil opposite the EMI filter and component bodies.
14. Add resistors on the auxiliary winding rectifier line.
15. Connect a damping resistor in parallel with the magnet coil.
16. Place a damping resistor across the output RF filter.
17. During PCB design, connect 1nF/500V ceramic capacitors or a series of resistors between the static and auxiliary windings of the primary transformer.
18. Keep the EMI filter away from the power transformer, especially avoiding placement at the end of the package.
19. If space allows, leave room for the shield winding and RC damper on the PCB, connecting the RC damper across the shield winding.
20. If space permits, place a small radial lead capacitor (Miller capacitor, 10 pF / 1 kV) between the drain and gate of the FET.
21. If possible, add a small RC damper at the DC output.
22. Avoid connecting the AC socket to the primary switch's heat sink.
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