Switching power supplies, as energy conversion devices operating in a switching state, experience high voltage and current change rates, which result in significant interference. The primary sources of this interference are located at the power switch, heat sink, and high-voltage transformer. These components are clearly identifiable in terms of their positions on the circuit board. The switching frequency typically ranges from tens of kilohertz to several megahertz, with the main forms of interference being 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 main interference is differential mode, and increasing X capacitors can help mitigate it. Between 1 MHz and 5 MHz, both differential and common-mode interference occur. Using X capacitors at the input and filtering out differential interference can help address the issue. Above 5 MHz, common-mode interference becomes dominant. Grounding techniques, such as using a magnetic shield with two turns around the ground wire, can significantly reduce interference above 10 MHz. For frequencies between 25–30 MHz, larger Y capacitors and copper shielding around the transformer can be effective. Adjusting the PCB layout, adding a small magnetic ring with at least 10 turns before the output line, and placing an RC filter at the output rectifier can also help.
Between 30–50 MHz, high-speed turn-off of MOSFETs is often the cause. Increasing the gate resistance, using an RCD snubber circuit with a slow 1N4007 diode, and replacing the VCC supply with a similar diode can resolve the issue. For 100–200 MHz, the reverse recovery current of the output rectifier is usually the culprit. Adding magnetic beads to the rectifier can help suppress this interference.
Most PFC MOSFETs and PFC diodes operate within the 100–200 MHz range. While horizontal placement of these components can help, vertical placement offers little improvement. Radiation from switching power supplies generally affects frequencies below 100 MHz. Adding an absorption loop to the MOSFET or diode can further reduce interference, though it may slightly lower 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 switching transistor, and the node between the primary and secondary windings.
2. Keep input and output terminals away from noise sources like transformers, cores, and heat sinks.
3. Position noise components, such as unshielded transformers or switches, away from the case edge, as it may come into contact with the ground wire during normal operation.
4. If the transformer lacks electric field shielding, ensure the shield and heat sink are kept 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 the gate feedback loop with the primary or auxiliary rectifier circuits.
7. Optimize damping resistance to prevent ringing during the switch’s dead time.
8. Prevent saturation of EMI filter inductors.
9. Keep corner nodes and secondary circuits away from the primary shield or switch heat sink.
10. Ensure the primary swing node and component body stay clear of shields or heat sinks.
11. Place the high-frequency EMI filter close to the input cable or connector.
12. Locate the high-frequency EMI filter near the output wire terminal.
13. Maintain distance between the opposite PCB copper foil and component bodies.
14. Add resistors on the auxiliary winding rectifier line.
15. Connect a damping resistor in parallel with the magnet coil.
16. Add a damping resistor across the output RF filter.
17. During PCB design, connect 1nF/500V ceramic capacitors or series resistors between the primary static and auxiliary windings.
18. Keep the EMI filter away from the power transformer, especially avoiding placement at the package end.
19. If space allows, leave room for the shield winding and RC damper on the PCB, and connect 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. Add a small RC damper at the DC output if possible.
22. Avoid connecting the AC socket to the primary switch’s heat sink.
Tower Type Line Interactive UPS
Line Interactive UPS
Shenzhen Unitronic Power System Co., Ltd , https://www.unitronicpower.com