“As an energy conversion device working in the switching state, the voltage and current change rates of the switching power supply are very high, and the resulting interference intensity is relatively large; the interference sources are mainly concentrated during the power switching period and the radiator and high-level transformer connected to it. The location of the circuit interference source is relatively clear; the switching frequency is not high (from tens of kilohertz and several megahertz), and the main forms of interference are conducted interference and near-field interference; while the printed circuit board (PCB) traces are usually wired manually. It has greater randomness, which increases the difficulty of extracting PCB distribution parameters and estimating near-field interference.

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As an energy conversion device working in the switching state, the voltage and current change rates of the switching power supply are very high, and the resulting interference intensity is relatively large; the interference sources are mainly concentrated during the power switching period and the radiator and high-level transformer connected to it. The location of the circuit interference source is relatively clear; the switching frequency is not high (from tens of kilohertz and several megahertz), and the main forms of interference are conducted interference and near-field interference; while the printed circuit board (PCB) traces are usually wired manually. It has greater randomness, which increases the difficulty of extracting PCB distribution parameters and estimating near-field interference.

Within 1MHz

The differential mode interference is the main problem, and it can be solved by increasing the X capacitor.

1MHz～5MHz

Differential mode and common mode mixing, use the input end and a series of X capacitors to filter out the differential mode interference and analyze which kind of interference exceeds the standard and solve it.

5MHz～10MHz

The common touch interference is mainly used, and the method of suppressing common touch is adopted.

10MHz～25MHz

For the case grounded, using a magnet around the ground wire for 2 turns will have a greater attenuation of the interference above 10MHz.

25～30MHz

It can be used to increase the Y capacitor to the ground, wrap copper skin on the outside of the transformer, change the PCBLAYOUT, connect a small magnetic ring with double-wire winding in front of the output line, at least 10 turns, and connect an RC filter at both ends of the output rectifier tube.

30MHz～50MHz

It is generally caused by the high-speed turn-on and turn-off of the MOS tube, which can be solved by increasing the MOS drive resistance. The RCD buffer circuit uses a 1N4007 slow tube, and the VCC power supply voltage is solved by a 1N4007 slow tube.

100～200MHz

It is generally caused by the reverse recovery current of the output rectifier, and magnetic beads can be stringed on the rectifier.

100MHz～200MHz

Most of them are due to PFCMOSFET and PFC diode. Now MOSFET and PFC diode string magnetic beads are effective. The horizontal direction can basically solve the problem, but the vertical direction is very helpless.

The radiation of the switching power supply generally only affects the frequency band below 100MHz. A corresponding absorption circuit can also be added to the MOS diode, but the efficiency will be reduced.

Measures to prevent EMI when designing switching power supplies:

1. Minimize the area of ​​PCB copper foil of noise circuit nodes; such as the drain, collector of the switch tube, the node of the primary and secondary windings, etc.
2. Keep the input and output terminals away from noise components, such as transformer wire packages, transformer cores, heat sinks of switch tubes, etc.
3. Keep noise components (such as unshielded transformer wire wraps, unshielded transformer cores, switch tubes, etc.) away from the edge of the enclosure, because the edge of the enclosure is likely to be close to the outside ground wire under normal operation.
4. If the transformer is not shielded by electric field, 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 circuit, auxiliary rectifier, etc.
6. Do not mix the gate (base) drive feedback loop with the primary switch circuit or auxiliary rectifier circuit.
7. Adjust and optimize the damping resistance 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 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 high frequency output EMI filter close to the output wire terminals.
13. Keep a certain distance between the copper foil of the PCB on the opposite side of the EMI filter and the component body.
14. Put some resistors on the lines of the rectifier for the auxiliary coil.
15. Connect a damping resistor in parallel with the magnet coil.
16. Connect a damping resistor in parallel at both ends of the output RF filter.
17. It is allowed to put a 1nF/500V ceramic capacitor or a series of resistors in the PCB design, which is connected across the primary static end of the transformer and the auxiliary winding.
18. Keep the EMI filter away from the power transformer, especially at the end of the wrap.
19. If the PCB area is sufficient, the pins for shielding windings and the position for placing RC dampers can be left on the PCB. The RC dampers can be connected across both ends of the shielding windings.
20. If space permits, place a small radial lead capacitor (Miller, 10 picofarads/1kV) between the drain and gate of the switching power FET.
21. Put a small RC damper on the DC output if space permits.
22. Do not place the AC socket against the heat sink of the primary switch tube.