Switching power supply EMI rectification frequency band interference causes and suppression methods

In the rectification of switching power supply EMI, the causes and suppression methods of interference in different frequency bands are as follows:

Within 1MHZ - mainly differential mode interference

1. Increase X capacitance;

2. Add differential mode inductance;

3. Small power supply can be processed by PI type filter (it is recommended to use larger electrolytic capacitors close to the transformer).

1MHZ---5MHZ---differential mode and common mode mixed

A series of X capacitors are connected in parallel at the input end to filter out differential interference and analyze which interference exceeds the standard and solve it.

1. If the differential mode interference exceeds the standard, you can adjust the X capacitance, add a differential mode inductor, and adjust the differential mode inductance;

2. If the common mode interference exceeds the standard, add common mode inductor and select reasonable inductance to suppress it;

3. The rectifier diode characteristics can also be changed to handle a pair of fast diodes such as FR107 and a pair of ordinary rectifier diodes 1N4007.

5M---Above are mainly common mode interference, and the method of suppressing common mode is adopted.

For the grounded shell, winding a magnetic ring 2-3 times on the ground wire will have a greater attenuation effect on interference above 10MHZ;

You can choose to stick copper foil close to the transformer core, or close the copper foil loop.

Deal with the size of the absorption circuit of the back-end output rectifier tube and the parallel capacitor of the primary large circuit.

For 20--30MHZ,

1. For a type of product, you can adjust the Y2 capacitance to ground or change the position of the Y2 capacitor;

2. Adjust the position and parameter value of the Y1 capacitor between the primary and secondary sides;

3. Wrap the outside of the transformer with copper foil; add a shielding layer to the innermost layer of the transformer; adjust the arrangement of the transformer windings.

4. Change PCB LAYOUT;

5. Connect a small common-mode inductor with two wires in parallel in front of the output line;

6. Connect RC filter in parallel at both ends of the output rectifier tube and adjust reasonable parameters;

7. Add BEAD CORE between transformer and MOSFET;

8. Add a small capacitor to the input voltage leg of the transformer.

9. You can increase the MOS drive resistance.

30---50MHZ is generally caused by the high-speed switching on and off of the MOS tube

1. You can increase the MOS drive resistance;

2. The RCD snubber circuit uses 1N4007 slow tube;

3. The VCC supply voltage is solved by 1N4007 slow tube;

4. Or connect a small common-mode inductor with two wires in parallel to the front end of the output line;

5. Connect a small snubber circuit in parallel with the DS pin of the MOSFET;

6. Add BEAD CORE between transformer and MOSFET;

7. Add a small capacitor to the input voltage leg of the transformer;

8. When laying out the PCB core, the circuit loop composed of large electrolytic capacitors, transformers, and MOS should be as small as possible;

9. The circuit loop composed of the transformer, output diode, and output smoothing electrolytic capacitor should be as small as possible.

50---100MHZ is generally caused by the reverse recovery current of the output rectifier tube

1. Magnetic beads can be strung on the rectifier tube;

2. Adjust the absorption circuit parameters of the output rectifier tube;

3. The impedance of the Y capacitor branch across the primary and secondary sides can be changed, such as adding a BEAD CORE to the PIN pin or connecting an appropriate resistor in series;

4. You can also change the MOSFET and the radiation of the output rectifier diode into space (such as clamping the MOSFET with an iron clamp; clamping the DIODE with an iron clamp, changing the grounding point of the radiator).

5. Add shielding copper foil to suppress radiation into space.

Switching power supplies above 200MHZ have basically very low radiation and can generally pass EMI standards

Additional Notes:

The first layer of the high-frequency transformer of the switching power supply is generally a shielded layer, which is not mentioned above.

The switching power supply is a high-frequency product, and the layout of PCB components is very sensitive to EMI. Please pay close attention to this point.

If the switching power supply has a mechanical housing, the structure of the housing has a great influence on the radiation. Please pay close attention to this point.

The parameters of the main switch tube and main diode produced by different manufacturers are somewhat different, which has a certain impact on EMC.