EMI suppression of switching power supply for different frequency bands

In the design of switching power supply, EMI is definitely a very important part. Many developers struggle with the problem of electromagnetic interference in order to successfully pass the test. However, the EMI in the circuit is very complex, and the frequency of interference is not the same. Some of the lower disturbances require only a few minor adjustments, while some of the larger ones require a multifaceted approach. In this paper, the EMI suppression method of switching power supply for different frequency bands is introduced.

1 MHZ

Within 1MHZ, mainly due to differential mode interference. The solution is as follows:

The voltage increases X capacity;

Adding differential mode inductance;

The smart power supply can be processed by PI filter (it is recommended that the electrolytic capacitors close to the transformer can be selected to be larger).

1 MHZ - 5 MHZ

1mhz-5mhz, differential mode common mode mix, a series of X capacitors connected in parallel at the input end are used to filter out the differential interference and analyze which kind of interference is out of tolerance and solve the problem:

The power capacity of X can be adjusted by the interference of difference mode, and the inductors of difference mode can be added to adjust the inductance of difference mode.

The common mode inductance can be added to exceed the standard of common mode interference, and reasonable inductance can be chosen to inhibit.

The device can also change the characteristics of the rectifier diode, a pair of fast diodes such as FR107 and a pair of ordinary rectifier diodes 1N4007.

More than 5 MHZ

The interference of common touch is mainly over 5M, and the method of suppressing common touch is adopted. For the case grounded, a series of magnetic ring around 2-3 turns on the ground wire will attenuate the interference above 10MHZ significantly. Can be close to the transformer of iron core adhesive copper foil, copper foil closed ring. The size of the shunt capacitance of the absorber and primary large circuits for processing the back end output rectifier.

20-30 MHZ

The switch can adjust the capacitance of ground Y2 or change the position of Y2 capacitance.

The Y1 capacitance position and parameter values between the first and second sides are adjusted by the margin.

Copper foil bread outside the transformer; The innermost layer of the transformer is shielded. Adjust the arrangement of the windings of the transformer.

Quadrature changed PCBLAYOUT;

Small common mode inductance wound with a double line in front of the main output line;

A RC filter is parallel at both ends of the output rectifier tube and reasonable parameters are adjusted.

In the moment, BEADCORE is added between the transformer and MOSFET.

Apply a small capacitor to the input voltage pin of the transformer.

Current can be driven by a large MOS resistor.