Solution to EMI interference source interference on switching power supply

Generally speaking, interference from external radiation, lightning strikes, or grid jitters on the components of power switches such as rectifier diodes, high-frequency transformers, power switches, and other external environments is the main manifestation of EMI interference sources in switching power supplies.

Firstly, introduce the transmission channels of radiation interference

(1) In a switching power supply, the components and wires that can form a radiation interference source can be assumed as antennas, which can be analyzed using the theories of electric and magnetic dipoles; Diodes, capacitors, and power switches can be assumed to be electric dipoles, while inductance coils can be assumed to be magnetic dipoles;

(2) When there is no shield, the transmission channel of electromagnetic waves generated by electric dipoles and magnetic dipoles is air (which can be assumed to be free space);

(3) When there is a shielding body, consider the gaps and holes in the shielding body and analyze and process according to the mathematical model of the leakage field.

Secondly, it is the transmission channel for conducting interference

(1) Capacitive coupling

(2) Inductive coupling

(3) Resistive coupling

a. Resistance conduction coupling generated by internal resistance of public power supply

b. Resistance conduction coupling generated by impedance of common ground wire

c. Resistance conduction coupling generated by impedance of public lines

The following are the suppression schemes related to EMI interference sources:

1. Shielding of high-frequency transformers

To prevent interference from the leakage magnetic field of high-frequency transformers on surrounding circuits, shielding tape can be used to shield the leakage magnetic field of high-frequency transformers. The shielding tape is generally made of copper foil, wound around the outside of the transformer and grounded. The shielding tape is a short-circuit ring relative to the leakage magnetic field, thereby suppressing the leakage of a larger range of the leakage magnetic field.

High frequency transformers may experience relative displacement between magnetic cores and windings, resulting in noise (whistling, vibration) during operation. To prevent this noise, reinforcement measures need to be taken for the transformer:

(1) Use epoxy resin to bond the three contact surfaces of magnetic cores (such as EE and EI magnetic cores) to suppress the generation of relative displacement;

(2) Bonding the magnetic core with "glass beads" adhesive provides better results.

Separately speaking, there are 9 measures for EMI suppression in switching power supplies:

(1) Reasonable PCB design

(2) Reasonable application of varistors to reduce surge voltage

(3) Reduce dv/dt and di/dt (reduce their peak and slope)

(4) Damping network to suppress overshoot

(5) Adopting a reasonably designed power line filter

(6) Diodes with soft recovery characteristics to reduce high-frequency EMI

(7) Active power factor correction and other harmonic correction technologies

(8) Effective shielding measures

(9) Reasonable grounding treatment