Solutions to EMI interference sources affecting switching power supplies
Generally speaking, external radiation, lightning strikes, or grid vibrations are the main manifestations of EMI interference sources in switch power supplies, which can interfere with related components such as rectifier diodes, high-frequency transformers, power switching tubes, and other external environmental factors.
Firstly, introduce the transmission channels of radiation interference
(1) In switch mode power supplies, the components and wires that can form radiation interference sources can be assumed to be antennas, and analyzed using electric dipole and magnetic dipole theories; Diodes, capacitors, and power switching devices can be assumed to be electric dipoles, while inductive coils can be assumed to be magnetic dipoles;
(2) When there is no shielding body, 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 of the shielding body, and analyze and process them 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) Resistance coupling
a. Resistance conduction coupling caused by internal resistance of public power supply
b. Resistance conduction coupling caused by impedance of public ground wire
c. Resistance conduction coupling caused by impedance of public lines
The following are suppression schemes related to EMI interference sources:
1. Shielding of high-frequency transformers
To prevent the leakage magnetic field of high-frequency transformers from interfering with surrounding circuits, shielding tapes can be used to shield the leakage magnetic field of high-frequency transformers. Shielding tape is generally made of copper foil, wrapped around the outside of the transformer and grounded. The shielding tape is a short-circuit loop relative to the leakage magnetic field, thereby suppressing the leakage of a larger range of leakage magnetic field.
High frequency transformers experience relative displacement between the 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 cores) to suppress relative displacement;
(2) Using "Glass beads" adhesive to bond the magnetic core yields better results.
