Introduction: The EMI interference source of the switching power supply is mainly reflected in the power switching tube, rectifier diode, high-frequency transformer, etc. The interference of the external environment to the switching power supply mainly comes from the power grid jitter, lightning strike, external radiation and so on.

1. EMI source of switching power supply
    The EMI interference source of the switching power supply is mainly reflected in the power switching tube, rectifier diode, high-frequency transformer, etc. The interference of the external environment to the switching power supply mainly comes from the power grid jitter, lightning strike, external radiation and so on.
    (1.1) Power switch tube
    The power switch tube works in the state of On-Off rapid cycle conversion, dv/dt and di/dt are sharply changing, therefore, the power switch tube is not only the main source of electric field coupling, but also the main source of magnetic field coupling interference.
    (1.2) High-frequency transformer
    The EMI source of the high-frequency transformer is mainly reflected in the di/dt rapid cycle transformation corresponding to the leakage induction, so the high-frequency transformer is an important source of interference in the field coupling.
    (1.3) Rectifier diode
    The EMI source of the rectifier diode is mainly reflected in the reverse recovery characteristic, and the break point of the reverse recovery current will produce high dv/dt in the inductance (lead inductance, stray inductance, etc.), resulting in strong electromagnetic interference.
    (1.4) PCB
    To be precise, PCB is the coupling channel of the above interference source, and the advantages and disadvantages of PCB directly correspond to the good or bad suppression of the above EMI source.

2, switching power supply EMI transmission channel classification
    (2.1) Transmission channels for conducting interference
    (2.1.1) Capacitive coupling
    (2.1.2) Inductive coupling
    (2.1.3) Resistance coupling
    2.1.3a. Resistive conduction coupling resulting from the internal resistance of the common power supply
    2.1.3b. Resistive conduction coupling resulting from the impedance of the common ground
    2.1.3c. Resistive conduction coupling resulting from common line impedance
    (2.2) Transmission channels of radiation interference
    (2.2.1) In the switching power supply, the components and wires that can constitute the source of radiation interference can be assumed as antennas, so that the electric dipole and magnetic dipole theory can be used for analysis; Diodes, capacitors and power switching tubes  can be assumed to be electric dipoles, and inductors can be assumed to be magnetic dipoles.
    (2.2.2) When there is no shield, the electromagnetic wave generated by the electric dipole and magnetic dipole is transmitted through air (which can be assumed to be free space);
    (2.2.3) When there is a shield, the gaps and holes of the shield should be considered and analyzed according to the mathematical model of the leakage field.

3, switching power supply EMI suppression 9 measures
    In switching power supplies, sudden changes in voltage and current, i.e. high dv/dt and di/dt, are the main cause of their EMI. EMC design technical measures for switching power supply are mainly based on the following two points:
    (3.1) Minimize the interference source generated by the power supply itself, use interference suppression methods or produce less interference components and circuits, and carry out a reasonable layout;
    (3.2) Suppress the EMI of the power supply and improve the EMS of the power supply through grounding, filtering, shielding and other technologies.
    Separately, the nine major measures are:
    (3.2.1) Reduce dv/dt and di/dt (reduce their peaks and slopes)
    (3.2.2) Reasonable application of varistors to reduce surge voltage
    (3.2.3) Damping network suppresses overshoot
    (3.2.4) Use of soft recovery characteristic diodes to reduce high band EMI
    (3.2.5) Active power factor correction, and other harmonic correction techniques
    (3.2.6) Adopt a reasonably designed power line filter
    (3.2.7) Proper grounding treatment
    (3.2.8) Effective shielding measures
    (3.2.9) Reasonable PCB design

4, high-frequency transformer leakage control
    The leakage of high frequency transformer is one of the important reasons for the peak voltage of power switching tube. Therefore, the control of leakage has become the first problem to solve the EMI caused by high frequency transformer.
    Reduce the leakage of high-frequency transformer two entry points: electrical design, process design!
    (4.1) Select the appropriate magnetic core to reduce leakage. Leakage induction is proportional to the square of the number of turns on the original side, and reducing the number of turns will significantly reduce leakage induction.
    (4.2) Reduce the insulation layer between windings. Now there is a kind of insulation layer called "gold film", the thickness of 20 ~ 100um, the pulse breakdown voltage can reach several thousand volts.
    (4.3) Increase the coupling degree between windings and reduce leakage.

5, high-frequency transformer shielding
    In order to prevent the magnetic leakage of the high-frequency transformer from interfering with the surrounding circuit, the magnetic leakage field of the high-frequency transformer can be shielded by a screen tape. The shielding belt is generally made of copper foil, wrapped around the outside of the transformer, and is grounded. The shielding belt is a short circuit ring relative to the leakage magnetic field, so as to inhibit the leakage of the leakage magnetic field.
    In high-frequency transformers, relative displacement occurs between magnetic cores and windings, resulting in noise (whistling, vibration) in the operation of high-frequency transformers. In order to prevent this noise, it is necessary to take reinforcement measures for the transformer:
    (5.1) The three contact surfaces of the magnetic core (such as EE and EI magnetic core) are bonded with epoxy resin to inhibit the generation of relative displacement;
    (5.2) Using "Glass beads" adhesive to bond the magnetic core, the effect is better.