EMI Design Experience in Switching Power Supplies
1. EMI source for switching power supply
The EMI interference sources of switching power supply are mainly reflected in power switch tubes, rectifier diodes, high-frequency transformers, etc. The interference of external environment on switching power supply mainly comes from grid shaking, lightning strikes, external radiation, etc.
(1) Power switch tube
The power switch operates in an On Off fast cycle transition state, with both dv/dt and di/dt rapidly changing. Therefore, the power switch is not only the main interference source of electric field coupling, but also the main interference source of magnetic field coupling.
(2) High-frequency transformer
The EMI source of high-frequency transformers is mainly reflected in the di/dt fast cyclic transformation corresponding to leakage inductance, so high-frequency transformers are important interference sources for magnetic field coupling.
(3) Rectifier diode
The EMI source of rectifier diodes is mainly reflected in the reverse recovery characteristics. The intermittent points of reverse recovery current will generate high dv/dt in inductors (lead inductance, stray inductance, etc.), leading to strong electromagnetic interference.
(4) PCB
To be precise, the PCB is the coupling channel of the aforementioned interference sources, and the quality of the PCB directly corresponds to the quality of suppressing the aforementioned EMI sources.
2. Classification of EMI transmission channels for switching power supplies
（1） 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
（2） Transmission channel 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
