EMI design experience for switching power supplies
1. EMI source of switch mode power supply
The EMI interference sources of switching power supplies are mainly reflected in power switching tubes, rectifier diodes, high-frequency transformers, etc. The external environment's interference on switching power supplies mainly comes from grid shaking, lightning strikes, external radiation, etc.
(1) Power switch tube
The power switch operates in an On Off fast cycling 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 rapid cyclic transformation of di/dt corresponding to leakage inductance, making high-frequency transformers an important interference source for magnetic field coupling.
(3) Rectifier diode
The EMI source of rectifier diodes is mainly reflected in the reverse recovery characteristics. The intermittent point of reverse recovery current will generate high dv/dt in inductance (lead inductance, stray inductance, etc.), resulting in strong electromagnetic interference.
(4)PCB
More precisely, PCB is the coupling channel of the above-mentioned interference sources, and the quality of PCB directly corresponds to the effectiveness of suppressing the above-mentioned 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) 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
(2) Transmission channel 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.
