Switch mode power supply EMI design experience
1. EMI source of switching power supply
The EMI interference sources of switching power supplies are concentrated in power switches, rectifier diodes, high-frequency transformers, etc. The external environment's interference to switching power supplies mainly comes from the jitter of the power grid, lightning strikes, external radiation, etc.
(1) Power switch tube
The power switch tube works in a state of On-Off rapid cycle conversion, and both dv/dt and di/dt are changing rapidly. Therefore, the power switch tube 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 concentrated in the rapid cyclic transformation of di/dt corresponding to leakage inductance. Therefore, high-frequency transformers are an important interference source of magnetic field coupling.
(3) Rectifier diode
The EMI source of the rectifier diode is concentrated in the reverse recovery characteristics. The discontinuity point of the reverse recovery current will produce high dv/dt in the inductance (lead inductance, stray inductance, etc.), resulting in strong electromagnetic interference.
(4)PCB
To be precise, PCB is the coupling channel for the above-mentioned interference sources. The quality of PCB directly corresponds to the suppression of the above-mentioned EMI sources.
2. Classification of switching power supply EMI transmission channels
(1) Transmission channel for conductive interference
(1) Capacitive coupling
(2) Inductive coupling
(3) Resistive coupling
a. Resistive conduction coupling caused by the internal resistance of the public power supply
b. Resistive conduction coupling caused by the impedance of the common ground wire
c. Resistive conduction coupling caused by common line impedance
(2) Transmission channels for radiation interference
(1) In switching power supplies, components and wires that can constitute radiation interference sources can be assumed to be antennas, and the electric dipole and magnetic dipole theories can be used for analysis; diodes, capacitors, and power switching tubes can be assumed to be Electric dipole, inductor coil can be assumed to be a magnetic dipole;
(2) When there is no shielding body, the electromagnetic wave transmission channel generated by electric dipoles and magnetic dipoles is air (can be assumed to be free space);
(3) When there is a shield, consider the gaps and holes in the shield and analyze and process it according to the mathematical model of the leakage field.
3. 9 major measures to suppress EMI in switching power supplies
In switching power supplies, sudden changes in voltage and current, that is, high dv/dt and di/dt, are the main causes of EMI. The technical measures to implement EMC design of switching power supply are mainly based on the following two points:
(1) Minimize the interference sources generated by the power supply itself, use interference suppression methods or produce components and circuits with less interference, and carry out reasonable layout;
(2) Suppress the EMI of the power supply and improve the EMS of the power supply through grounding, filtering, shielding and other technologies.
Speaking separately, the nine major measures are:
(1) Reduce dv/dt and di/dt (reduce their peak value and slow down their slope)
(2) Reasonable application of varistor to reduce surge voltage
(3) Damping network suppresses overshoot
(4) Use diodes with soft recovery characteristics to reduce high-frequency EMI
(5) Active power factor correction and other harmonic correction technologies
(6) Use a properly designed power line filter
(7) Reasonable grounding treatment
(8) Effective shielding measures
(9) Reasonable PCB design
