What is the transmission channel of EMI technology in switching power supply
（1） Transmission channel for conducted 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
EMI Technology Suppression for Switching Power Supplies
(1) Reduce dv/dt and di/dt (reduce their peak and slope)
(2) Reasonable application of varistors to reduce surge voltage
(3) Damping network to suppress overshoot
(4) Diodes with soft recovery characteristics to reduce high-frequency EMI
(5) Active power factor correction and other harmonic correction technologies
(6) Adopting a reasonably designed power line filter
(7) Reasonable grounding treatment
(8) Effective shielding measures
(9) Reasonable pCB design
Switching Power EMI Technology Interference Sources
(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) 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) 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, pCB is the coupling channel of the aforementioned interference sources, and the quality of pCB directly corresponds to the quality of EMI source suppression.
Control of Leakage Inductance in High Frequency Transformers
The leakage inductance of high-frequency transformers is one of the important reasons for the generation of power switch off peak voltage. Therefore, controlling leakage inductance has become the primary problem to solve the EMI caused by high-frequency transformers.
Two entry points for reducing high-frequency transformer leakage inductance: electrical design and process design!
(1) Choose a suitable magnetic core to reduce leakage inductance. The leakage inductance is proportional to the square of the original side turns, and reducing the number of turns will significantly reduce the leakage inductance.
(2) Reduce the insulation layer between windings. There is now an insulation layer called "gold thin film", with a thickness of 20-100um and a pulse breakdown voltage of several thousand volts.
(3) Increase the coupling between windings and reduce leakage inductance.
