EMI suppression of switching power supply
(1) Reduce dv/dt and di/dt (reduce its peak value and slow down its slope)
(2) Reasonable application of varistor to reduce surge voltage.
(3) Damping network suppresses overshoot.
(4) Diodes with soft recovery characteristics are used to reduce high-frequency EMI.
(5) Active power factor correction and other harmonic correction technologies.
(6) Adopt a reasonably designed power line filter.
(7) Reasonable grounding treatment
(8) Effective shielding measures
(9) Reasonable pCB design
EMI interference source of switching power supply
(1) Power switch tube
The power switch tube works in the state of On-Off fast cycle switching, and both dv/dt and di/dt are changing rapidly. Therefore, the power switch tube is the main interference source of both electric field coupling and magnetic field coupling.
（2）
The EMI source of high-frequency transformer is mainly reflected in the fast cyclic transformation of di/dt corresponding to leakage inductance, so high-frequency transformer is an important interference source of magnetic field coupling.
(3) Rectifier diode
The EMI source of rectifier diode is mainly reflected in the reverse recovery characteristics, and the intermittent point of reverse recovery current will produce high dv/dt in inductance (lead inductance, stray inductance, etc.), which will lead to strong electromagnetic interference.
（4）pCB
Accurately speaking, pCB is the coupling channel of the above-mentioned interference sources, and the quality of pCB directly corresponds to the quality of EMI suppression.
Control of leakage inductance of high frequency transformer
The leakage inductance of high-frequency transformer is one of the important reasons for the peak voltage of power switch tube, so controlling the leakage inductance has become the primary problem to solve the EMI caused by high-frequency transformer.
Two breakthrough points to reduce leakage inductance of high frequency transformer: electrical design and process design!
(1) Select proper magnetic core to reduce leakage inductance. The leakage inductance is proportional to the square of the number of turns in the primary side, and reducing the number of turns will significantly reduce the leakage inductance.
(2) Reduce the insulation layer between windings. Now there is an insulating layer called "gold thin film" with a thickness of 20 ~ 100 μ m and a pulse breakdown voltage of several thousand volts.
(3) Increase the coupling between windings and reduce leakage inductance.
