Switch mode power supply EMI technology Transmission channel
EMI suppression technology for switching power supply
(1) Reduce dv/dt and di/dt (decrease their peak and slow down their slope)
(2) Reasonable application of varistors to reduce surge voltage
(3) Damping network suppresses overshoot
(4) Using diodes with soft recovery characteristics to reduce high-frequency EMI
(5) Active power factor correction and other harmonic correction techniques
(6) Adopting a well-designed power line filter
(7) Reasonable grounding treatment
(8) Effective shielding measures
(9) Reasonable pCB design
Switching power supply EMI technology interference source
(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) 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 aforementioned interference source, and the quality of pCB directly corresponds to the effectiveness of suppressing the aforementioned EMI source.
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 peak voltage when power switches are turned off. Therefore, controlling the leakage inductance has become the primary problem to solve the EMI caused by high-frequency transformers.
reduce
Two entry points for leakage inductance of small high-frequency transformers: 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 edge turns, and reducing the turns will significantly reduce the leakage inductance.
(2) Reduce the insulation layer between windings. There is now an insulation layer called "golden film" with a thickness of 20-100um, and a pulse breakdown voltage of several thousand volts.
(3) Increase the coupling degree between windings and reduce leakage inductance.
