SMPS generates electronics noise due to high dv/dt and high di/dt transition during the switching. The high frequency SMPS typically generates three types of EMI: low frequency noise (<30MHz) as harmonics of the switching frequency; mid-frequency broadband noise (30-300MHz) from switching node voltage ringing and high frequency pulsating current; high frequency noise from reverse recovery. The low frequency EMI is easy to be filtered by the LC filters. The aim of this design guideline is to minimize mid-to-high frequency radiation EMI which is most difficult to deal with, and pass the vert strict CISPR25 level5 EMI test.

In this application note, the root cause of the EMI noise is analyzed. Then a list of design guideline for minimizing the EMI noise is presented. The guideline contains the layout level tips, which involves components placement, layer stackup, ground plane routing. The guideline also contains circuit level design, which involves the frequency dithering, snubber circuit and the filter circuit design.