Resonance of Ferrite Bead

[hossein_atp]

Hello Friends
I have used LM2575 as my power supply for 3.3v and I have added Ferrite beads of MURATA (BLM18PG300SN1) in my board for noise suppression.
Approximately 80pcs of ferrite beads are used the board and I put one 100nf capacitor after each ferrite bead , each pair of ferrite bead and capacitor is put for one IC .

My problem is resonance of these ferrite beads , When I remove the ferrite beads there is no resonace but when they are,I have 100mv peak-to-peak resonance on my supply 3.3v.
How can I remove these ripples? or it is normal?

 

[jiripolivka]

Ferrite beads come in various types designed for the specific uses. Your observation indicates that those you selected resonate with the 100 capacitor and generate the ripple you observed.
A good advice is difficult. I would order several bead types and test them over the frequency range of interest. If they fail, simply use only the capacitor if the results are good.
Ripple can also be reduced by a RC in series, e.g. 5-10 Ohms in series with a 0.1 to 10 uF capacitor.
Apparently, similar situation happens with many DC-DC converters on the market I observed such ripple on some I needed to use, and the RC combination as above served me well.
To power sensitive circuits like microwave PLOs, I recommend to use "linear" voltage regulators and rectifiers. The ripple is often difficult to reject.