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Reducing Switching Regulator noise and Conducted interferance

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rocky79

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I have a switching regulator that operates at switching frequency of 260Khz that steps down a 28vdc to 3.3Vdc. The switching regulator output will supply current as high as 1 amp for the initial 5 minutes on power up then it falls off to several mAs.

Can you please suggest an input and output filter that dramatically reduces the input and output ripple as well as the high frequency noise?
HF noise is measured to be in the 200~300Mhz range. I have attached the schecmatics as well as the input and output waveform as captured by the scope
 

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  • Output-combined.jpg
    Output-combined.jpg
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  • WaveInput@Cbps_Frequency.JPG
    WaveInput@Cbps_Frequency.JPG
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  • Schematics-SwitchingRegulator.JPG
    Schematics-SwitchingRegulator.JPG
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Add the snubber across DS2. say 75 ohms and 680 pf. You may need to experiment for the best values.
Add a 1 or 2 ufd. ceramic cap to the output.
The input just looks like it needs a much larger electrolytic.
In simulaton this was like 10 to 1 better.
 

C1 and Cbps will dominate the noise reduction, providing the track layout is OK. I would use a 1nF ceramic across both as an RF bypass. Adding the snubber will reduce the risetime but not the absolute amplitude of the noise pulses.
Frank
 

There is so much of ringing in the waveform. I guess there will be noise in your input also and I just want to know from which source you are feeding the input supply. Even try to filter out the input supply by adding few electrolytic supply at the input side.
 

rocky79 said:
I have a switching regulator that operates at switching frequency of 260Khz that steps down a 28vdc to 3.3Vdc. The switching regulator output will supply current as high as 1 amp for the initial 5 minutes on power up then it falls off to several mAs.

Can you please suggest an input and output filter that dramatically reduces the input and output ripple as well as the high frequency noise?
HF noise is measured to be in the 200~300Mhz range. I have attached the schecmatics as well as the input and output waveform as captured by the scope
Click to expand...

I couldn't find the schematics in attachments. So I am just going to add my few points which may help you -
1. You can use a C-L-C (PI) filter to reduce the ripples at the input side. You may have to simulate and use proper values for cap and inductor. In place of inductor, a ferrite bead can be used.
2. You can use a L-C-L filter at the output side, here as well you can use a ferrite bead which will be more effective. The cap here will be separate than the output cap which will support the peak load currents.

Wish you the best :)
 

ronv said:
Add the snubber across DS2. say 75 ohms and 680 pf. You may need to experiment for the best values.
Add a 1 or 2 ufd. ceramic cap to the output.
The input just looks like it needs a much larger electrolytic.
In simulaton this was like 10 to 1 better.
Click to expand...

Thank you for your replies. These waveform are measured with a snubber across the diode DS2. R= 45ohms & C= 220Pf in series
Here is the resulting waveform before and after the snubber was added (measured across DS2).I will try adding the 1nF at the input and output and see how that goes.
 

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  • SwitchingNode-With&WithoutSnubber.jpg
    SwitchingNode-With&WithoutSnubber.jpg
    575.2 KB · Views: 155

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