Ripple Filter Damping Techniques

Found this very useful article by Kevin M. Tompsett on damping of a PSU output filter:

http://www.analog.com/media/en/technical-documentation/technical-articles/Designing-Second-Stage-Output-Filters-for-Swit… 

In Fig 4 there are 3 techniques shown to damp oscillation in the output C-L-C pi filter:

1. Resistor across the inductor

2. RC load on input

3. RC load on output

I can understand how techniques no. 1 & 3 work as they are loading the output of the filter to damp the oscillation, but I do not understand how no. 2 works. How can a RC load on the input of the filter stop oscillations on the output? It seems to me that the output LC can still resonate.

Any clarification of how technique no. 2 works would be well appreciated!

Thanks, Ken

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  • Thanks so much for reading my article.  It works just like the others and damps the filter.  Though you cannot completely damp it like you can with the other techniques it is very cost and size efficient, and does not compromise transient response or filter effectiveness like the other techniques.  Take a look at the bode plots in the paper.  You will see a small rise at resonant frequency, but not a large peak.  Take one of the ADP2386 Simpower Excel spreadsheets and export to Simplis the design with the output filter and play around a bit.

     

    Regards,

     

    Kevin

Reply
  • Thanks so much for reading my article.  It works just like the others and damps the filter.  Though you cannot completely damp it like you can with the other techniques it is very cost and size efficient, and does not compromise transient response or filter effectiveness like the other techniques.  Take a look at the bode plots in the paper.  You will see a small rise at resonant frequency, but not a large peak.  Take one of the ADP2386 Simpower Excel spreadsheets and export to Simplis the design with the output filter and play around a bit.

     

    Regards,

     

    Kevin

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