Tuesday, August 20, 2013

Buck converter noise reduction

Two-stage LC filter for buck converter



In this post we will address output voltage ripple reduction in buck converters through use of two-stage LC filter.
When designing a buck converter output filter section we calculate the inductor L1 based on desired ripple current, selected to be usually between 20% and 40%, and the output capacitor C1 based on desired output ripple voltage requirements. In case of C1 its ESR is the dominant parameter that affects ripple voltage, thus C1 is chosen based on its ESR in practice. An example of such circuit is Circuit 1 where the ripple voltage is around 0.2V or 5% of 5V.

Circuit 1 -- if embedding does not work click here
Click on Open Project, see Transient Analysis tab or Run the simulation.
--use the zooming tool to inspect circuit and see BOM tab for parts values--            

Voltage ripple with single-stage output filter.
What if the noise is still to high and we want to reduce it significantly? Since it is not practical to increase the inductor or capacitor values by a factor of, let's say, ten the most obvious way to do this is by adding a second high-frequency noise filter after the main one. This approach is not correct however and can lead to stability issues. The issue here is adding C and L with smaller values after the main filter (consisted of C and L of bigger values). If we refer to circuit 2, we have two resonant frequencies, it's higher resonant frequency is omega_h = 1/sqrt(L2*Cs), where 1/Cs = 1/C1 + 1/C2 (series connection). Clearly omega_h is dependent on the smaller between C1 and C2. That means that the resonant frequency (omega_h) will change a lot with load capacitance since if C2 is small adding anything in parallel with it will increase the total capacitance considerably.
Interestingly, if C1 is the smaller of the two then the resonant frequency is less affected by load capacitance changes. As a rule of thumb, C1 is usually ten times smaller than C2, L2 is usually ten times smaller than L1. The values should however be fine tuned for good control loop stability and proper damping factor.
Voltage ripple with two-stage output filter
The reduction of voltage ripple by a factor of ten, using a second-stage LC filter, can be seen in the graph on the left. Using this technique we can attain considerable voltage noise reduction without compromising stability. Note that C1 should be capable to sustain the full ripple current from the inductor L1, that means it's ESR should be comparable to C2. Being C1 a smaller capacitor it should often be film type or ceramic. Film type capacitors have a smaller ESR for equal capacitance than electrolytic ones.


Circuit 2 --  if embedding does not work click here  
Click on Open Project, see Transient Analysis tab or Run the simulation.
--use the zooming tool to inspect circuit and see BOM tab for parts values--
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5 comments:

  1. Power Converter Loved this article. Would surely take the advantage

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    Replies
    1. Yap, sometimes this technique could be a great help to reduce ripple noise.

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