## 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--

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

Interestingly, if **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.**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.

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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Power Converter Loved this article. Would surely take the advantage

ReplyDeleteYap, sometimes this technique could be a great help to reduce ripple noise.

DeleteIts a great blog

ReplyDeleteThanks Emmanuel !

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