why we filter smps

[ginetto61]

.....And if one wants extremely low noise then a simple CapMx after it will do just that.

Hi I think you are the first and only to mention that
I wonder why capacitance multipliers are not so popular
They All talk only of passive filter networks
I would try a basic cap multiplier with a nice Darlington first and see it's impact on noise
My main issue is to filter the output of switch mode DC step up converters

 

[ABSNimes]

Hi all,

Have a look there : https://www.diyaudio.com/community/...-cap-multiplier-for-smps.406233/#post-7534342
You'll find Single and Dual SMPS Filter with CapMx. (Dual means bipolar)
Each PCB could be used as Pos or Neg ; some polarised parts to reverse.
Same power and voltage as Mark's ones (3A/48V - per rail if you use Dual version)
Worth a try 😉

 

[Mark Johnson]

Have your circuit consultant create a SPICE simulation of a capacitance multiplier, and ask her to plot its noise attenuation vs frequency. At frequencies where SMPS emit the greatest amount of noise, the capmul has become ineffective. So, if you insist upon using a capacitance multiplier as part of an SMPS post-filter, make sure it's only a part of the filter. Make sure there are other parts of the filter that provide lots of attenuation between 20 kHz and 200 kHz.

 

[ABSNimes]

Mark, my knowledge is too weak to give you an answer, but schultzsch could.
He gaves some VFet amp measures in Post #9 of his thread. (20Hz to 100KHz)