Lets say I have a critical device, which requires almost no ripple in voltage and draws a very high current. What are some ways, other than a simple capacitor which take out the ripple from the full-wave rectified ac voltage?
Techniques for removing mains ripple
ripple
Related Solutions
Your output ripple will be a triangular voltage waveform at the converter switching frequency; the triangular current waveform given to you by the output inductor superimposed onto the output capacitor ESR. What you have on your scope is huge CM (common-mode) pickup which is swamping the actual ripple.
Turn your scope bandwidth limiter on. Your true ripple will be orders of magnitude lower than 20 MHz.
Measure directly across your output capacitors with a short probe to minimize pickup.
Use your scope cursors to ignore the junk at the switching transitions and just measure the triangular part of the waveform.
The ripple current in the output caps is a significant compared to the output current in a buck convertor running in current mode with a not too large output inductor In fact the peak to peak ripple current could be say 30% of the output current in this orthodox setup SO at 40 amps dc you need caps to handle almost 7 amps peak now the approx. current waveform is triangle so you are up for multiple caps in parallel Use big smd packages and remember that your control loop will change so you may get instability which mill mean changing compensation component values OR leaving the output electro in parallel with your new ceramic caps
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Best Answer
The low frequency of the ripple is the problem. Should the ripple frequency be 10kHz it's a lot more easily solved with components that are small.
I'd use an on-line switching power supply and forget about conventional transformers, bridge rectifiers and big capacitors. The output you get might have some higher frequency ripple and noise but this could be removed with low pass filtering from inductors and capacitors that are a fraction of the cost of filtering at 120Hz (full wave 60Hz).