If it's an adjustable linear regulator, you won't need an output filter.
If you have a DC regulator with some low frequency ripple on its output, then the proper solution is to improve its feedback loop performance, not just add an enormous filter on the output.
I wonder what "DC ripple" is? Ripple is always an AC quantity, I think.
As already explained, a linear voltage regulator can give a nearly perfect DC output voltage if designed respectively. It has usually output bypass capacitors that reduce higher frequency noise and provide a low output impedance for frequencies near and above the feedback loop cutoff frequency. Even a high performance voltage regulator shows a certain amount of electronic noise, slow voltage fluctuations and long-term drift. But it's effectively impossible to remove it with LC filters.
Linear regulators do an excellent job of removing low frequency stuff like mains ripple (100Hz or 120Hz), but don't cope well with high frequency garbage at the input, so a good plan is to put a passive filter before the linear regulator to prevent any high frequency garbage reaching it.
So basically you're talking about two things, PSRR and output noise. A decent regulator should have extremely high PSRR at low frequencies (like below 1kHz), enough to practically eliminate any low frequency ripple. The regulator itself is a filter for this sort of ripple. If you're seeing poor PSRR, then the problem is with the regulator itself. Noise on the other hand is a trickier issue. The low frequency output noise can't be effectively filtered, but high frequency noise can, using an external LC or RC filter. What sort of noise levels are you looking for?
Ultimately the quietest DC source is a plain old battery, it's pretty much impossible to beat it.
I am planning to build a multistage LC low pass filter for the output of an AC to DC converter.
The output of the supply will be adjustable between 6V - 12V at 2A of steady current. The required cut off frequency required for the multi-stage filter is 2Hz.
What is your budget for this 12V, 2A (24W) power supply? A 2 Hz LPF seems reasonable to get low ripple until you find out how many thousand uF and uH you need to get 2Hz and how much that costs to supply 2Adc then you will give up and choose Buck regulator instead which will cost <1% of the Linear filter since it operates at 1000x the frequency.
View attachment 108407
How much peak current does a 2A DC inductor use ? Answer 350%x average. How much does this cost
Now go find an inductor rated for 7A (>7A saturation) It will be an steel laminated core as large as the transformer. Choosing a &A Ferrite Choke only gets you 10uH at reasonable cost.
If you rely on an RC filter instead how much ripple current does a Capacitor see with 15% ripple voltage. Answer AC current increases as AC ripple decreases, thus with 15% ripple voltage then the AC ripple current shown below is +10A,-2A =12Ap-p or about 6x the Adc.
This is not an optimal solution to design an LC filter around a 50Hz AC to DC conversion. This is why Buck converters are preferred.
Your question is about an LC filter yet all you want is a clean variable lab supply. If I were to suggest a solution, there are many assumptions not stated, so I will expect you to make a list of requirements on availability of all other parts such as a transformer solution or better, a full spec.
Generally one does not design an LC filter without knowing what is being filtered. 100Hz or 100kHz or ?
But for very clean lab supplies, there are many solutions which depend on safety, input voltage, regulation accuracy, noise in uV or mV.
I cannot give an explicit answer until ALL the requirements are completed. I suggest you look around to see HW lab supplies are created or specified and decided what you want to make/buy and expected compliance to safety, cooling, regulation etc etc.
- a linear regulator suppresses noise well, but at high current the voltage drop between input and output wastes heat that requires a large heatsink.
- SMPS using Buck mode or step-down is 80-95% efficient but can be noisy at light loads so a pre-load and good LC filter or an Linear regulator is needed.
-low ESR Caps are essential for any low noise filter supply, but also short circuit protection.
- here are some schematics on power supplies
- here is some terminology required in a spec.
- some general discussions
i suggest you consider this question (answered) and do some research and come up with a better question.
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