Filter Power Supply Noise for sensitive electronics - Page 1
Forum section: electronics / projects

author: jusaca
author_rank: Unknown
timestamp: June 23, 2023, 12:28:01 pm
content: Typically I'm more involved with battery powered electronics and I don't have much experience with grid supplied devices. But currently I'm trying to supply some noise sensitive electronics with a general purpose power supply.The goal would be to build a dedicated power filter, that enables me to use basically any supply, wether its a linear supply or an SMPS (most supplies used will have no earth connection, if that is important).The output will typically be loaded by a few mA only, but on heavy use current draw might go up to peaks of 4.5A for ~100ms with a "worst-case average" of something like 1A.Specs:Vout= 7.4VVnoise,rms< a few tens of µV (DC ... ~20 MHz) (I'm not entirely sure about what is needed yet, I will see when the first prototype is build)Ityp= ~25 mAIpeak= 4.5 AIavg,worst-case= 1AI decided to limit the input voltage to 12V supplies, which gives me plenty of "wiggle" room for voltage drop in the filter stages, in my use case I can live with the power dissipation.I came up with the following schematic and annotated my intentions of the different filter stages. Does this make sense? I assume I should filter differential mode noise first, to have a low-impedance return path in the ground line. And after that I filter for common mode noise by two ferrites + a CM choke.I would be glad if someone with more experience with this stuff can glance over the schematic and give some feedback =)

author: Vovk_Z
author_rank: Super Contributor
timestamp: June 23, 2023, 08:53:11 pm
content: Quote from: jusaca on June 23, 2023, 12:28:01 pmThat sounds like you want to build an ultimate ideal filter, so I'm not sure that's an easy or doable task.From my small experience with low-noise circuits I may say that it is typically much more easier to use suitable dedicated low-noise power supply than try to build some ultimate filter for excessively noisy power supply.E.g. common mode noise is much harder to get rid of, and mains-powered SMPS are known sources of high level of common mode noise.I mean, if you plan to work with some low noise device it is oftien easier to put some effort in design dedicated low-noise power supply.But, of cause, it is better to have you own experience with power line filtering, so from another side it is better if you try. You'll definitely gain some experience which is the most important thing.

author: jusaca
author_rank: Unknown
timestamp: June 26, 2023, 07:23:29 am
content: Thanks for your reply. Yes, a dedicated power supply would be best, but due to a combination of factors I would like to be flexible. And I'm just not comfortable designing my on supply, I would like to work with safe isolted voltages only.I will definitly make my own experiences with this one, I will build the posted filter circuit and test different components assembled and not assembled.But I was hoping for some more experiences here whether the filter makes sense in general or if there are some obvious shortcomings I missed.

author: Vovk_Z
author_rank: Super Contributor
timestamp: June 26, 2023, 03:03:22 pm
content: One of the shortcomings is AC-DC SMPS typically give a lot of common mode noise. And a common mode noise is such a thing that's hard to filter or eaxpencive to filter. We don't know how strict your noise requirements are so we can speak only about general rules.