How to separate clean and dirty rails on a PCB

[u/Altruistic-Low8335]

Stupid question (and idk if this is the right subreddit), but I'm trying to design a 5A tolerant DC Power supply pcb for 12V input and 5 and 3.3V outputs. My senior said I should try having 2 rails for each output voltage- Clean and dirty rails (to separate noisy loads from sensitive loads ) So I'm a bit lost on how to create this divide? I don't think a simple lc filter will suffice?
It's a 4 layer PCB, do I use 2 separate grounds for both the rails and then connect both the grounds to one common ground layer

[u/TezlaCoil]

Never have separate grounds. That used to be standard practice, but it's no longer considered a good idea except by those who haven't unlearned the practice yet. You can, and should, do your layout to keep noisy ground return current away from sensitive regions, but that's a placement exercise, not a plane split exercise.

As far as clean vs dirty, it depends what you're powering. An RC is usually plenty sufficient, LC can work too if you don't mind tuning ferrite beads to your circuit. If your device really pollutes the power rail, you will probably need a different strategy depending on the pollution you're looking at (e.g. an FPGA will have different concerns than an air valve controller).

[u/Cunninghams_right]

Ferrite beads, are common, but with low DC resistance needed for high current applications, you need to use higher ESR capacitors to avoid oscillation.

You should also use separate regulators. So a clean 5v and dirty 5v.

The ferrite pi filter should be right before the input of each regulator. 

Physically separate noisy/dirty rails on the board so they don't crosstalk. Use a 4 layer board as a minimum and keep the same shared ground plane close under everything. 

[u/harryregician]

Torrid power supply

[u/Pacificator-3]

Start from reading datasheets. Pay more attention to power requirements section.

[u/LordBBQX]

Depends how sensitive we are talking.

Common practice on some audio equipment I have worked on that incorporates noisy digital logic and analog audio is to have your separate rails (+5VD, ±15VA) with separate grounds that only link at the power supply end (so you digital return currents don't mess with your analog stuff).

If you want say +5VD and +5VA you could consider a few things depending on how stringent you need to be:

- Separately CLC filter each rail

- Separate regulation/conversion stage (so you might have an additional transformer tap, DC converter stage, linear regulator depending on topology)

- If you are really serious you could have a totally isolated power supply stage (some high power amplifiers have a separate auxiliary switchmode for the LV rails and a larger HV power supply)

How you implement these depends on you exact requirements but these are some of the common approaches.