I have a circuit with a couple of bucket converters. The bucket converters are either standalone or getting a PWM signal from a processor and give a feedback to the processor (which is on another PCB). During my bachelor studies I heard a lot about grounding issues. I learned that a star topology is quite useful to avoid EMI problems. How would I do this correctly in KiCad? What happens when I use the predefined GNDPWR and GND? How can I bring them together to one ground plane? I think one needs one power grounding point connected to the GND plane for every amplifier? That’s what I have in mind. What is your opinion? Do I even have to separate the ground signals with an transformer or a optocoupler?
It depends. Some application need isolated power supplies. (Just think about an electric vehicle. There is a regulation that states that the high voltage side needs to be isolated from the low voltage side.)
If your application needs such an isolated power supply you need to use Transformers/ Isolated DC/DC converters, optocouplers (or some other means to isolate digital signals).
If your application does not require a separation of the power supplies than a better option is to use one ground plane.
We had a discussion about something similar recently:
You need to connect them in one way or another yourself.
As stated in the discussion linked above, the best way to do this is via an inductor. (See post of @1.21Gigawatts in the given thread.)
If you want to separate different grounds, PWRGND is a good name in your case.
(Power symbols in KiCAD are very similar to global labels but combined with a nice symbol.)
In your case this might not even be necessary. Your GND and PWRGND are on different layers, so you don’t have the problem of overlapping planes.
To be honest i’m not sure if separating your grounds is a good idea. Maybe design both and compare them? (Might be interesting to see which one is really better and how big the difference really is.)
Another remark: edit: I got it completly wrong!!! its exactly the other way round! Sorry about that. The placement of the devices in your screenshot might not be ideal. You should look into the details of L_out. Place the Capacitors C_out as close as possible to normal parrallel on the field lines of the inductors stray field. Also use a shielded inductor. If the shield is open towards the bottom, make sure you have a ground plane there to shield the near field.
Example for bad placement (assuming the axis of the axis of the inductors coil is parallel to the pcb):
If the axis of the coil points up the capacitors placement of your example is ok.
In your example the grounds are not separated, they are the same net. To be separated they would need to be connected together using a ferrite bead or choke, but they are instead connected using vias. This is just a layout technique used to try to localize the switching currents of the power supply to the track labelled as “PGND”. PGND is the ground connection from the incoming power supply. This can help to keep the switching currents off of the ground plane. The track is labelled “PGND” for illustration purposes mainly, it is the same ground as the others.
I made a mistake in my post above.
Having something normal to the magnetic field lines leads to an induced current so the original placement of the capacitors was correct. I fixed my post so that nobody gets the wrong idea.
Sorry about that.
(Note to self: Your memory doesn’t work very well. Check everything.)