Reference plane

Thread Starter

BRAID94

Joined Jul 18, 2022
3
Hi,
I'm trying to understand eletrical dynamics in pcb boards.
In a mix of power and ground planes, the return currents always flow in the closest plane and have to go through many bypass capacitors. (High Speed Digital Design - Prentice Hall - Johnson- Graham).
I have a board with different power planes (1.5V, 3.3V and 2.5V). In case an internal trace layer is between a 3.3V power plane and a ground plane, which plane uses the return current of a trace betwwen 2 components powered by 1.5V? Near the components there are only 1.5V bypass capacitors.
3.3V bypass capacitors are away from the trace. The distance between the signal layer and the 3.3V plane is less than the distance from signal trace to GND.

Thanks for the support.

Bryan
 

WBahn

Joined Mar 31, 2012
27,893
The return current wants to follow the signal as much as possible. Where it comes to a discontinuity it will travel out to the nearest path that will let it come back (and that isn't a unique path, it will likely split along many available paths, but not equally). You can get some idea of what the likely dominant path is by pretending that you are the return current and finding the "best" route home that deviates the least from your ideal path.

To minimize the deviations, use stitching capacitors between the planes that the signal crosses as close to the crossing points as possible.
 

Thread Starter

BRAID94

Joined Jul 18, 2022
3
So in the example shown, the return current will flow in GND and +3V3 power plane through the bypass capacitors. Is it correcet?
 
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