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PCB layout help - splitting copper planes

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armandas:
Hi.

I'm in the process of designing a PCB which can be seen in the attachment. This will be a four-layer board, with the two inner layers dedicated to power and ground. All of the circuitry runs on 3.3V, so initially, I had a solid plane for that. Now, since I have to also distribute 24V to most of the connectors on the board, and all of the circuitry is located in one place, I decided that it would be better to split the plane into two parts.

The unfilled area in the top-left corner is where the 3.3V plane will be, the rest is the 24V plane. My question is thus: is it a bad idea to have this loop shaped plane? Would it be better if I only let current flow through the top part of the board? The power inlet is located at the top-left corner, you can see three holes in there.

Please let me know what you think. Thanks!

Zad:
If it is all digital then I wouldn't worry too much. If it supplies analogue too then you need to be thinking about current paths and splitting it into separate segments with more "linear" current flows.

ejeffrey:
Well, the art of engineering is finding your place on the scale beween "optimal" and "good enough."

What you need depends on how fast your signals are, how sensitive to noise you are, and whether you are going to need to pass an EMC test.  There is a big window between what is probably good enough and the best possible situation.

Do you really need a 24 V plane?  If its only purpose is to power devices on the other end of the cable the plane doesn't really get you anything.  You can just run 24V as traces to the connectors.

In any case, the biggest concern with a split plane is if you run fast signals across the break.  You want to avoid that if possible.  If you can't keep all high speed traces on the side with the ground plane you should put bypass capacitors between the 3.3 and 24 V planes next to those signals.

Gall:

--- Quote from: armandas on September 07, 2011, 07:38:58 am ---My question is thus: is it a bad idea to have this loop shaped plane?

--- End quote ---
How much current flows through it? At which frequency? How much does it have in AC?

armandas:
Thanks for the replies, guys.


--- Quote from: Zad on September 07, 2011, 05:18:12 pm ---If it is all digital then I wouldn't worry too much. If it supplies analogue too then you need to be thinking about current paths and splitting it into separate segments with more "linear" current flows.

--- End quote ---

The 24V DC is going to power both analogue and digital devices.


--- Quote from: ejeffrey on September 07, 2011, 07:39:30 pm ---Do you really need a 24 V plane?  If its only purpose is to power devices on the other end of the cable the plane doesn't really get you anything.  You can just run 24V as traces to the connectors.

--- End quote ---

Routing 24V tracks on either top or bottom layers would be quite awkward, that's why I decided to use the internal plane.


--- Quote from: ejeffrey on September 07, 2011, 07:39:30 pm ---In any case, the biggest concern with a split plane is if you run fast signals across the break.  You want to avoid that if possible.  If you can't keep all high speed traces on the side with the ground plane you should put bypass capacitors between the 3.3 and 24 V planes next to those signals.

--- End quote ---

There are no high-speed signals on this board. There are digital signals, like I2C, but they are contained in the 3.3V area.


--- Quote from: Gall on September 08, 2011, 07:26:38 am ---How much current flows through it? At which frequency? How much does it have in AC?

--- End quote ---

The current is going to be rather low, since high-power devices will have their own mains supply.

I've done some changes and will probably stick to what I have now.

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