When to use a copper region vs a trace - Page 1
Forum section: electronics / projects

author: drksy
author_rank: Regular Contributor
timestamp: January 16, 2021, 04:52:37 am
content: Hi everyone, I guess this is my debut to EEVBlog forum. Always been a big fan of Dave's channel on Youtube, so here we are. Full disclosure, I'm not a professional hardware engineer by trade. I do software for a living, python and things of that nature. But I have a special love for electronics and tinkering, and lately, I've been obsessed with designing my own lab power supply.In laying out my first four-layer pcb, It dawned on me that I just don't understand the subtle art of copper regions and when to use them in relation to traces. So let me explain to you what I mean.I've based my lab power supply on two LT3081's in parallel. You can see a simplified version of it herehttps://www.analog.com/en/technical-articles/high-performance-portable-dc-bench-power-supply.html. In my version, I'm using a dac and an opamp to adjust the set voltage. LT3081 is a nice regulator that acts as a follower to whatever voltage is applied at its set pin. Because I have two of them, I want the two set voltages to be as close to identical as possible. The question is whether I should pour a large copper region for the two set pins, or have sort of a "star" point in the pcb where thin traces attach to the two set pins.On my prototype PCB, the components are placed like this:Now, in my design I have poured a copper region for the set voltage to reach the two set pins like so, with 10n close to the set pins:But I am curious whether I should have done something similar to this:And whether or not there are any advantages or disadvantages to one or the other. When I look at other people's pcb layouts, I often see things such as star grounding instead of a single ground layer, or a thin trace snaking around a copper region instead of being a direct connection. Like for the example, look at this amplifier circuit designed by a professional engineer:Notice the places that I marked as red. There is a thin trace mysteriously going around a copper region where it could just attach somewhere close by. This is the dark magic part of pcb layout design that is puzzling to me.For the sake of additional information, the current flowing out of the set pin from the regulator is on the order of 100 microamps.

author: rs20
author_rank: Super Contributor
timestamp: January 16, 2021, 05:57:56 am
content: I'm going to go out on a limb here and claim that it's very unusual to use a copper pour for anything other than ground or power. It'd be much more common to go with your third image.Parallel pours (i.e., pours on different layers but otherwise "on top of each other") form a capacitor with very low value but rather good high frequency performance. This can be desirable for power supplies, as it assists the decoupling capacitors a little (loosely speaking).But given that this is a DC set signal, low current, no spiky current surges expected, and with decoupling at the other end anyway, I think that you'd be hard pressed to identify any difference in behaviour between the two approaches at all, let alone a difference worth of making one decision rather than the other.I'd personally go for the third image, not because it's inherently better, but because it will allow you to expand your ground planes in between those traces, which will theoretically improve the heatsinking provided by that ground plane to the LT3081.As for the fourth image, that's either an extreme form of 'star grounding', or literally a mistake/meaningless accidental detail.PS/ You messed up your link to Analog's by including the dot in the URL. And you inline the images by taking the URLs to the posted images (which you can grab from this page right now), and going back and editing your post and just straight inserting a new image with that URL. But you should have made the images half the size or even less because they'd be too big for that.

author: drksy
author_rank: Regular Contributor
timestamp: January 16, 2021, 07:33:04 am
content: I see. My hunch was that there shouldn't really be a difference seeing a copper region is essentially a fat trace, but I'm not confident in my judgement from lack of experience. Because this is a four layer board, I have the liberty to have a solid ground plane under the top layer, and I do have an almost uninterrupted ground plane on the bottom side as well. So I thought to make the set node just a big polygon.Thanks for suggestions about the image size. Resized them and the link is fixed.

author: fourfathom
author_rank: Super Contributor
timestamp: January 16, 2021, 08:06:08 am
content: In that photo of the circuit board, that skinny trace may be part of a "Kelvin" connection, as you often see in current-sensing circuits. To be honest, I don't see anything that looks like the other half of that possible current-sense connection, but I haven't tried to trace it all out. But in a connection like that you are trying to measure the voltage drop across a current-sensing resistor, and you don't want the this current flowing through your measurement traces and creating an additional voltage drop.

author: rs20
author_rank: Super Contributor
timestamp: January 17, 2021, 02:10:27 am
content: Quote from: fourfathom on January 16, 2021, 08:06:08 amThat's a great theory that I hadn't considered, makes a lot of sense as it's connecting to the terminal of a big resistor. I think you're right that this is probably what the designer was going for, however IMHO that particular PCB shouldnotbe taken as an example of how to do Kelvin connections correctly - far from it!

author: james_s
author_rank: Super Contributor
timestamp: January 17, 2021, 06:19:13 am
content: I use copper pours for heatsinking surface mount power semiconductors, that's about the only time I use one for anything other than a power or ground plane. Occasionally I'll use one to beef up a trace that is carrying a lot of current, if I have the space I may as well leave that copper there instead of etching it away, but that's pretty rare.