80A on a 1oz PCB - Page 1
Forum section: electronics / projects

author: drakejest
author_rank: Regular Contributor
timestamp: June 22, 2021, 05:31:53 pm
content: Is a 3mm x 15mm x 150mm copper bar strip enough for 80A soldered to a 1oz PCB/I am in a weird predicament where i can only go 1oz PCB. JLCPCB currently dont offer 2oz on 4 layer with black colored pcb. I am making a LED driver circuit with 8 channels and each of them consumes about 10A. I am spaced constrained on the main trace delivering to each of the channel with only about 180mm x 25mm.I was thinking of not placing a solder paste on the trace and solder a 3mm thick 15mm x 150mm copper strip.Would that be enough? if not if i double the thickness to 6mm will it still be not enough ? I did have a look at thecurrent carry capacity of the copper barand i am below 1/2 of its limits (45mm2 is about >130A at wost). But is it really that straight forward?Also the bottom copper is mostly ground do i need to place a buss bar there too ?

author: Alti
author_rank: Frequent Contributor
timestamp: June 22, 2021, 06:37:02 pm
content: How are you going to solder a 3mm thick copper bar into a PCB? Also thermal expansion of FR4 is huge when compared to copper so you cannot put thick copper on one side of FR4 because you get a banana effect after it cools down. But maybe 150mm bar is stiff enough to hold PCB straight...I suggest you should power it with a bunch of 10 x 0.5mm2 stranded wires of equal length instead of using one thick bar. Then you do not need thicker Cu, 10A is nothing serious. Also, 10x0.5mm2 is going to fit to standard 6mm2 terminal (use ferrule).

author: Renate
author_rank: Super Contributor
timestamp: June 22, 2021, 06:51:04 pm
content: You got me thinking on a tangent.Asquare1 ounce PCB should measure 0.45 milliohm from one edge to the opposite edge.Regardless of size!

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 22, 2021, 06:54:03 pm
content: 80A is a serious amount of current, and if you combine that with thin copper, you may get into problems with localized (over) heating near connectors.At the moment it looks like you're injecting 80A into a corner of the PCB, and then transport (a part of it) to the other extreme end. Compare that with putting the 80A connector in the middle (halves the distance) and you have 40A going up and 40A going down. On top of that, you have better heat spreading around the high current connector.Have you thought about making a dual PCB project?For example, Use one PCB with thick copper for Power distribution and some MOSfets, and another PCB for small stuff such as a microcontroller, and then combine them with either a connector or use castellated edges for the digital PCB.Maybe you can even keep both dual-layered, but even if you want to keep the "microcontroller" PCB as 4-layers, it's likely a much smaller PCB.

author: poorchava
author_rank: Super Contributor
timestamp: June 22, 2021, 07:00:13 pm
content: I once did something of similar magnitude - 8 phase, 200A current source, max 5V. The PCB was 2oz with a 5x5mm square bar soldered on top. This is about 8A/mm.sq and we had no problems with this. We are soldering 4 of those in a completely standard SMD reflow process (with some additional steps before and after, but i can't discuss the details).AFAIK there's a couple hundred of those in the field, and we had no problems whatsoever with overheating.Mind you, the device is a T&M instrument and does not work at 200A constantly. Maximum effective duty cycle is about 45%

author: drakejest
author_rank: Regular Contributor
timestamp: June 23, 2021, 05:31:24 am
content: Quote from: Doctorandus_P on June 22, 2021, 06:54:03 pmthat is indeed a great idea,Quote from: Doctorandus_P on June 22, 2021, 06:54:03 pmthis is indeed a multi pcb circuit, im using the middle layers as a pathway for the mosfet gates and current sensing, so the outer layers can be focused for the power traces.

author: Siwastaja
author_rank: Super Contributor
timestamp: June 23, 2021, 07:37:09 am
content: Change the arrangement so that input power goes in the middle, fuse holder holes spread out in all four directions (up, down, left, right) so there are just two in each direction instead of all eight on up right.

author: macboy
author_rank: Super Contributor
timestamp: June 24, 2021, 07:46:38 pm
content: You didn't say so, but I assume you are using all four layers for this? Note the inner layers may be 0.5 oz, you will need to check.

author: Siwastaja
author_rank: Super Contributor
timestamp: June 25, 2021, 06:57:06 am
content: Note that generally, minimizing distance is the best way to deal with large currents. This is, think about the component placement.If 80A is flowing between points A and B and these points are just 10mm away, and there is excess copper around it, the extra copper works as a heatsink.1oz copper has sheet resistance of 0.5 mOhm per square. This means, if you draw a grid (choose any grid size, 1x1mm or 10x10mm or whatever), each square is a 0.5mOhm resistor regardless of its dimensions as long as it's a square. These squares then act in series or in parallel. Make the number of series squares as small as possible, and number of parallel squares as large as possible. People remember the importance of wide tracks / fills but forget about making them short as well because current handling rules of thumb are given based on width alone assuming infinite length and ignoring heatsinking from the ends of the tracks.