Author Topic: High current PCB design  (Read 7063 times)

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Offline zxsq

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High current PCB design
« on: December 19, 2016, 03:42:23 am »
So I have this project I'm working on, where I want to design a pcb to adapt the solder lugs on a motor my robotics team uses to screw terminals. The lugs tend to break off, to the point that after losing 4 motors in a 3 day competition, we started soldering, zip-tying, and hot-snotting them to try and provide some means of strain relief, to little avail.

So my question is, how can I get a pcb to safely conduct a current of circa  15 to 20 amps, and do so without catching fire, melting solder, letting out the angry pixies, magic smoke, etc. (read: safely)?

http://files.andymark.com/CCL-9015-motor-curve-am-0912.pdf Here's the motor curve, with amperage graphs etc, I'm not designimg to operate at stall torque, but failing safely might be ok.

Thanks,
zxsq
 

Offline pyroesp

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Re: High current PCB design
« Reply #1 on: December 19, 2016, 04:28:25 am »
You can find online pcb trace calculators where you specify the current needed and thickness of the trace, and it tells you how wide your trace needs to be.
 

Offline zxsq

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Re: High current PCB design
« Reply #2 on: December 19, 2016, 04:36:42 am »
The online calculator gives 18.7 mm wide trace, I don't think that is possible in this situation. Are there other ways to increase the current capacity?
 

Offline Brumby

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Re: High current PCB design
« Reply #3 on: December 19, 2016, 05:31:32 am »
You could simply solder some heavy gauge wire onto the tracks like this....



Doesn't look pretty, but could do the job.

Just make sure you run the heavy gauge connection from one high current point to the next without gaps ... and don't short anything out, of course.
« Last Edit: December 19, 2016, 05:35:06 am by Brumby »
 

Offline salbayeng

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Re: High current PCB design
« Reply #4 on: December 19, 2016, 05:53:03 am »
You can get thicker copper, it's normally 1Oz/sqft (=38um). With e.g. 6oz  you should get the same current through a 6mm track. You can also put a track on both sides of the PCB (with adequate insulation).
You need to glue the PCB to the motor with some decent glue (polyurethane?) or you will still break off the lugs.  Sometimes copper braid (with a sleeve) can be more flexible than round wire, although solder wicks easily up it and makes it stiff, spot welding braid onto the lugs would be a solution in a manufacturing situation.
 

Offline T3sl4co1l

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Re: High current PCB design
« Reply #5 on: December 19, 2016, 06:14:16 am »
20A over two sided 2oz PCB needs about 300 mil (7.5mm) width, for I think around 20C temp rise, modest but not crazy.

You want to find a calculator that does IPC-2152, like:
http://www.smps.us/pcb-calculator.html

You can select temp rise and such.

Note that the heat conductivity goes up, and therefore the current handling goes up, if there is an internal plane (4+ layers) to carry heat away from the trace.

So, this assumes there isn't a lot of stuff immediately around the trace.  If you need to use the whole board to carry current, then there's nowhere for the heat to go, and the whole thing will get much hotter than a single trace alone.  Same to say -- if the trace is really wide, it'll be noticeably hotter in the middle, because the heat generated there can't find its way out sideways (by conduction).

And tacking stuff on, as above, is definitely an option for one-offs. :)

Tim
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Bringing a project to life?  Send me a message!
 
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Offline salbayeng

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Re: High current PCB design
« Reply #6 on: December 19, 2016, 08:50:23 am »
Thanks for looking up the numbers Tim,  I've got a business card PCB on my desk, with a table of the widths on the back for different weights, but couldn't find it.

Also regarding hot spots, you can get a bottleneck at the pads where your lugs attach , so you need to ensure you fill the entire hole with solder.
It's also common to surround the pads with multiple vias, and fill these with solder, although the solder has much worse resistivity than copper, the cross-section through the solder in the via is 1.6mm thick , compared to 0.07mm for 2oz solder. Solder through the vias also helps anchor the pad (like tree roots), so you don't lift the pad off when the PCB flexes. (That's probably a concern for you as you are already cracking the lugs).
 

Offline TheDane

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Re: High current PCB design
« Reply #7 on: December 19, 2016, 06:46:09 pm »
Solder wick can be found in a lot of different widths, and if enough solder is applied - it conducts large amounts of current without blowing or heating up due to the 'huge' thickness of it compared to a regular PCB track.
Apply a little solder at the end and at bends, and it's somewhat easy to route on a board. Solder it to the right places (beware of shorts! - also at higher temps) and then apply lots of solder. IMHO easier to apply than a regular wire, single or multi stranded. The entire length must be covered - otherwise it's just thin Cu wire that blows like a fuse  :palm:
It's kind of PCB design - it just requires touch-up in/under the end  :-DD
 

Offline Alex Eisenhut

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Re: High current PCB design
« Reply #8 on: December 20, 2016, 12:19:51 am »
Since RC cars can safely manage many times more amps, why not look into RC car parts?
 

Offline timb

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High current PCB design
« Reply #9 on: December 20, 2016, 12:32:12 am »
You could do a 4 layer PCB with 2oz copper. Place the same trace on all for layers and use via stitching along the trace to link the layers together. This will increase current handling ability and heat dissipation.
Any sufficiently advanced technology is indistinguishable from magic; e.g., Cheez Whiz, Hot Dogs and RF.
 

Offline zxsq

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Re: High current PCB design
« Reply #10 on: December 22, 2016, 02:11:19 am »
ok thanks all! Very goods ideas here, I'll try the solder wick as a prototype, and probably go wity 4 layer 2oz for the final product.
 

Offline max_torque

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Re: High current PCB design
« Reply #11 on: December 22, 2016, 11:26:56 am »
Resistance turns electricity into heat.  If that heat can be removed from the trace at the same rate as it is created, the trace stays at the same temperature.  As temperature difference drives heat flux, the higher the temp difference, the more heat that flows between hot and cold.

One interesting test, is to get a spare pcb, that has various trace widths and lengths on it, and connect a trace across a currently limited supply.  You can then turn up the current in stages, and leave it for the temps to stablise at each current level.  In a normal ambient (say 25degC) with a pcb in air (air acts as conductor) you might be suprised at just how much current a very small trace can carry before it fails.  The failure mechanism is generally the trace heating beyond the de-lamination temeprature of the FR4 pcb material, which bulges up, and then the trace becomes un-laminated and lifts up from the FR4, which means it then looses a large amount of its conductive cooling path, and suddenly, that local area of that trace quickly rockets up in temperature and the copper then actually melts, opening the trace.

On a pcb i tested, i could actually push around 12amps down an 8mil trace before it actually failed!  (20degC, un-obstructed airflow)


It would actually be an interesting video for Dave to do, get some pcbs with different traces, break out the thermal camera, and torture some traces  :-+   

« Last Edit: December 23, 2016, 11:41:54 am by max_torque »
 

Offline salbayeng

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Re: High current PCB design
« Reply #12 on: December 22, 2016, 10:36:54 pm »
I did some research on PCB necking , the idea being to make a controlled fusing track section,
and also for using traces as current sense resistors.
See page 6 of the ZXCT1010 (current sense amp) datasheet http://www.diodes.com/_files/datasheets/ZXCT1010.pdf
and http://www.diodes.com/_files/datasheets/ZXCT1009.pdf
You can also save $$ by using BCV61 or BCV62 as the sense amplifiers.
The copper has a large tempco, so useless for accurate metering, but OK for overload protection where a higher current limit is acceptable at lower temperatures.

If you take a short section of  (say 8mil) track and use it to join two large areas of copper together then it can carry significantly more current than a long section of trace.
From memory it was a rule of 3's , Say for example a trace is 10mil wide, then if you use 3 times this length (30mil) and join it to fills (pads) 3 times as wide e.g. 30mil , that were at least 3 x trace  long (30mil)   then you would double the trace current capacity.
So you could make a serpentine shunt, and put blobs at every turn.
I generally found any kind of PCB trace shunt impractical for generating 100mV type signals.
But 1000mils of 100mil (in 1oz) with a gain of 100 amplifier was  pretty good for detecting current in the 5 to 10A range

These shunts are used for chopper MOSFET protection on actuator drivers , these might be 4A at 12v nominal load, but should be able to ride through 8A surges and go into protection mode above 10A or a short circuit.

I've actually gone away from the shunt approach now, and instead measure the VDS of the MOSFET, including a PTC thermally bonded to the drain pad, this provides overtemperature, underdrive, overcurrent protection , and the PTC provides failsafe protection; I'm measuring the VDS using a 2N7002 in semiparallel to the big MOSFET, surprisingly simple, yet accurate, the VDS measured is large enough to measure directly with  a CPU, (4A = about 100mV = 50bits on ADC)
 

Offline T3sl4co1l

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Re: High current PCB design
« Reply #13 on: December 23, 2016, 12:24:01 am »
Note that copper shunts are essentially useless at high frequencies, above about 10kHz.  Around that frequency, the inductive reactance dominates, no matter how you Kelvin-connect it.

(The high frequency "figure of merit" of a resistive structure is proportional to:
- How similar its resistivity is to the impedance of free space (scaled by a geometry factor),
- Its equivalent line length, as a transmission line (at low frequencies, the equivalent inductance thereof; less is better)
- Ratio of transmission line impedance to circuit impedance (i.e., nominal resistance!)
Metals are all relatively conductive, so it's generally advantageous to choose the metal or alloy with the highest possible resistivity.  Composite materials, with even higher average resistivity, can go "too far" and the problem becomes capacitance instead.  Hence why high value resistors appear capacitive at high frequencies, and low values appear inductive.)

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 


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