I wanted to know is there a calculation to determine the ideal size of a polygon-poured heatsink for an SMD voltage regulator. Also related to this, how many vias should I use, and is there an easy way to solder on a regulator that has a large heatsink?
For example, I have a voltage regulator will be putting out somewhere around 5.5W of heat. I was thinking to do a double sided polygon pour to use both sides of the pcb board, and thus increase the surface area of the heatsink. Im using EAGLE 5.11.0 Light.
several example in this: http://www.ti.com/lit/ds/symlink/lm1117-n.pdf
but I don’t think 5.5W is doable
So maybe I should be using a switching regulator instead of a 7805 in my situation. That should reduce my heat output from 5.5w to 0.625W. Thanks for the link.
While you could do 5.5W with large pours and lots of thermal vias (preferably with thicker copper), I concur that you need to look at switching regulators at this power level.
To solder components directly to a ground plane I’d suggest solder paste and either use a hot-plate or hot air (even a paint-stripper gun). Even the best soldering irons would REALLY struggle with this.
MichaelN:
While you could do 5.5W with large pours and lots of thermal vias (preferably with thicker copper), I concur that you need to look at switching regulators at this power level.
To solder components directly to a ground plane I’d suggest solder paste and either use a hot-plate or hot air (even a paint-stripper gun). Even the best soldering irons would REALLY struggle with this.
Well it would be its own dedicated grounding plane, and wouldnt cover the entire board. I managed to solder an LM317 to a grounding plane just the other day, but I have to admit it took a tad longer to get the part soldered on than it usually does.
Is there a way to determine the number of thermal vias I should use? I dont think I can do thicker copper, since Im doing my boards through OSH Park.
UNTEngineer:
Is there a way to determine the number of thermal vias I should use? I dont think I can do thicker copper, since Im doing my boards through OSH Park.
You can calculate the thermal resistance of the vias if you know the copper thickness. I think this is normally about half the 35 micron that the board itself is plated to (i.e., say 18 micron).
The thermal conductivity of copper is around 400W.m-1.K-1.
[EDITED to correct formula]
Thermal resistance = (Thickness in meters) / (Thermal Conductivity X Cross sectional area in square meters).
For example, a 0.6mm via with 18 micron copper, through 1.6mm PCB, would have a thermal resistance of about 118°C / watt. That is, if you had 100 vias like this conducting a total of 1W, there would be about 1.18°C temperature difference across the vias.
This could be improved by filling the vias with solder (thermal conductivity = 50W.m-1.K-1 for tin/lead solder and 60W.m-1.K-1 for lead-free solder). In the above example, you would approximately halve the thermal resistance by filling the vias with solder.
Thanks for the analysis. Do you have any reading material that covers this more extensively?
UNTEngineer:
Do you have any reading material that covers this more extensively?
Not really - it's mostly knowledge I built up over time. You could try the datasheets / application data for various high-power LEDs, as this often gives good info on designing PCBs to readily remove heat from the chip. For example, Luxeon Rebel:
http://www.philipslumileds.com/uploads/10/AB33-pdf
My calculations above only cover the thermal resistance of the vias. You should also consider the thermal resistance of the semiconductor package (junction to pads - should be in the datasheet) and between the pads and the vias.
You don’t normally have to go overboard though, as the thermal resistance from the heatsink surface (in this case, the PCB copper) to ambient is normally much higher than all the other factors. Unfortunately, it is also the most difficult to calculate, as it is highly dependent on airflow, PCB orientation etc. I know there are some “rules of thumb” for estimating this, but I don’t have one at hand - Google will help find them.
I managed to find a few more documents provided by TI that walk through how to calculate these figures. I think the most useful is titled “AN-2020 Thermal Design By Insight, Not Hindsight”, document ID: SNVA419B.
Thanks again for the info.
UNTEngineer:
I managed to find a few more documents provided by TI that walk through how to calculate these figures. I think the most useful is titled “AN-2020 Thermal Design By Insight, Not Hindsight”, document ID: SNVA419B.
Thanks - looks like a useful document.
http://www.ti.com/lit/an/snva419b/snva419b.pdf
