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Old 03-03-2010, 05:38 PM   #3051 (permalink)
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Paul,

I have used FreePCB before with PCBcart and everything worked just fine. They were able to read the gerber files and everything went well. FreePCB has both a layout portion and a routing portion. It isn't as good as the ones you can buy but I laid out a 12 channel BMS design and it didn't require too much hand routing. Let me know if I can help.....

Rick

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Old 03-03-2010, 06:54 PM   #3052 (permalink)
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Holy cow, are you serious!?!? I have that downloaded right now! Go China Go! hahahaha! Thank you!!!!1
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Old 03-03-2010, 08:23 PM   #3053 (permalink)
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Originally Posted by ElectricZX2 View Post
Hard anodizing the heat spreader seems like a great idea for electrical isolation. The only thing I would worry about would be the degradation of thermal conductivity. I get what you're saying about the layer being very thin Camlight, but it might be nice to have some numbers. Type III anodizing is used in industry to provide a thermal barrier in addition to an electrical barrier in some applications (which makes sense since thermal and electrical conductivity are generally directly correlated in metals). For example, part of the reason pistons in ICE engines are hard anodized is to reduce the thermal conductivity of the piston and therefore mitigate thermal expansion allowing for tighter tolerances....
With the heat sinks used in my 400W electronic loads, using four TO-264 MOSFETs, the sink-to-ambient thermal resistance (of my heat sink and fan combo) was increased approximately 0.04C/W over the bare aluminum number. This is at least 10x better than any other material that can be used.

Don't underestimate the power of being thin.
Type III anodizing, while being a VERY efficient insulator (electrically and thermally) is so thin when applied to a heat sink that it really doesn't matter. Yea, if you increase the thickness you can probably block a blowtorch, but when thin, it allows enough heat to flow vs. its dielectric strength (ability to electricall insulate) that there's just about no other material that can match it. Certainly not any thermal pad, mica, kapton tape, etc. They might be able to if they were 1/2 mil thick...but they're not.

Do you know how thick the Type III lining was for those ICE piston? I suspect it's more for reduced friction and wear-resistance than heat-flow blocking...unless a very thick layer was applied.
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Old 03-03-2010, 08:31 PM   #3054 (permalink)
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Originally Posted by williamson View Post
Has anyone researched a CPU cooler? It uses heat-pipes to transfer the heat. The pipes contain a liquid, most likely alcohol, that evaporates and absorbs a lot of heat. The fan then condenses it back. I would guess this must be operated with a designated side down, so the liquid flows. They are sold cheap on ebay. Such as # 160407436678. Of course, it's small. BTW, in case anyone wants to know trivia, the miller capacitance is the multiplication of the real gate to drain capacitance, by the drain voltage rise. It slows the turn-off.
Liquid-cooling, heat-pipes, and other similar cooling methods can be very effective if built and set up properly. Unfortunately, every single liquid-cooling and heat-pipe device for CPUs are no more effective than a good heat sink with a fan. And I did days and days of research on this when selecting a cooling method for my electronic loads.

These coolers look great, but the tolerances are just too loose to be of any use. Effective units are VERY expensive and are only of use where the point sources of heat are so intense that it's worth the extra space, time, sensitivity to vibration and shock, etc.

Also, these units only take the heat away from a point source. You still have to get the heat transferred to the air and that takes space and $$$ vs. a heat sink and fan that do it directly and are much more robust in an environment where vibration or shock might be present.
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Old 03-03-2010, 08:42 PM   #3055 (permalink)
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Originally Posted by chapper View Post
The lower drawing (The Off-Road Controller) shows the same configuration, but it’s held together with anodized aluminum clamps for true electrical isolation and equalized surface clamping forces. It avoids screw threads abrading their tapped holes and potentially contacting different voltages. (In the upper drawing, note the 3 screws holding the buss bars to the heatsink. What happens if…?)

Silver plate the buss bars and PCB with “Cool-Amp” Conducto-Lube Co.
Between the MosFETs, diodes and buss bars use an electrically and thermally conductive compound like “Conducto-Lube”. Conducto-Lube Co.
Between the circuit board copper and the buss bars, use an electrically conductive compound like “Conducto-Lube”
Between the buss bars, and the aluminum heatsink, use a thermally conductive compound like: Aavid “Ultra Stick”
All anodizing is Type III
- The silver plating might help reduce any galvanic corrosion that might occur between two metals that are far apart on the activity scale but the coating has to be very, very smooth (almost like a mirror). Any change in the surface roughness of the surface can significantly affect its ability to transfer heat. It can prevent the oxidation of copper surfaces exposed to air. But, if that surface is exposed to air, it's not touching the other surface it need to in order the transfer heat or current.
- Is Conducto-Lube rated for the current density you're looking at (amps/cm-squared or similar)?
- How does the electrical conductivity of Conducto-Lube compare to that of copper itself? Might more pressure along the metal-to-metal interface be better than using Conducto-Lube? I don't know, just thinking out loud.
- Could the bus bars be made of aluminum? Well, they can, of course, but is it worth it? Then selectively plating the heat sink can allow it to be both the bus bar and heat sink. It's then easier to assemble and the heat-shedding efficiency goes up.
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Old 03-03-2010, 08:48 PM   #3056 (permalink)
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Originally Posted by jyanof View Post
At cruising speeds, ~200 motor amps, the total calculated heat generation was roughly 300W. Roughly 2/3 of that were from the 10 mosfets, so that's only 20W per mosfet. When I went through the calculations, I think I was predicting a junction temp somewhere around 80C at these steady state conditions in the hot AZ weather. The heat generation is roughly proportional to motor amps (duty cycle has an effect too), so conceivably 400A would have twice the heat generation. Accelerations would also create excursions into higher temperatures, though they often occur after coasting or sitting still for several tens of seconds which allow enough time for the controller to cool down a little bit.
Hi Joe,
Thanks for the links, I'll check them out.

Sounds like you have a good cooling system already in place...no need for heat pipes.

I'm a bit sleep-deprived right now but isn't the heat generated by the static resistance of a MOSFET proportional to the square of the current? That is, going from 200A to 400A would increase the power (heat) that needs to be dissipated by four times (Power = Current-squared times Resistance)?
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Old 03-03-2010, 08:48 PM   #3057 (permalink)
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Originally Posted by CamLight View Post
Don't underestimate the power of being thin.
Don't tell that to Oprah...

OK, McMaster Carr sells 2"x1"x12" pieces. That's a little longer than I would want, and it wouldn't be drilled yet, so I'm a little worried about that. I read that there are a few materials that can remove the coating over the areas where I'd have to drill it. Like Lye (which doesn't get along with Aluminum at all) and some other weird name like TPE or something. I don't know! man!
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Old 03-03-2010, 08:55 PM   #3058 (permalink)
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Originally Posted by MPaulHolmes View Post
I'm almost done with a power section layout for the SR controller. I'm sending it to Advanced Circuits, since they do stuff with 4 ounce copper now. The plan is to drill holes and mill the sides and top flush of an approximately 10"x1"x2" aluminum block, send it in to be type 3 anodized, and use that as the heat spreader. The company that does it can do 1 to 2 mil thick anodizing, and they say it's 2000-3000V/mil of isolation, and as hard as saphire! The SR control boards should be here either tomorrow or Thursday.

Oh I just did some figurin' about how much energy the aluminum block can absorb. That will give me a loose idea as to how much energy can be dumped into it in short bursts:

Specific Heat Capacity of Aluminum is 900 J/(m*degC). My Aluminum block will have a mass of 0.889 kg. If I consider a change of 30 degC for the aluminum block to be acceptable for short periods of time, the amount of energy the block can absorb is:

900*0.889*30 J = 24000 J.

Now, at 1000amps, if it's wasting 2000 watts of power (oh please don't be that bad!), it will take the heat spreader 12 seconds before the temperature has raised by 30 degC. At that point, the software can throttle back a bit, and allow the energy to slowly seep away.

I'm not sure if my reasoning is right, but it makes sense to me! Please tell me if I'm not understanding something! ya!
Great news about the anodizing! Get the thinnest layer they offer.

I'm worried about using energy calculations for cooling design. But, as mentioned earlier, I'm sleep deprived and may be missing something. The thermal resistance of the heat spreader will cause the temperature near the MOSFETs to be much higher than that at the other end of the spreader. Don't the energy calculations assume that the heat is spread through the entire header immediately?

If true, then the aluminum spreader can take 24KJ but the temperature at the MOSFETs might be 300C and the other end of the spreader might be 30C...until it all reaches thermal equilibrium, that is.
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Old 03-03-2010, 08:58 PM   #3059 (permalink)
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Hey! The control boards came today for the SR controller!

There are a couple details that I'm going to redo next time, but nothing serious so far.

Don't worry! It's lead free solder they used for the plating.
Great news!
For both the project and your son.
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Old 03-03-2010, 09:08 PM   #3060 (permalink)
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Don't tell that to Oprah...

OK, McMaster Carr sells 2"x1"x12" pieces. That's a little longer than I would want, and it wouldn't be drilled yet, so I'm a little worried about that. I read that there are a few materials that can remove the coating over the areas where I'd have to drill it. Like Lye (which doesn't get along with Aluminum at all) and some other weird name like TPE or something. I don't know! man!
You mean removing hardcoat anodizing?
Don't do it with chemicals! Very, very nasty. Actually, I'm amazed it can be done that way.

Just use a center punch to mark the hole and penetrate the anodizing and then drill your holes. Use high-quality hardened bits (not HSS!) and have a few of them laying around as they'll dull quickly if you have a lot of holes to drill.

I'm slightly worried about using McMaster's anodized aluminum as they don't mention the surface thickness of the anodizing. If it's 2mils+, as is often done for just hardening a surface, the thermal resistance will be much higher than the 0.5mil surface (0.5mil penetration) thickness recommended for heat sink use.

But, a few tests can easily determine the thermal resistance of their anodized aluminum vs a raw Al bar.

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