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Chip Amps Amplifiers based on integrated circuits

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Old 22nd August 2012, 03:29 PM   #171
bcmbob is offline bcmbob  United States
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Andrew, I shoot for that line (intersection)where the top of the chip meets the heat sink to be consistent. There is a range of readings on different areas on the chip itself. Is there a better way to get closer to the exact temp?
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Last edited by bcmbob; 22nd August 2012 at 03:35 PM.
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Old 22nd August 2012, 03:43 PM   #172
AndrewT is offline AndrewT  Scotland
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the temperature gradient along or across a conductor is determined by the rate of heat flow and the cross sectional area and the thermal resistance of the material.
Most of the heat flow of the metal heatsinking tab is across the thickness, i.e. from Tj to heatsink.
There is some heatflow from Tj near the middle of the chip to the big area formed by the tab backplate. But the thermal resistance and the area for conducting result in a lowish thermal gradient across the tab. If you could measure at different points on the tab you will find that they are at similar temperatures. This quite effectively confirms for you that the Tc of the device is within a couple of degrees of any of the tab, whether you measure the front face of the tab or the back face of the tab.
What I would not do is measure out at the corner of the tab farthest away from Tj.

But when looking at the plastic package the thermal gradients are very much different.
The thermal resistance is orders of magnitude higher and that gives rise to very large differences in temperature around different parts of the package. The surface of the package is generally much cooler than Tj and usually cooler than Tc. It is for this reason that I cannot see the logic in placing a temperature compensating device on top of a plastic packaged device. The thermal gradient completely defeats the accuracy of monitoring either Tj or Tc.
D.Self did experiments and published recommendations for how to locate the Vbe compensating device on the metal packaged To3 output device. Never try to extrapolate that to To247/To264/To3p devices.
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Old 22nd August 2012, 04:13 PM   #173
bcmbob is offline bcmbob  United States
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Well as you know, I'm all about what I hear. Since I have enough leftovers from a couple PC liquid cooling projects to build another rig, I am going to apply that same technique to a MyRef pair (probably the stock V1.2s). The pump has a wide range speed/flow adjustment that will allow everything from cool to almost no heat transfer at all.

I don't have too much of a concern about the amps stability as related to heat, but I dying to find out if I can hear a difference or notice a performance change at various settings. I will contact you directly when I get the system built.
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Last edited by bcmbob; 22nd August 2012 at 04:34 PM.
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Old 22nd August 2012, 07:28 PM   #174
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Are you sure you don't mind the noise caused by these machines (fans and pumps)? The LM3886 in very hot days here never went hot on average listening levels, it was warm. I did use small heatsinks for LM317s as I did not want to follow the datasheets, TBO.

What is the story behind keeping the ICs super cool Then you can use thermoelectric cooling, I am sure it is a silent colleague :P

Next step, I want to wind my own transformers, I read too much about it, and learnt how to correctly do it, it will take sometime before I finish two transformers.
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Old 22nd August 2012, 08:08 PM   #175
bcmbob is offline bcmbob  United States
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Metal, I cheated a bit and you caught me. What is shown in the pic is well oversized and will only be used for the base experiment, That stuff is on a 120 cm fan base/standard. If there is a sensible advantage everything would be downsized possibly all the way to 60 cm. I have built some coolers that can't be heard 3 feet away. Wont know if it's even worth the effort till I play around a bit. These pumps are virtually silent.

And no, even though I know how to use it, I'm not going to try liquid nitrogen.

Ok Andrew - Talk to me This is 82 db at 12 feet

Ta Temperature Ambient
Tc Case Temperature
Tj Junction Temperature
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Old 23rd August 2012, 07:57 AM   #176
AndrewT is offline AndrewT  Scotland
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Those temps on the tabs are good guides to the TC of the chip.
These are quite low @ 111F to 115F.
The markings on the left diagram are wrong.
The TC are plastic case surface temperatures. They have no close relationship to Tc.
The Tj are tab temperatures. Thes are close to Tc, certainly not Tj which is the junction temperature of the chip, buried way inside the device.
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Old 23rd August 2012, 08:27 AM   #177
bcmbob is offline bcmbob  United States
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Quote:
Originally Posted by AndrewT View Post
certainly not Tj which is the junction temperature of the chip, buried way inside the device.
So there is actually no way to measure Tj without a formula, correct? And I would assume the self-protection is looking at Tj also?
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Last edited by bcmbob; 23rd August 2012 at 08:30 AM.
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Old 23rd August 2012, 08:35 AM   #178
AndrewT is offline AndrewT  Scotland
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The manufacturers may have ways of measuring Tj, we don't.
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Old 23rd August 2012, 08:56 AM   #179
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Hi Bob,

there is another way for "silent" cooling of the chips. You can combine two alu-extruded heatsinks so that the fins are "looking" towards each other.

For example this one and another one rotated 180.

Click the image to open in full size.

The two gaps on the outer side can be sealed with adhesive tape. On one of the short sides you mount the cooler. The other short side leads straight out of the case. You get the idea looking at the picture below

Click the image to open in full size.

Just an idea.

Greets
Sven
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Old 23rd August 2012, 08:58 AM   #180
bcmbob is offline bcmbob  United States
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Thanks Andrew.

metal - I didn't comment on part of your question. Theoretically and as suggested by the data sheet, there should be a temperature or small range where the chip operates at optimal performance. I'm just curious to see if that can be both controlled and if it is detectable by ear.

bK - That looks effective, but for my experiment I need something that is not passive as in the sentence above.
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Last edited by bcmbob; 23rd August 2012 at 09:03 AM.
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