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Old 26th January 2013, 01:39 PM   #11
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I've heard of a 5 second rule, that if you can't keep your fingers on it for 5 seconds it's too hot. But this is DIY. Wouldn't you like a greater helping of reliability than you need for a 90 day warranty? Make the sink big enough to hold onto indefinitely, under any conditions.

Actually, if you were testing that sink flat on the bench and it barely passes the 5 second test, it will probably be OK with the sink and fins mounted vertically in free air.

What really matters is case temperature. I usually do the finger test on the transistor plastic case, right over the die.

Last edited by Andrew Eckhardt; 26th January 2013 at 01:46 PM.
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Old 26th January 2013, 02:10 PM   #12
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Originally Posted by Fusion916 View Post
is there a way to tell if I can dial up the bias current more without killing the output stage?
Calculate die temperature.

If die temperature is kept under 87.5C, long term reliability is in the clear for the C5200.
(87.5C => (150-25)/2 + 25C ambient)
It doesn't count how one deals with winning, but how to handle a loss (© DjT)
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Old 26th January 2013, 03:19 PM   #13
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The hot water out of your home taps is generally set to about 60 Degrees (Not always). You can hold your hand under the hottest tap water for about 5 secs.

The same rule is applied to using the human body to measure the heatsink temp. If you can hold on for 5-10 seconds the temperature is likely to be about 60-65 Degrees. This is considered good for long term reliability.

Tj (the die temperature) can be considerably more than this. Tcs (the case temperature marginally more). Most guys would aim to keep Tcs below about 70 Degrees - (80 Degrees is OK but long term reliabilty then comes into play).

Last edited by KatieandDad; 26th January 2013 at 03:22 PM.
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Old 26th January 2013, 03:52 PM   #14
sbrads is offline sbrads  United Kingdom
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I don't know why class A builders don't use fans. A slowed down large PC one with a reputation for quietness won't be audible and it will make a huge difference, although granted fans can fail so an over-temperature trip would be a good idea.
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Old 26th January 2013, 03:59 PM   #15
EUVL is offline EUVL  Europe
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Linear Audio Vol 3, "Design Considerations for a Class A Amplifier Enclosure"

Free download.

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Old 26th January 2013, 04:38 PM   #16
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60° hot heatsink is a sure way to degrade lifetime for ALL components in an Amp.
Regardless wich class.

I would never let the heatsink be the item to save money on when building a Amo.
If there is too little space for a good enough heatsink, the enclosure is too small.

I am avoiding using fans unless I am building amp for PA/stage-use.

Also I try not to use the enclosures other metal area as a part of the heatsink, especially in Class A-Amps. General component degrade is the reason.

Keeping enclosure in a pretty low temp is only obtained by designing the heatsink big enough to do the job all the time. I always try to keep any touchable surface on a temperature lower than 50°C
Sooner or later you end up with TANDBERG
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Old 26th January 2013, 06:38 PM   #17
EUVL is offline EUVL  Europe
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> 60° hot heatsink is a sure way to degrade lifetime for ALL components in an Amp.
> Regardless wich class.

That I do not agree, and I have explained that in detail in the article.
Above all if you just look at heat sink temperature, and ignore the thermal path all the way to the semiconductor substrate, you may well come to the wrong conclusions.
It is in the end the junction temperature that is of critical importance, and the Toshiba reliability handbook has illustrated this clearly and scientifically.

I do agree that lower operating temperatures is always beneficial.
But the benefits have diminishing returns at some point.
So my personal design guideline is still <=100°C junction temperature.
And this is much more meaningful than defining heat sink temperature.
You can start discussing where the heat sink temperature should be measured ....

But that is only my personal opinion,

Last edited by EUVL; 26th January 2013 at 06:45 PM.
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Old 26th January 2013, 07:09 PM   #18
sreten is offline sreten  United Kingdom
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How hot you can run a heatsink is not just to do with heatsink temperature.

How hot you should run an external heatsink is to do with safety and burns.

What really matters is the difference between device die temperature and
the heatsink temperature, the former is always more, its just like ohms law,
the temperature difference (voltage) creates the thermal flow (current)
through the devices and any insulation used total thermal resistance.

i.e. For 4 output devices you can run the heatsink hotter than for 2, as
the thermal resistances are halved, so are the temperature differences.

rgds, sreten.

Doubling output devices for class A when they are basically not
needed in terms of power handling, is a very good idea thermally.

Last edited by sreten; 26th January 2013 at 07:26 PM.
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Old 26th January 2013, 08:01 PM   #19
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Originally Posted by Fusion916 View Post
...this class A amp... ...I have be playing music for about a hour straight...
A class A amp runs cooler when there is music playing.
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Old 26th January 2013, 08:04 PM   #20
dmills is offline dmills  United Kingdom
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The device datasheets will usually quote thermal resistance numbers for various mounting methods, from which it is trivially to calculate how far above heatsink temperature the die is (Don't forget to add the resistance of any insulating washer).

For a non accessable heatsink, how hot it gets is mostly irellevant, it is die temerature that matters, and data on that will also be in the datasheet.

I tend to feel that touch tests are a very poor way to figure this as if the thermal resistance from the die is very low (A big device on a copper heat spreader as the RF power guys do, say) then a heatsink hot enough to cause second degree burns is not necassarily a problem, where a device with a poor thermal path may run too hot even with the heatsink only slightly above ambient.

Do the math, you might be surprised by what modern sand will run at with good reliability, electromigration is not the MBTF killer at high temperature that it once was.

And yes, more devices will allow the heatsink to run hotter for a given die temperature, which given the price of heatsinks Vs the price of sand can argue for quite extensive parallelism if it means the total heatsink area can be smaller.

Of course if the heatsink is external then touch temperatures come into it, but for internal heatsinks do the math and don't be afraid to run hot (Just not too close to the main capacitors).

73 Dan.
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