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Old 3rd December 2011, 07:01 AM   #1
jitter is offline jitter  Netherlands
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Default Can a heatsink be too big?

Heatsinks that are too small obviously increase the risk of components failing due to overheating, but I was wondering if heatsinks could also be too big.
Sure, big heatsinks are expensive and bulky, but for this topic, lets focus solely on the electronic issues.

Do you know of any situations and/or circuits in which a bigger than neccessary heatsink is undesirable?

Last edited by jitter; 3rd December 2011 at 07:06 AM.
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Old 3rd December 2011, 07:22 AM   #2
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No. There are none.
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Old 3rd December 2011, 07:35 AM   #3
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Quote:
Originally Posted by jitter View Post
Do you know of any situations?
Try this :

Click the image to open in full size.

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Old 3rd December 2011, 07:49 AM   #4
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Do you know of any situations and/or circuits in which a bigger than neccessary heatsink is undesirable?
In case of semi-conductors resistance is decreasing as temperature of device increases. So for an output transistor of a power amplifier or IC like LM3886 might be better operates on a warm side within specification limits of course. Larger heat sink will not let run BJTs hot enough for just a bit more appealing sounding. But this is very ethereal subject actually.
Technically too big heatsink will not affect device operation but will increase size, weight and cost without any positive benefit.
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Old 3rd December 2011, 08:33 AM   #5
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You might like to place a Vbe multiplier transistor a bit closer to output devices for better thermal coupling when using a very huge heatsink, waste of good aluminium aside.
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Old 3rd December 2011, 09:30 AM   #6
AndrewT is online now AndrewT  Scotland
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I agree with Dark & Zeon. Heatsinks can never be "too big".

I completely disagree with Sun.
Quote:
So for an output transistor of a power amplifier or IC like LM3886 might be better operates on a warm side within specification limits of course. Larger heat sink will not let run BJTs hot enough for just a bit more appealing sounding.
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Old 3rd December 2011, 10:24 AM   #7
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If you can afford it, go for the heatsink that doesn't require any fan cooling.

Fans are noisy.
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Old 3rd December 2011, 11:28 AM   #8
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BJTs like to run COOL.

MOS-FETs can run HOT.

COOL is better - THERMAL STABILITY is paramount.
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Old 3rd December 2011, 11:30 AM   #9
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Quote:
Originally Posted by jitter View Post
any situations and/or circuits in which a bigger than neccessary heatsink is undesirable?
Merely theoretical jive, but a Class A power amp with an infinite size heatsink is pretty unsmart.

(any amp that doesn't reach thermal stasis in less than an hour is unpractical, save for folks who do not hear a difference anyway )
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Old 3rd December 2011, 11:31 AM   #10
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I am not so sure, if the heatsink is too large, the thermal rise of the output devices will be dominated by die to case and insulation thermal resistance. This means that an external Vbe multiplier cannot sense the die temperature effectively and runaway is possible. Devices with built in diode sensors like Sanken used to do avoid this problem, chipamps should also be safe.
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