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?
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?
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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.
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.
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
)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.
Well, I think that having a too large heatsink and having it thermally connected to the case is not so good. An exampe is the MBL 9011 that only sounds good if never turned off. But maybe that depends on something else.
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One device on one heatsink. Then YES the heatsink can cease to do anymore than a smaller heatsink.
The case of the IC or the Semiconductor can only remove so much heat from the silicon wafer.
If you want to increase power then increase the number of devices on a large heatsink. Each device will be dissipating less and the heatsink will be getting rid of the heat.
The case of the IC or the Semiconductor can only remove so much heat from the silicon wafer.
If you want to increase power then increase the number of devices on a large heatsink. Each device will be dissipating less and the heatsink will be getting rid of the heat.
How can something thermally run away that dissipates the energy as fast as you apply it.
You are limited to the Safe Operating Parameters of the device regardless of heat sink size.
You can still have thermal runaway on a huge heatsink if the SOAR is exceeded.
It's all down to how much heat can be disspated from the case to the heatsink.
If the heatsink is COOL, ie massive, and the junction is trying to disspiate too much power then it will still overheat.
It's all down to how much heat can be disspated from the case to the heatsink.
If the heatsink is COOL, ie massive, and the junction is trying to disspiate too much power then it will still overheat.
You could mount a 200W transistor in the middle of the Antartic which would keep the case cold. It will stiil fail at the SOAR ie 200W.
Have a look at the datasheets. Case temp is simply a de-rating factor.
Have a look at the datasheets. Case temp is simply a de-rating factor.
I am not saying it is true. This is my absurd "theory" why some users might prefer sound of LM1875, LM3886 while running them hot. Related only to an ethereal part of DIY nothing more
Agreed. And thermal noise is another beast to handle. So I believe both would benefit from being cryogenically cooled.
Otherwise at room temperature heatsink that dissipates sufficient power to prevent internal thermal damaged would be OK for both everything else going to be overkill imho.
Yes. Cordell gave gave us the equation and the method for determining just that.
Ethereal about an LM3886 intagrated amplifier?
Sounds like plating a hammer with gold.
Nope that was originally related to heatsink - temperature - transconductance not to IC vs discreet.
Once again that was my pretty voluntary assumption regarding possible explanation why some listeners prefer sound of semiconductors being running hot.
I am primerly a tube guy BTW but respect modern ICs a lot
If a system is temperature-compensated (ie a typical class AB amplifier), a heatsink could be too big.
Under usual conditions, the tempco of a Si junction is ~2mV/K.
Thanks to the exponential relationship between voltage and junction current, the 2mV will translate into a exp(2/26) increase per degree, ~8%/K.
If the collector voltage is ~constant vs unwanted current variations (normally a good assumption), the increase in dissipation for each degree will be 0.08*Pstatic.
Thus, without any other limiting mechanism, and on an infinite heatsink the system will become unstable if:
Rth(j-C)>1/0.08*Pstatic.
Such a situation could be encountered in amplifiers having no emitter degeneration resistors, as there are some prominent examples on this forum.
Note that the instability condition does not have to be present for a long time for the runaway to occur: the thermal capacity of the junction and its surrounding is very small, and a thermal transient of milliseconds could be sufficient to trigger the thermal runaway.
Under usual conditions, the tempco of a Si junction is ~2mV/K.
Thanks to the exponential relationship between voltage and junction current, the 2mV will translate into a exp(2/26) increase per degree, ~8%/K.
If the collector voltage is ~constant vs unwanted current variations (normally a good assumption), the increase in dissipation for each degree will be 0.08*Pstatic.
Thus, without any other limiting mechanism, and on an infinite heatsink the system will become unstable if:
Rth(j-C)>1/0.08*Pstatic.
Such a situation could be encountered in amplifiers having no emitter degeneration resistors, as there are some prominent examples on this forum.
Note that the instability condition does not have to be present for a long time for the runaway to occur: the thermal capacity of the junction and its surrounding is very small, and a thermal transient of milliseconds could be sufficient to trigger the thermal runaway.
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