Help with choosing / making heat sinks - Page 1
Forum section: electronics / beginners

author: Dan123456
author_rank: Regular Contributor
timestamp: November 12, 2023, 04:39:07 am
content: Hey allI am back working on my LM3875 56W audio amp project.As I want to crank it as hard as I can, the IC is going to need a heat sink to keep it cool.From what I can gather from the “power dissipation vs output power” graph on the top of page 9 of the data sheet (below), it can output ~45W of heat (or 48.8 using the formula on page 11) when being maxed out.https://www.digchip.com/datasheets/download_datasheet.php?id=514503&part-number=LM3875I have a bunch of old heat sinks that I have taken off old PCBs, CPU’s GPU’s etc. but, according to the heat sink calculator I found below, none of them are quite big enough (apart from the big CPU ones but they would be bigger than the entire board).https://celsiainc.com/resources/calculators/heat-sink-size-calculator/I do however have some big chunks of aluminium and a little mill in the shedWould making a heat sink be as easy as taking a Xcm3 chunk of aluminium and just milling a bunch of slots in it?I.e. using the numbers I put into the calculator of (from top to bottom) 50, 80, 30, 150, it says I need a heat sink of 91.4cm3. So if I just slotted the snot out of a 100cm3 chunk of aluminium, would that be sufficient/ correct?Apologies if I am completely misunderstanding this whole concept! This is my first “high power” project that really requires an appropriately sized heat sink so all of this is brand new to mePlease let me know if I am reading anything wrong / misunderstanding anything here

author: mariush
author_rank: Super Contributor
timestamp: November 12, 2023, 05:05:28 am
content: For that amount of heat dissipation you need active cooling.It would be safer / cheaper / easier to just get a cpu cooler with heatpipes and somehow drill a hole (without breaking heat pipes) in the base to be able to screw the chip to the base

author: helius
author_rank: Super Contributor
timestamp: November 12, 2023, 05:13:54 am
content: Instead of focusing on size, you should calculate the required thermal conductance that you require. A heatsink is simply one part of a cooling system the object of which is to provide sufficient thermal conductance.Thermal conductance expressed in W/K is a figure that describes how much heat power will flow across a gradient of one degree Kelvin. The reciprocal figure, thermal resistance in K/W is also used. A degree Kelvin is the same as a degree Celsius, so W/°C is also used; only the temperature difference matters, not the absolute temperature.Your amplifier IC data sheet specifies its maximum operating temperature, above which the device will not meet its specifications. Chips are binned in different grades depending on what temperature they can correctly operate at: Commercial grade being the lowest at up to 85°C, Industrial grade being higher and Military/Space grade being rated at 150°C. Typically engineers try to meet requirements using the lowest rated parts and then may specify a higher grade part to provide a safety margin. Equipment that will not be used in a metal foundry or a missile should be doable using commercial-grade parts.Now, knowing the maximum temperature of the IC, you must find the maximum ambient in which the device will be used. A sealed enclosure will have high ambient temperatures because of the lack of airflow between the inside and the environment. A vented case, and even better, a case with forced airflow will have ambient temperatures closer to what is outside in the environment.The difference between max operating temperature and max ambient provides the denominator in W/K. The numerator is the heat output that may be expected under maximum load conditions. This fraction is the figure of thermal conductance that you must engineer with your cooling solution. Thermal conductance, or the reciprocal thermal resistance, can be calculated from each element of the cooling apparatus. For example. the thermal resistance of the heatsink should be added to that of the thermal pad or grease used to make contact to the IC. These materials are commonly specified by their thermal resistivity (or thermal conductivity) which is also relative to their thickness in meters.This is all a way of saying that you won't be able to simply choose a block of metal based on its volume. The "heat sink" is not theultimatesink; it is just a part of a chain to provide sufficiently low thermal resistance to the heat flowing from the IC to the environment. You can sink away hundreds of watts using very tiny heatsinks:ifthe temperature gradient and the conductivity permits that. For example, if the latent heat of a material phase change is employed, then the effective temperature gradient becomes greater and more heat can be transferred. This can be as simple as placing an ice cube on your IC! (not necessarily a good plan due to ionic contaminants in water) or the use of engineered heat pipes which achieve similar effects.

author: coppercone2
author_rank: Super Contributor
timestamp: November 12, 2023, 05:54:23 am
content: honestly check digikey because the time you need to make a stupid heat sink vs the amount of time you need to trouble shoot an audio amp means that buying a heat sink might be quite worth it.Its mad work for something thats not electronics. Break 1 good end mill working in some deep trench and its not worth it at all.you just can't compete with extrusion

author: Ian.M
author_rank: Super Contributor
timestamp: November 12, 2023, 07:06:23 am
content: There is another fly in the ointment! Pure aluminium has good thermal conductivity but is so soft and gummy that it is a bitch to machine. Harder and tougher alloys are much easier to machine but have drastically lower thermal conductivity - some high strength alloys having less than half the thermal conductivity of pure aluminium.There is no way that milling a complete heatsink from billet will ever be cost effective unless you need to prototype one with very unusual size and shape constraints. Machining a heat spreader block to allow the use of (possibly multiple) existing heatsinks is a different prospect, as the machining is much much simpler - you only really care about getting the mounting holes in the right places and two faces smooth and parallel.You can get power resistors for heatsink mounting. A 15 ohm 100W one wont set you back too much, and in combination with a bench PSU and some reasonably accurate means of measuring surface temperature, can be used to characterise your existing heatsinks to determine their thermal resistance.

author: BrianHG
author_rank: Super Contributor
timestamp: November 12, 2023, 07:19:46 am
content: Like it or not, if you want overkill, sometimes going to a junk shop where you can find a broken high power stereo amp, power-supply, or even a ham radio transmitter will lead you to an existing heatsink and maybe even an enclosure / spare transformer & bulk DC caps.Even very old micro-computers which had huge analog supplies would contain huge heatsinks, transformers and caps.\All for a few $.

author: Dan123456
author_rank: Regular Contributor
timestamp: November 12, 2023, 01:39:24 pm
content: Quote from: helius on November 12, 2023, 05:13:54 amThank you so much for the detailed reply!When I was doing the calculations to get the 48.8 number I used the info from the data sheet that says:“It is also known that typically the thermal resistance, θJC (junction to case), of the LM3875 is 1 ̊C/W and that using Thermalloy Thermacote thermal compound provides a thermal resistance, θCS (case to heat sink), of about 0.2 ̊C/W as explained in the Heat Sinking section.”You are also correct the the max IC temp is 85CI just used 80C in the heat sink calcs as a bit of a “fudge factor” as figured it is better to overshoot the size I need than undershoot (same thing with upping the heat sink size from ~90cm3 to 100cm)I also set the “Volumetric Thermal Resistance (Rv)“ in the calculator to 150 which it calls “gentle air” as I plan to have a fan on the side of the case (maybe 2 - 1 sucking, 1 blowing like with a PC) so haven’t completely ignored the airflow situation either.With all that said, it sounds like machining a heat sink is not an option.In that case, how do I know which heat sink to buy?Do I just need something better than that 1C/W number (or the 0.2C/W number?) mentioned above?For instance, I found this 4.0W @ 20°C one below. Does that mean it can cool 4 times (or 20 times) more than I need? I.e. at 20C, it will take 4W to increase its temp by 1C?https://www.digikey.com.au/en/products/detail/aavid-thermal-division-of-boyd-corporation/6400BG/1625171Or is that the max it can handle so I would need to find a 50W @ 80C heat sink?Sorry again if I am completely misunderstanding this. This is all still half Greek to me… quite literally if you include all the symbols in the maths

author: mariush
author_rank: Super Contributor
timestamp: November 12, 2023, 03:12:00 pm
content: It says on the digikey page :Power Dissipation @ Temperature Rise 4.0W @ 20°CThermal Resistance @ Forced Air Flow 1.50°C/W @ 400 LFMThermal Resistance @ Natural 2.70°C/WUsing the one at the bottom, in a very simplified way, if your chip dissipates 10 watts, then heatsink temperature would rise 27 degrees above ambient.... so heatsink would be at around 50CWith forced airflow ... the page says 1.5 C/w at 400 LFM ... that's around 28 CFM for a 80x80 fan according tohttps://myheatsinks.com/calculate/lfm-to-cfm/Linear Flow: 400 LFMDuct Width: 80 mmDuct Height: 80 mmVolume Flow: 27.56 CFM (13.00 liter/sec)So you'd have to use 6-10$ fan -https://www.digikey.com.au/en/products/detail/sunon-fans/MF80251V3-1000U-A99/6198724- and still get only 1.5C/w ... let's say you're comfortable with heatsink going to 80c ... 80c-30c = 50c / 1.5 = ~33 wattsVery basic math, not accounting for thermal paste and other things...

author: Dan123456
author_rank: Regular Contributor
timestamp: November 13, 2023, 01:57:35 am
content: Quote from: mariush on November 12, 2023, 03:12:00 pmOoooh thank you so much! I was completely misunderstanding how to work with those heatsink numbers!I think I now understand enough to work out what I needThank you so much again!Just for my own curiosity, what does the Power Dissipation @ Temperature Rise of 4.0W @ 20°C number I was looking at mean then?Does that mean that, in ideal lab conditions / theoretically, 4W of power would raise the temperature from 0C to 20C or something like that?Edit: or the other way round. I.e. in perfect lab conditions it is theoretically able to dissipate 4W @ 20C?

author: coppercone2
author_rank: Super Contributor
timestamp: November 13, 2023, 02:17:03 am
content: it means that when there is 4 watts of heat flow, the temperature will increase 20 degrees compared to ambient.so 45C if your ambient is 25@ on digikey means "above ambient".