How much heat sink do I need to dissipate 100W? (FAN okay) - Page 1
Forum section: electronics / projects

author: NEDM64
author_rank: Contributor
timestamp: October 04, 2021, 08:41:59 pm
content: I want to dissipate 100W out of a TO-220 MOSFET, but I don't know how much heat sink do I need. I've watched Dave Jones video and it's a good one, but most heatsinks don't have any data with them.So, if you have any experience with using heatsinks with mosfet, please can you tell me how much do I need? I'm okay with adding a 80mm fan and divide the load with multiple mosfets.Target temperature of 80C or below.

author: Benta
author_rank: Super Contributor
timestamp: October 04, 2021, 09:28:56 pm
content: First, please explain how you intend to dissipate 100 W from a TO-220 package. Cryogenic cooling?Thenwe'll help you with heatsink calculations. What you're trying to do is totally unrealistic.

author: tunk
author_rank: Super Contributor
timestamp: October 04, 2021, 09:34:13 pm
content: Couldn't say anything about realism, but IIRC many CPU heatsink+fans are around there.

author: bdunham7
author_rank: Super Contributor
timestamp: October 04, 2021, 09:43:29 pm
content: Is your goal to have a Tjof 80C? You'll need to look at the device datasheet to see the number for junction-to-case W/K to see if that would work even if you kept the case at room temperature. Assuming not, you'll need multiple devices. As mentioned, big CPU coolers approximate this performance, but that's with heat pipes, a lot of fins a and a powerful fan.

author: TimFox
author_rank: Super Contributor
timestamp: October 04, 2021, 09:56:52 pm
content: You must start with the datasheet of the actual TO-220 device you want.A typical value of junction-to-case (absolute minimum thermal resistance for the part) is around 0.5 K/W for a TO-220 package, so a perfect heat sink (the hull of a battleship in the North Atlantic) at 25oC would get the junction to 25 + 50 = 75oC. If you are trying to keep the heat sink itself at 80oC, then the junction would get to at least 130oC, which is not conservative for a best case scenario. Don't forget to add the thermal resistance of a greased flat joint (not much) or that of a greased insulating pad (more) to the estimate of total thermal resistance.

author: Benta
author_rank: Super Contributor
timestamp: October 04, 2021, 10:02:46 pm
content: I think you all missed my point:The maximum power dissipation from a TO-220 package is 70...75 W at acasetemperature of 25 C.The OP wants to dissipate 100 W. Completely utopic.

author: TimFox
author_rank: Super Contributor
timestamp: October 04, 2021, 10:15:42 pm
content: Actually, there are some MOSFETs in TO-220 packages that are rated for substantially more than 100 W at 25oC case (actually, tab) temperature. That’s only a parameter to calculate the safe power by derating, using the maximum junction and the thermal resistance calculated from that power and the two temperatures.

author: bdunham7
author_rank: Super Contributor
timestamp: October 04, 2021, 10:17:31 pm
content: Quote from: Benta on October 04, 2021, 10:02:46 pmWe didn't miss it, we were explaining it. And while TO-3P or TO-247 might be better choices (and I suspect the OP would be fine with those instead of TO-220 specifically), here's an example of a Power MOSFET that does 500W and 4W/K (in TO-3P) and 2W/K in actual TO-220, at a Tjof 175C. So that is 300W continuous with a 150C gradient even in TO-220, provided you have a battleship hull and push the margins to the limit.https://www.mouser.com/datasheet/2/240/ixyss05255_1-2272147.pdfAnd he did say multiple devices were OK. He just needs to select a device and read the datasheet.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: October 04, 2021, 11:46:57 pm
content: if you find an old desktop PC , Pentium 4 3Mhz - heatsink+fan suppose to dissipate 80W at +65C. it quite large one, it will handle 100W at higher temp. so you can, in overall, figureout the size. more airflow, less heatsink size. more airflow and more fins -> less size.i have no idea of application, but count a thermal resistance between crystal and copper tab of package, and resistance between tab and heatsink. Means: instantaneous apply of some power on the crystal will increase the temperature beyond the safe range, due to heat can not dissipate instantaneously from the crystal to the tab to the heatsink, so the crystal will irreversibly damaged

author: langwadt
author_rank: Super Contributor
timestamp: October 05, 2021, 12:43:19 am
content: Quote from: bdunham7 on October 04, 2021, 10:17:31 pmI read an IRF app note on how the tests are done (I assume everyone does it that way)The transistor and test fixture is submerged in a liquid that boils at 25'C

author: Miyuki
author_rank: Frequent Contributor
timestamp: October 05, 2021, 07:14:11 am
content: Quote from: TimFox on October 04, 2021, 09:56:52 pmBeware!It is a lotGreased TO-220 to the heatsink is 0.5 K/W (this is a number for conservative calculation and is in most datasheets)

author: rs20
author_rank: Super Contributor
timestamp: October 05, 2021, 07:18:03 am
content: Just quoting the OP, lots of folks seemed to miss that they already mentioned:Quote from: NEDM64 on October 04, 2021, 08:41:59 pm

author: Siwastaja
author_rank: Super Contributor
timestamp: October 05, 2021, 07:31:57 am
content: Dividing the load over multiple parts is generally a good idea because today (even despite recent shortages), silicon is cheap and aluminium is expensive.By paralleling multiple parts, their RthJunction-Case and RthCase-Heatsink are paralleled. The result is, you can run the heatsink hotter while retaining low enough junction temperature.For example, for an imaginary part with RthJ-C (internal bonding from die to the tab) = 0.5 K/W and RthC-HS (your heatsink coupling imperfections, grease, etc...) = 0.5 K/W, by using one part, temperature gradient from junction to heatsink is defined by total RthJ-HS = 1 K/W, i.e., 100W causes 100K temperature difference. To keep Tj=125degC, heatsink must be at 25degC.Now if you parallel two parts, theyeachstill have total RthJ-HS = 1 K/W, but now each only dissipates 50W, so you have 50K temperature difference, so the heatsink can now run at 75degC. What a difference!You can calculate this using parallel thermal resistances as well and get the same result (two times 1K/W in parallel = 0.5K/W): 100W over 0.5 K/W = 50 K.Fan is exceptional to keep heatsink size and temperature down. Passive cooling is quite fine if you can accept high heatsink temperature, but then again you need to parallel even more MOSFETs. This is because the cooling effect of a passive heatsink is quite non-linear because natural convection increases cooling when the heatsink heats up. With a fan, you have strong cooling at low heatsink temperatures already.You can use a modern CPU cooler if you want. Pick one designed for a CPU with designed thermal load of approx. 100W, read the ratings of some PC enthusiasts who actually run benchmarks, you'll likely get actual Tcore readings which would equal to the resulting Tj quite nicely. Modern CPUs have heat spreaders which could kind-of equal to the surface area of two TO-220 parts in parallel.

author: MathWizard
author_rank: Super Contributor
timestamp: October 07, 2021, 12:02:44 am
content: I have a fairly small 1cm*6cm*12cm Aluminum heatsink with about 22 fins, the average thickness is 2mm on the base, and 1mm finsIn open air, it would very quickly go over 60C, with 5-10W entering it.Now with a small fan, I can pretty easily keep it at 45C, while dumping 35W into it. And very little noise, the fan was only using around 0.4W itself.The source of the 35W is 4 BJTs, I could go to 3 and be fine for my PSU build, but any less and they get too hot for my liking.I'm working a BJT temp controlled, fan circuit right now, and I'm surprised how little cooling this heatsink needs

author: Vovk_Z
author_rank: Super Contributor
timestamp: October 08, 2021, 10:45:18 pm
content: 1. 100 W is too much for a single TO220 case but 50 W is ok. So 100W is ok for two TO220 cases.2. I used an 80x80x26 mm heatsink with an 80 mm fan for 100 W active load and it was ok.

author: David Hess
author_rank: Super Contributor
timestamp: October 09, 2021, 02:02:18 am
content: Quote from: Benta on October 04, 2021, 10:02:46 pmThat pretty much covers it. I think International Rectifier was the first one to make TO-220 transistors, power MOSFETs obviously, with a 125 watt power dissipation but they relied on the largest lead-frame they could fit into the area of the TO-220 package. Now you can find many 125 watt parts but otherwise more typical TO-220 power transistors are in the 70 to 83 watt range.IXYS makes alleged 400 to 500 watt TO-220 parts but I do not think that represents any sort of continuous rating.As far as the heat sink, look at what is required for a 125 watt CPU, and realize that it would be overkill with the limited area on the back of a TO-220 package.

author: TheMG
author_rank: Frequent Contributor
timestamp: October 11, 2021, 06:19:14 pm
content: In my experience any more than about 50W continuous on a TO220 starts to get challenging, especially if the tab must be isolated from the heatsink due to the added thermal resistance. 75W doable with no insulator and a good thermal compound on a copper heatsink with a smooth surface finish.Above that, I would spread the heat dissipation between multiple devices, or go with a larger part such as TO247, which can easily do 150W in a practical application.This can be achieved easily enough with decent CPU coolers that are commonly available, but the limiting factor with TO220 is the small surface area of the package not the heatsink itself.