Fanning that heatsink! - Page 1
Forum section: electronics / projects

author: kiltro
author_rank: Contributor
timestamp: February 24, 2023, 05:03:56 pm
content: Hello everyone,for a future project I'll be using two power mosfet, IRFP250https://html.alldatasheet.com/html-pdf/22407/STMICROELECTRONICS/IRFP250/3247/2/IRFP250.htmlThey should disspiate 60W each... So if I'm not wrong heatsinking a mosfet would take(Tj-Ta)/P - Rjc- Rcs(100-50)/60 - 0.66 - 0.1 = 0.073°C/W .... and I need to mount both on the same heatsinkNow, how do people more expert then me usually solve this?I was thinking of mounting a big 12V fan on the heatsink but how can I estimate the new thermal resistance of the heatsink? Is there some formula for fans or similar?Thanks

author: jonpaul
author_rank: Super Contributor
timestamp: February 24, 2023, 05:11:07 pm
content: 120w...what's the device, allpliccation? if possible consider a switcher not linearread papers and book,on thermal management and design for electronicsj

author: kiltro
author_rank: Contributor
timestamp: February 24, 2023, 05:24:06 pm
content: Current sinkingDo you know any good book title?

author: macboy
author_rank: Super Contributor
timestamp: February 24, 2023, 08:17:31 pm
content: I've used a recycled heat pipe based CPU cooler for over 150W with minimal temperature rise. Mounting can be critical to effective heat transfer. I'd recommend a clamp rather than a simple screw.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: February 24, 2023, 08:31:21 pm
content: As JonPaul says, the best way to get rid of heat is not to generate it in the first place.Another degree of freedom to consider is a review of the specs. Often the allowable junction temperature can be pushed by 10-20 degrees. Maybe dissipation is lower when ambient is higher? Is the duration of the 60W event short? Is the high temperature diurnal (The daily variation) so that you can add heat capacity (waxes, or other phase change materials) reduce the high temp at the heat sink.If your case is extremely pressing you could consider lowering the heatsink ambient with active cooling (refrigerator of some sort).

author: Siwastaja
author_rank: Super Contributor
timestamp: February 25, 2023, 01:22:39 pm
content: Thermal simulation is complicated, there is no easy formula.Unless you want to buy a complete solution (heatsink + fan) which has Rth value (or graphs), testing is simplest, by far, for us mere mortals.Tj is hard to measure, but because As Rth_junction-case is known, and Rth_case-heatsink can be estimated (basically Rth of the thermal interface material), you can calculate maximum heatsink temperature you can live with, and measure heatsink temperature. Then you can try adding a fan to see if the temperature is OK under full load. If not, change to larger heatsink and/or bigger fan.

author: dietert1
author_rank: Super Contributor
timestamp: February 25, 2023, 02:48:25 pm
content: For example at ebay: "Fischer Elektronik Hochleistungs Lüfter LA-HL1 100mm Aggregat Kühl-Körper NEU" with 0.14 K/W.In order to improve mount, one can use more Mosfets. If there needs to be insulation, one can use a copper heat spreader to increase the coupling area, with insulation between copper and heatsink.Regards, Dieter

author: Siwastaja
author_rank: Super Contributor
timestamp: February 25, 2023, 04:19:26 pm
content: Quote from: edavid on February 25, 2023, 04:05:35 pmWhy not? DC SOA says 30A*5V = 150W, which is of course at +25degC and thus optimistic, as always, but I can see 60W could be actually viable.But it's marginal. With RthJ-C of 0.66 degC/W, that is 40degC already though, so OP needs nearly perfect heatsink as they have little temperature difference left. OP could raise Tj to 120degC, but then even more derating of SOA curve would be needed.So I kinda agree using less power per MOSFET (i.e., get one MOSFET more, or dissipate some of the power in resistors instead) would be a good idea, but 60W is not impossible.

author: bdunham7
author_rank: Super Contributor
timestamp: February 25, 2023, 07:03:28 pm
content: Quote from: kiltro on February 24, 2023, 05:03:56 pmWill you settle for a less expert opinion?Here's how I would work it out, one layer at a time:First, if you take a max Tjof 150C and a thermal resistance of 0.66C/W from junction to case, you get a drop of 40C and a maximum Tcaseof 110C. Or, if you take the part of the datasheet where it gives you a maximum power of 180W @ Tcase=25C with a derate of 1.44W/C, derating 120W (from 180 to 60) gives you an additional 83C to work with, or a maximum Tcaseof 108C. So I'd pick a design goal of an absolute maximum Tcaseof 100C. Toasty perhaps, but within specs. If this was something that was always going to run at this power level and I wanted it to have a long lifetime, I might lower that a bit.So now you look at the thermal resistance from the case to the heatsink and then from the heatsink to air. The datasheet says 0.1C/W, so for 60W that is 6C. Make it 10C just to be on the safe side and you now know you need the heatsink to be 90C or less at the point where the devices attach. If you assume an ambient air temperature of 30C or less (an assumption you'd have to consider, of course) which results in a delta of 60C, and then a total power dissipated of 120W, you see that you need a heatsink that can do 0.5C/W or better. Then you can just look at various commercial heatsink solutions to see what that looks like. In the case of a fan-cooled heatsink, it would be on the order of a large-ish CPU cooler. In a passive version, it would be a pretty big finned bar of aluminum.https://www.mouser.com/ProductDetail/Advanced-Thermal-Solutions/ATS-61600W-C2-R0?qs=Wj%2FVkw3K%252BMAbqk8JoMlu7g%3D%3Dhttps://www.mouser.com/ProductDetail/Wakefield-Vette/125460?qs=2WXlatMagcGW88U0l6XzPg%3D%3DUsing a device that had lower thermal resistance would help somewhat because you could run the heatsink hotter, but the difference wouldn't be so huge--less than twofold. Keep in mind that I haven't left a lot of safety margin and I could have made errors.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 25, 2023, 07:12:47 pm
content: Quote from: edavid on February 25, 2023, 05:43:54 pm"Too high" is subjective, here - 40degC, as shown above. Given OP's Ta=50degC, it is just barely workable. It's not impossible, but places pretty high requirements for the heatsink.But OK, let's say you disagree. What would you like to see instead of 40degC? 30? 20? Maybe 10?Improvement in RthJ-C goes into the heatsink requirement directly; so it's a tradeoff whether to use better/larger/more MOSFETs vs. using better/larger/more heatsinks. You can find cost optimum somewhere.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: February 25, 2023, 10:42:00 pm
content: Quote from: Siwastaja on February 25, 2023, 07:12:47 pmAlways remembering that cost may not be the only or most important requirement. The reality is that mathematical optimization frequently is impossible because of missing data or extreme complexity or .... Which is where terms like 'engineering judgment' and 'good enough' step in.

author: dietert1
author_rank: Super Contributor
timestamp: February 26, 2023, 07:55:30 am
content: In the datasheet of that "advanced thermal solutions" fanned heatsink at mouser the size table stops at 0.5 K/W. Times 120 W nominal gives you 60 K above ambient. Ambient temperature requirement is an assumption or from some legal prescription. Let's assume 50 °C. Then there is little headroom for the mount, i mean the thermal resistance from silicon to heatsink.Differences to that cost optimized CPU cooler are the coupling area, the melting pad and the spring loaded mount. If you won't have that, you wil probably need two of those mouser fanned heatsinks.Also when i look at a 120W chinese peltier cooler at ebay, it appears about twice the size.Regards, Dieter

author: cantata.tech
author_rank: Regular Contributor
timestamp: February 26, 2023, 08:26:30 am
content: Hint #1:Use passive cooling.If you have heat, then you can make a heat pump. You really don't need to consume extra electricity.Hint #2:Heat risesMake a system where cool air is pulled in from the bottom and goes over the hot part and rises. The air will pump itself.Use a one way valve at the bottom if you like to get more airflow.