Electronic load - heatsinking / managing thermals - Page 1
Forum section: electronics / beginners

author: ifonlyeverything
author_rank: Regular Contributor
timestamp: October 30, 2022, 04:42:27 pm
content: I want to built a DIY electronic load using FDL100N50F since I believe this is best bang for the buck (please correct me if I'm wrong.) I have a few questions on estimating thermals before I order anything or blow something up.https://www.mouser.com/datasheet/2/149/FDL100N50F-1007460.pdf1. TO-264 lacks a conductive body tab like TO-220 so I don't need to worry about isolation pads or the like? I can just use a good thermal compound like Arctic Silver?2. Has anyone tried using server CPU heatsinks?https://www.mouser.com/ProductDetail/Delta-Electronics/DHS-B9292-04A?qs=%2FW4LtXOBxKu49uJSI41V0Q%3D%3Dis 80x80mm and lists ~0.24 C/W. TO-264 dimensions are roughly 20mm wide. I figure I could comfortably fit two MOSFETs per heatsink. My thoughts are to buy some of these low profile heatsinks (probably used/salvage parts on eBay, pretty cheap) and use them in sets with powerful fans, sandwiching the MOSFETs in the center and clamping everything together with bolts. Is anyone aware of a better options in terms of cooling performance/$, while remaining relatively compact and conducive to easy mechanical layout?3. I've read reports of people pushing 200-400W through a single FDL100N50F. Not sure how realistic this it, but I'm guessing roughly 0.47 C/W total REAL WORLD thermal resistance using something similar to the above heatsinks and some high quality thermal paste. At a target of 150 W dissipation per MOSFET, this gives me a junction temp of 95.5 C compared to a maximum of 150 C per the data sheet. Data sheet also claims 2500 W @ 25C junction with a 20W/C de-rating above 25 C, so 2500 W - (95.5 C * 20 W/C) = 590 W maximum. Does 150 W per MOSFET then sound reasonable or am I missing something?5. Any other tips or tricks that I should know of, specific to managing the heat / not blowing up the FETs? I'm going to start small with just 2 FETs and I know that each FET needs its own op amp for proper load sharing. The project will be controlled by a MCU so I'm also thinking about thermally gluing a "naked bead" thermocouple directly to the edge of the FET, to hopefully shut things down if something goes wrong.

author: ledtester
author_rank: Super Contributor
timestamp: October 30, 2022, 05:28:01 pm
content: Quote from: ifonlyeverything on October 30, 2022, 04:42:27 pmIt doesn't seem that the FDL100N50F isn't available (that is, in stock) from any of the major distributors (Mouser, Digikey, Newark, etc.)It is available for purchase on aliexpress but a lot of the pics show a metal tab on the back, e.g.:I would be wary of buying them on aliexpress. It is quite possible you won't get a genuine device.

author: Vovk_Z
author_rank: Super Contributor
timestamp: October 30, 2022, 05:40:56 pm
content: Quote from: ifonlyeverything on October 30, 2022, 04:42:27 pmMostly they are not sufficient for high power or are too bulky. I know only one type, I know it as 'IBM Server Heathink' which suits fine up to several hundreds of watts with a powerful noisy 70mm fan.

author: ifonlyeverything
author_rank: Regular Contributor
timestamp: October 30, 2022, 05:44:49 pm
content: Quote from: ledtester on October 30, 2022, 05:28:01 pmOctopart shows them in stock at a bunch of "non-authorized" sellers. I'm hoping I could buy from one of the more reputable ones, without getting killed on minimum order quantity or outrageous shipping & handling costs. I'd prefer to use the FDL100N50F but if that fails, I think my next choice is IXTK90N25L2 or similar (Kerry Wong's videos.) My only fear is screwing something up during testing and blowing up a $40 FET, rather than a $15 FET.

author: ifonlyeverything
author_rank: Regular Contributor
timestamp: October 30, 2022, 05:47:36 pm
content: Quote from: Vovk_Z on October 30, 2022, 05:40:56 pmWhy is a server heat sink insufficient, assuming I can find something comparable to the Delta heatsink I linked to? The Delta model shows a thermal resistance of just 0.24 C/W which should be fine -- or is there more to it than just thermal resistance? Sorry if this is a dumb question.

author: Vovk_Z
author_rank: Super Contributor
timestamp: October 30, 2022, 05:54:26 pm
content: Quote from: ifonlyeverything on October 30, 2022, 05:47:36 pmI haven't tried that one, so can't say much. It depends on how much power you want to dissipate on it and depends on a fan. Possibly it is fine up to 200W, IDK about more power.

author: Vovk_Z
author_rank: Super Contributor
timestamp: October 30, 2022, 05:59:43 pm
content: Quote from: ifonlyeverything on October 30, 2022, 04:42:27 pmTO-264 is like any other non-isolated case. It has a conductive tab too. For a high-power device, I would not isolate the transistor from a heatsink, but I would rather isolate the heatsink from a case (if the case will be metal type).

author: pqass
author_rank: Super Contributor
timestamp: October 30, 2022, 06:10:19 pm
content: Quote from: ifonlyeverything on October 30, 2022, 04:42:27 pmTO-264 have a metal tab underneath. See attachment.QuoteEvery FET should have its own opamp and shunt. If you make it a module then you can parallel a number of them to meet your current and power requirments. See my implementation here:https://www.eevblog.com/forum/testgear/example-of-why-people-say-you-sholdnt-use-mosfets-in-parrallel-as-dummy-load/msg4251337/#msg4251337. I've since attached it to an old AMD Athlon CPU heatsink. It should be good for 120W.A single MCU DAC output can feed all modules (opamp positive input). Each module's readback output is fed into an ADC input of its own or a non-inverting summing amplifier then a divide by, say, 2 (depends on # of modules) divider into one ADC input.An LM35 is easier to interface than a thermocouple; ~1°C accuracy.

author: ledtester
author_rank: Super Contributor
timestamp: October 30, 2022, 06:11:19 pm
content: You may some useful info in this thread:https://www.eevblog.com/forum/testgear/cheezeball-dc-load-dl24p-pump-or-dump/The "DL24P" uses a CPU heatsink and a few people have replaced the MOSFET with a FDL100N50F -- just search the topic for that keyword.

author: ifonlyeverything
author_rank: Regular Contributor
timestamp: November 01, 2022, 03:40:16 pm
content: I might just go with tried and true IRFP250N/IRFP250M because the shipping costs on these non-authorized FDL100N50F sellers are ridiculous. I'll target 4A and 40W per FET maximum since this is in line with commercial units (BK 8500/8514, Rigol DL31021/3031, Maynuo M9711, etc.)I plan on starting small with a CPU heatsink but my goal is to eventually scale this up to 20 FETs for 800W peak, which is obviously a lot of heat to dump. I would appreciate some help with my estimates or guesses. For heat sinking I am looking at 2" sections ofhttps://www.heatsinkusa.com/10-080-wide-extruded-aluminum-heatsink/per bank of 5 FETs with the goal of dissipating 200W. I keyed the specifications intohttps://myheatsinks.com/calculate/plate-fin-heat-sink-calculator/with the following assumptions and results.For calculation I assumed the FETs would be lumped into a single contiguous block of 16.5 mm x 21.5 mm per FET, whereas in practice they would be spaced out evenly and bolted to the heat sink with a thin copper backing and no insulating pads. I haven't thought about fans but I could fit 3 x 80mm case fans (high CFM models) per each 10" heat sink bank.My main concern with this estimate is the manufacturer's suggestion of 0.80 C/W for convection and "20% better for forced convection" per their FAQ, compared to my "estimate" of 0.25 C/W. This is a massive difference -- which do I believe?

author: Vovk_Z
author_rank: Super Contributor
timestamp: November 01, 2022, 05:13:33 pm
content: The hearhink on a picture is for a convective cooling and won''t work much-much better with a fan. I am on a weak internet (on a smartphone) so possibly don't see pictures good enough and may be wrong. What I can see - is ok for about 100W per heatsink with an 'average' fan cooling.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 01, 2022, 06:02:31 pm
content: Big transistors aren't economical. Thermal performance is better anyway, spread out over a few transistors.Hmm, the FQA9N90C I've used has since gone obsolete, it seems. Well, if you find some for cheap, no reason to complain, but as for currently stocked today...TO-220: AOT8N50. Surprisingly powerful, quite cheap, DC SOA. Should get close to ratings with flat greased heatsink (as mentioned above: prefer to isolate the heatsink; all transistors can be common-drain on the same heatsink, no worries).TO-247: FCH165N60E. Much higher current rating than needed, and a bit more expensive I think than the 9N90s I had. SJ design, good performance if you ever happen to find a need for half a kilowatt or more of switching power. But despite the power density, shows DC SOA.IRFP450, 460: Not good design-in choices as they don't show DC SOA anymore. I believe old datasheets have shown those data before; likely they haven't been die-shrunk and are the same crummy old designs as ever. Crummy for switching performance, that is, but they may indeed still be suitable for linear despite what the datasheets seem to imply these days.Related: I've tested a Siliconix IRF740PbF at apparently -- oh I forget what it was exactly, anymore -- 1.4, 1.6, 1.8 times the minimum rating? It wouldn't surprise me if others of that age are suitable, too.The catch is, I wouldonlydo a design with one after performing an eval test (to check that a given part is likely to work), plus preferably sampling during production to guard against adverse product changes.As for design tips:Parallel transistors with common gate voltage (use a modest say 100R gate resistor to each just in case) and individual source resistors. Which double as source degeneration for better stability. Use a low-value sense resistor on the common / ground return to measure / feedback total load current.Consider adding resistance in the drain circuit. This can be chosen so that drain voltage reaches minimum when load current reaches maximum, thus maximizing available SOA -- about doubling it in fact. This comes at the expense of low-voltage operating area, of course -- basically the Rds(on) is increased accordingly. A compromise might drop 50 or 100V this way, still extending SOA modestly. As a bonus, the resistance also takes the brunt of a short circuit, in the event the transistor(s) cook off.The bulk of the load can also be taken up by whole resistors switched in. An array of which can be used as a power DAC. The number of resistors required, for a given operating point, is easily computed: in fact, the LM3914 bargraph display driver can perform precisely this function. Too bad it's obsolete (or nearly). A linear sink can then take up the excess, regulating on total current.Tim

author: ifonlyeverything
author_rank: Regular Contributor
timestamp: November 01, 2022, 08:00:51 pm
content: Thanks for the detailed response T3sl4co1l. Do you have any personal experience with FCH165N60E? I'm wondering how hard it can pushed in the real world. I see ~2A DC at 100V but who knows what this is in reality.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 01, 2022, 11:09:16 pm
content: It... will most likely do what it says it will do.Keep in mind that includes correct thermal design. Add up thermal resistances in the system, and that determines maximum power rating. A 200W transistor is useless without a 200W heatsink to keep it cool, after all. The nameplate figure is calculated by extrapolating to Tc = 25°C (or not extrapolating at all, but measuring with the device immersed in boiling freon at 25.0°C(!!)), an extraordinarily unrealistic figure with real (passive or forced-air) heatsinks. If Tjmax is 150°C, then that's a drop of 125°C between case and junction; halve that (i.e., at Tc = 87°C) and maximum power likewise halves, etc.With insulators, it's feasible to get on the order of 20W from a TO-220F, 50W plain TO-220, 100W TO-247 or TO-3P, or 150-200W from TO-264. With direct greased surfaces, a bit more can be done, but I wouldn't push more than double that (or, a bit less than that even) even with greased joints only (no insulators) and water cooling.Tim

author: mvs
author_rank: Frequent Contributor
timestamp: November 02, 2022, 01:55:52 pm
content: Quote from: ifonlyeverything on October 30, 2022, 04:42:27 pmTO-264 MOSFETs have conductive pad connected to drain. It is not a big deal, since drains of the MOSFETs in an electronic load are usually connected together.If you still wish to isolate MOSFETs rather then heatsink, you can use thin alumina or mica pads.Quote from: ifonlyeverything1U server heatsinks are designed for high airflow. They are used in combination with multiple high speed 40mm fans, which are very noisy.You may look for 2U/3U server heatsinks or even larger desktop heatsinks. They usually need less airflow to achieve same performance.Quote from: ifonlyeverythingIt should be sufficient to measure only the base of the hottest heatsink in your design. You can use 4-pin fans, so you are able to detect failed fan and control fan speed.

author: JohanH
author_rank: Frequent Contributor
timestamp: November 02, 2022, 02:26:32 pm
content: Anyone looked at FGH60N60SMD? Good DC SOA (imagine that!) and easily paralleled.https://www.onsemi.com/pdf/datasheet/fgh60n60smd-d.pdf

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 02, 2022, 03:05:49 pm
content: Impressive, an IGBT with full DC SOA -- I would test it first, personally, but it's likely to work.PTC too, easy for paralleling. Not that that matters here, that's paralleling in switching operation.It's $6.50 in singles at Mouser though; you're not likely to get all the nameplate watts out of it -- maybe not the most economical. Still, about $1/100W is a nice showing at least in raw numbers!Tim

author: ifonlyeverything
author_rank: Regular Contributor
timestamp: November 02, 2022, 03:56:10 pm
content: Quote from: JohanH on November 02, 2022, 02:26:32 pmThat's a really nice find. Maybe I'll buy a few of these.

author: JohanH
author_rank: Frequent Contributor
timestamp: November 02, 2022, 05:33:29 pm
content: Quote from: T3sl4co1l on November 02, 2022, 03:05:49 pmI was comparing it to Onsemi MOSFET FCH041N60E that has somewhat similar specs (also very good) and noticed the IGBT is almost half the price and better power/temp rating. I'm planning to build something with it.

author: mvs
author_rank: Frequent Contributor
timestamp: November 02, 2022, 06:44:04 pm
content: Quote from: JohanH on November 02, 2022, 02:26:32 pmIGBTs do have non-zero Vce(sat). This will limit low voltage operation.Combined IGBT/MOSFET power stage may solve this problem, but design will be a bit more complex.

author: ifonlyeverything
author_rank: Regular Contributor
timestamp: November 02, 2022, 08:06:22 pm
content: Quote from: mvs on November 02, 2022, 06:44:04 pmThis is probably a foolish question -- but in what way will it limit low voltage operation?

author: mvs
author_rank: Frequent Contributor
timestamp: November 02, 2022, 08:26:37 pm
content: IGBTs and bipolar transistors do have collector-emitter saturation voltage (Vce(sat)). It is a voltage drop between collector and emitter of fully open (saturated) transistor.If you make an electronic load with IGBT power stage, it will work down to around 2-2.5V.

author: ifonlyeverything
author_rank: Regular Contributor
timestamp: November 03, 2022, 03:45:20 pm
content: Quote from: mvs on November 02, 2022, 08:26:37 pmI found a SPICE model for this particular IGBT and played around with it a bit. The behavior is weird at <5V. I want something I can run down fairly low, to load test single cell batteries (in addition to higher voltages for power supply testing) so this might not be ideal.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 03, 2022, 04:13:42 pm
content: Why not make two, one for low voltages?That's what I did, though that's a bit roundabout as my low-voltage unit is a bidirectional switching power supply...Tim

author: Vovk_Z
author_rank: Super Contributor
timestamp: November 04, 2022, 09:26:09 am
content: I found a partnumber for a good powerful heatsink, which works fine with a powerful noisy 70 mm fan. Here it is: IBM HEATSINK (40K7438) IBM X3400 / X3500 / X3650 (FRU 40K7438).I have two of them. Tested one - it works fine with 200 Watts and 4500 rev/min 70mm powerful fan (and TO-247 FETs). A man who sold me them, said it may dissipate up to 600 W with a TO-264 FETs (?).

author: ifonlyeverything
author_rank: Regular Contributor
timestamp: November 04, 2022, 11:55:01 am
content: Quote from: Vovk_Z on November 04, 2022, 09:26:09 amThose look pretty nice. How did you mount your FETs? Clamping, or drill & tap a hole in the heatsink base?

author: Vovk_Z
author_rank: Super Contributor
timestamp: November 04, 2022, 07:53:39 pm
content: drill & tap. It is made of something like pure copper (or copper-like alloy). It is pretty soft and drills very easily.

author: MathWizard
author_rank: Super Contributor
timestamp: June 04, 2023, 04:37:39 am
content: I've been wanting to make a DCEL to test some computer PSU's. I have a couple of real schematics that are similar to a lot of the low power ones you'll see. And I would probably combine that with something like Scullom Hobby Electronics DCEL, that and the 2 sch's are both digital.Last year I tried some big iron sheets as heatsinks, and they can't even dissipate the heat fast enough for 1 fet at 20W. Maybe I was doing something bad to those modfets, or they were knockoff's. But I haven't tried anything since.Now I have some nice op-amps, but for the cost of all the other parts, like a giant heatsink, I'm back to looking at brand-name ones.I have a +10yo 1,200W PSU. I should just give up on testing that, I'd need a multi-rail tester built for 2,000W.But then there's those low voltage, high power heating elements, like 12V 400W, and cheap. IDK if they are really suitable, since I don't see anyone else using heater elements. But for a resistive load, with pass mosfets trying to control it, is that a good or bad idea ? Like if I wanted to test a 750W PSU, may I'd only use 600W of heaters, and the rest could goto just power mosfets on a heatsink.

author: barshatriplee
author_rank: Regular Contributor
timestamp: June 04, 2023, 07:38:17 am
content: Using server CPU heatsinks can be a good option for cooling the MOSFETs in your electronic load. The Delta Electronics heatsink you mentioned (DHS-B9292-04A) with dimensions of 80x80mm and a thermal resistance of ~0.24°C/W can provide adequate cooling. You can fit two MOSFETs per heatsink, as you suggested. Additionally, using powerful fans and clamping everything together with bolts will help enhance the cooling performance.

author: Jwillis
author_rank: Super Contributor
timestamp: June 04, 2023, 01:05:12 pm
content: I run 8 FDL100N50F in the ELoad I built. It can sustain around 900W before the thermal shut down kicks in at around 100C. Eventually I would like to improve the cooling but it supports my needs for now.

author: Jwillis
author_rank: Super Contributor
timestamp: June 04, 2023, 01:28:12 pm
content: Quote from: ledtester on October 30, 2022, 05:28:01 pmI bought several from Digikey , Ebay and Aliexpress and they all work the same. Didn't see any difference between them. Aside from price.Mind you I got them before the Covid thing so there was lots of them around then.

author: MathWizard
author_rank: Super Contributor
timestamp: June 07, 2023, 07:16:32 am
content: Quote from: Jwillis on June 04, 2023, 01:05:12 pmThat heatsink is not very big, but that can almost handle 900W ? Where do people get large heatsinks from anyways, what did it cost? I had a quick look around and I was seeing prices like $350 for anything big. Insane price markup I'm sure.I know I'd need the fans to have the right spec's too.

author: Jwillis
author_rank: Super Contributor
timestamp: June 07, 2023, 10:53:34 pm
content: Quote from: MathWizard on June 07, 2023, 07:16:32 amWith forced air it will sustain 900W for the 5 minute without further temperature increase on the test i did. I won't kid you , It gets hot . But most of the of the time I use it at around 400W to 600W.I lucked out on these heat sinks. Found them for $40 a piece . I check junk stores, pawn shops and scrap yards. Sometimes I get lucky. Even metal shops have scraps they will sell you for peanuts. After all what is a heat sink but a chunk of metal.I do have an advantage of having lots of petroleum companies around that have scrap yards. Sometimes they will just let you take it. Saves them from hauling it away.