Empirical mosfet DC SOA testing - Page 1
Forum section: electronics / projects

author: mzzj
author_rank: Super Contributor
timestamp: September 21, 2025, 09:19:04 pm
content: We pretty much know already that older generation mosfets are typically better for DC SOA. For a DC load project I tested some chips laying around but did not expect this huge differences:IPP70N10S3 (no DC SOA curves) = Failure at 20v 2.5A at case temperature of 77 Cel20 year old-stock MTP52N06VL = manages 10A 20V until plastic case temperature reaches 165 CelGood old times.. MTP52N06VL handles more than rated power mounted to 90 Cel heatsink. Die must be at 250 C when it failed.

author: mzzj
author_rank: Super Contributor
timestamp: September 21, 2025, 09:56:43 pm
content: And FDP8443. This has DC SOA on datasheet but it is piss-poor at about 20v 1AFails in no time at 20V 3A, barely had time to warm up.

author: Slh
author_rank: Regular Contributor
timestamp: September 22, 2025, 06:44:32 am
content: Have a look at superjunction FETs (400V+). They appear to generally have decent DC SOA.I haven't run any to their limits yet but I got 20W out of one at 500V for a few seconds.

author: Smokey
author_rank: Super Contributor
timestamp: September 22, 2025, 06:48:39 am
content: There is still a whole line of Ixys (littlefuse?) FETs designed for higher linear SOA.Also, keep in mind that the whole idea of SOA is keeping the die from melting. What that means is it's not just a DC power limit. Short pulses of power can exceed the DC limits by quite a bit if they are short enough, and the FET starts out cool enough. What you are really trying to do is estimate the die temperature based on time, power input, and case temperature (which is the only direct measurement you can get usually, unless you have one of those fancy FETs with a temp sensor on the die).

author: tszaboo
author_rank: Super Contributor
timestamp: September 22, 2025, 09:03:27 am
content: You have to differentiate between SOA and FBSOA.FBSOA is Forward Bias safe operating area. Forward bias is what's killing newer MOSFETs. Instead of one big FET think about it as several smaller ones in parallel. There are parts in the transfer function, where with increasing temperature the conductivity decreases. So parts of the FET will conduct more current, get hotter, conduct more, get even hotter and thermal runaway. Staying below this critical temperature, FET can indefinitely stay stable, above it, some will eventually self destruct. But basically it will be unreliable and it's a bad practice to run the FET there.So the same FET with different amount of cooling, or different ambient temperature will behave differently.

author: mzzj
author_rank: Super Contributor
timestamp: September 22, 2025, 01:14:41 pm
content: Quote from: Smokey on September 22, 2025, 06:48:39 amLinear-operation specific mosfets are bit pricy so I'm looking for alternatives.Die temperature estimates are more or less useless with the modern mosfets in DC linear load operation as local hotspots (invidual mosfet cells) melt long before average dissipation would be a problem. (Spirito effect)Quote from: Slh on September 22, 2025, 06:44:32 amI remember someone mentioning that the SJ mosfets handle suprisingly good DC SOA but haven't tested any. At the moment looking for lower voltage options with lower RDS-on.Quote from: tszaboo on September 22, 2025, 09:03:27 amThat was the whole idea of testing "small" TO-220 mosfets. But there is a limit how small I'm willing to go as the goal is 1kA @3v

author: tszaboo
author_rank: Super Contributor
timestamp: September 22, 2025, 04:18:38 pm
content: Quote from: mzzj on September 22, 2025, 01:14:41 pmWhat I mean is, that a TO220 FET is going to act like a bunch of tiny FETs in parallel. The effect that I described happens within one FET.For TO220 I found the practical limit to be 75W, above that it's a dice roll. And this is only for the FETs that have very good internal thermal resistance. You might get twice as much if you can select a really good FETs. IXYS maybe even more.

author: thm_w
author_rank: Super Contributor
timestamp: September 22, 2025, 10:06:53 pm
content: Probably didnt help that the Rjc of the IPP70N10S3 is much worse as well, I guess sign of a smaller die.You can find some previous threads with lots of good ideas.https://www.eevblog.com/forum/projects/mosfet-dc-soa-collection/https://www.eevblog.com/forum/projects/linear-mosfets-for-kunkin-kp184/https://www.eevblog.com/forum/projects/igbt-vs-mosfet-for-dummy-load/

author: mzzj
author_rank: Super Contributor
timestamp: September 23, 2025, 07:29:11 pm
content: Quote from: thm_w on September 22, 2025, 10:06:53 pmIPP70N10S3 fails at 50W 20V or tested at lower voltage it can handle short-term 200W at 4.5 volts and fails with a puff of smoke at little over 48ARather extreme example of Spirito effect. Seem like not any problem to find mosfets for low voltage electric load.After trolling around Digikey I found some "too good to be true" mosfets.Namely anything in Fairchild/Onsemi unifet or qfet series but until proven otherwise I suspect the datasheets are not correct.

author: mzzj
author_rank: Super Contributor
timestamp: September 23, 2025, 08:33:16 pm
content: Some failure points at 1.0A currentST SPW20NB50 165v =165WIntersil HGTG30N60B3 IGBT = 136WInfineon 20N60C3 Superjunction mosfet = 172WIR IRFP250 1.0A 185W , did not fail, not tested any higher voltage as this is only 200v partIR IRFP250 tested to failure at 2.0A 242W

author: thm_w
author_rank: Super Contributor
timestamp: September 23, 2025, 09:31:05 pm
content: IRFP250 is the classic FET, manufacturers usually limit it to 40-60W, so to see 242W is a good sign.https://www.eevblog.com/forum/beginners/electronic-load-heatsinking-managing-thermals/How long are you letting it sit before increasing the power?

author: mzzj
author_rank: Super Contributor
timestamp: September 23, 2025, 10:02:46 pm
content: Quote from: thm_w on September 23, 2025, 09:31:05 pmI increased the voltage in constant current tests at roughly 1-2 volts per second to keep the heatsink temperature rise below 20CFor IRFP250 50% derating for reliability and another 50% to account for higher heatsink temperature during actual use would put me also at 60W.With good cooling(ie CPU heatpipe cooler) I'd dare to push 120WI should have bunch of Non-punch through (NPT) IGBTs somewhere, possibly interesting option for higher voltages.

author: Vovk_Z
author_rank: Super Contributor
timestamp: September 26, 2025, 05:36:24 am
content: I you are interested in TO220 case then I can say that electronic load manufacturers design their devices with a rule of about 30 W (+-) for one TO220 case.TO247 have a rule about 45-50 W/case if we are talking about real good reliability.If we talk about extreme cases then almot all transistors can withstand extreme heat at least once in their life.For example, when I was doing my first transistor power amplifier I didn't have a heatsink first. So, I bolted output transistors to a sheet of a paper, just for they don't fly in the breeze. And they unsoldered from their wires quite fast - faster then I could measure the collector current and set it properly. And then I soldered them to wires again and they were ok (or at least they worked). But that is not how it must be done.

author: ifonlyeverything
author_rank: Regular Contributor
timestamp: September 26, 2025, 12:56:05 pm
content: Quote from: mzzj on September 23, 2025, 10:02:46 pmI ran an IRFP250 at 120W for 30 minutes+ steady state with this heatsink (which was surprising because the heatsink definitely doesn't look like it was rated for 120W.)https://www.ebay.com/itm/317314242349They're also sold as "A70" model number heatsinks on Alibaba.

author: Smokey
author_rank: Super Contributor
timestamp: September 27, 2025, 01:32:36 am
content: A little forced air cooling airflow goes a LONG way with any heatsink. Without airflow you just get a hot air layer around the heatsink and are relying on convection, which sucks compared to any fan.

author: Vovk_Z
author_rank: Super Contributor
timestamp: September 27, 2025, 12:04:02 pm
content: Quote from: ifonlyeverything on September 26, 2025, 12:56:05 pmNot sure how high it is, but actually, most box CPU coolers and lookalike ones are really capable of 100-120 W heatsinking. I prefer used AMD AM2 heatsinks because they are square and easy to use.

author: David Hess
author_rank: Super Contributor
timestamp: September 27, 2025, 03:34:21 pm
content: Quote from: mzzj on September 22, 2025, 01:14:41 pmI have had problems identifying which power MOSFETs are superjunction parts and which are not; it is not always clear.Annoyingly some datasheets show a square FBSOA when that is extremely unlikely.

author: macboy
author_rank: Super Contributor
timestamp: September 28, 2025, 11:54:46 pm
content: Lower voltage, high current, high speed, very low RDSon MOSFETs are highly likely to be unsuitable for DC operation.The high voltage parts (several hundred V or higher) with moderate to high RDSon (100 milliOhm or more) are much more likely to have generous DC SOA.

author: mzzj
author_rank: Super Contributor
timestamp: September 29, 2025, 12:57:21 pm
content: Quote from: macboy on September 28, 2025, 11:54:46 pmMy need was mainly low-voltage high current, ie 50A 2v or so but if I can use same part for higher voltage I'd rather stock just one modelIRFP4768 would be an prime example of a part that does fine at low voltages but is disasteriously bad at higher voltages.Published SOA is 500W @ 5 volts but dropping like stone to less than 20 watts at 20v. And this is a 250 volt part.https://www.infineon.com/assets/row/public/documents/24/49/infineon-irfp4768-datasheet-en.pdf?fileId=5546d462533600a40153562c959b2021

author: Doctorandus_P
author_rank: Super Contributor
timestamp: September 29, 2025, 12:59:14 pm
content: Does SIC have similar problems asthe silicon switching FET's?I vaguely remember the "HEXFET" from somewhere in the '90-ies. It's was probably one of the ways to mag big fets switch faster, but it also effectively results in a lot of parallel fets as the hex grid cuts up the die into lots of little channels. And if one of those fets has a slighty lower threshold voltage then the others, it would die very quickly when it's used in linear applications. For switching applications this does not matter, as the difference in all the parallel fets switching on will be very small.So if you want reliabilty. Then go get a bit more money and buy the Fet's that are rated for linear operation.

author: mzzj
author_rank: Super Contributor
timestamp: September 29, 2025, 01:52:58 pm
content: Quote from: Doctorandus_P on September 29, 2025, 12:59:14 pmAnd today the original "HEXFET" is the proven solution for large SOA. O tempora! o mores.

author: voltsandjolts
author_rank: Unknown
timestamp: September 29, 2025, 02:28:40 pm
content: For anyone like me who isn't familiar with linear mosfets, here are some made by Littelfuse (IXYS);https://www.littelfuse.com/products/power-semiconductors-control-ics/mosfets-si-sic/n-channel-linear/standardInDigiKey stockthey range from £5 to £30

author: voltsandjolts
author_rank: Unknown
timestamp: September 29, 2025, 02:35:38 pm
content: FWIWI have adc resistance welderwhich uses 40 of IXFH75N10 (obsolete) (in parallel, on large fan cooled heatsink) as the linear control element. 10-1000A. 12V cap bank.

author: tszaboo
author_rank: Super Contributor
timestamp: September 29, 2025, 09:28:32 pm
content: Quote from: David Hess on September 27, 2025, 03:34:21 pmI've seen DC "defined" as 10ms pulse length on some FET datasheets for these purposes.

author: Smokey
author_rank: Super Contributor
timestamp: September 29, 2025, 09:35:56 pm
content: Here:https://www.ti.com/lit/an/slva158/slva158.pdfThat does a good job of showing the thermal RC model for a FET and how those thermal impedances effects heat flow in the device. That is what SOA is trying to tell you. SOA is not just "how many DC watts can I do" (which is dominated by your heatsink anyway).If the junction has a tiny thermal mass (capacitance in this model) then it will heat up and melt faster even with short pulses of power. If there is a huge thermal impedance between the junction and case, then ya, heat will build up and the junction will melt. If your case to ambient (ie heatsink) has a huge thermal impedance then DC power will be highly limited in all conditions.Another thermal model schematic (some reason the forum won't let me show that with an img tag).https://www.researchgate.net/profile/Sinan-Zengin/publication/269405459/figure/fig2/AS:295156780683271@1447382268741/Thermal-model-of-Mosfet-and-heat-sink.png

author: Slh
author_rank: Regular Contributor
timestamp: September 30, 2025, 07:22:41 am
content: Quote from: Doctorandus_P on September 29, 2025, 12:59:14 pmSiC is even worse than Silicon! All manufacturers have wildly different thresholds and the positive/negative regions of the Vgs curve vary accordingly. Last time I looked, infineon probably had the smallest difference between hot and cold. I think the only parts worth looking at for electronic loads are the On Semi JFETs.Just for fun I tried using a 1200V 40A SiC MOSFET in an electronic load. I managed to get a few 10s of mA at 500V for a few seconds. Any more and they would die quickly.

author: Vovk_Z
author_rank: Super Contributor
timestamp: September 30, 2025, 09:45:10 am
content: Quote from: mzzj on September 22, 2025, 01:14:41 pmDon't waste your time. Just use40xIRF3205 and that's it. Or at least25-30xUpdate: I misread "1kA" as "1kW" (I was reading it from a phone). So, for 3 V and 1kA you possibly need more IRF3205 I guess. IDK, I guess, something like 100 IRF3205 can be ok with 3 kW of power.Actually, it looks like it can be easier done with a bit larger case (e.g. TO247). Mount and solder 50 x TO247 instead of 100 x TO220 must be twice faster.

author: tszaboo
author_rank: Super Contributor
timestamp: September 30, 2025, 10:17:13 am
content: Quote from: Vovk_Z on September 30, 2025, 09:45:10 amWhat a terrible advice.I was designing systems where FETs were used in DC mode, and we were buying them by the reels. You don't know why the OP wants to select a part for his design.

author: Vovk_Z
author_rank: Super Contributor
timestamp: September 30, 2025, 11:44:31 am
content: I guess TS wants to select MOSFET by himself becase he don't believe in 40-50 years of people experience with MOSFETs. He definitely don't want to just google which types are mostly suited for DC mode and are mostly used in electronic loads.

author: tszaboo
author_rank: Super Contributor
timestamp: September 30, 2025, 11:56:37 am
content: Quote from: Vovk_Z on September 30, 2025, 11:44:31 amThe terrible part is to "Just do X". Especially if you don't know most of the design criteria.The part advice is good.

author: rikkitikkitavi
author_rank: Contributor
timestamp: September 30, 2025, 12:22:14 pm
content: Quote from: mzzj on September 29, 2025, 12:57:21 pmI think that you need to parallell several higher voltage parts to allow for safety margin regarding Tj/Tc and power dissipation and then your 10-20A parts have a combined current capacity that meets your requirement. If not, parallell more and you have even more safety marginBTW great thanks to contributors in this thread. Much information.

author: Vovk_Z
author_rank: Super Contributor
timestamp: September 30, 2025, 04:33:20 pm
content: Quote from: mzzj on September 29, 2025, 12:57:21 pmIf task is 3 V and 1000 A then this one looks like suites well. It definitely can dissipate something like 100 W in real life case so for 3 kW you need only 30 x IRFP4768 in terms of power.Another thing is a current. 1000 A / 30 = 33 A/leg. 33 A looks like a bit too much for usual PCB but may be easier done with wired design. Actually, I don' know how to handle such large kilo-ampere currents right.

author: Vovk_Z
author_rank: Super Contributor
timestamp: September 30, 2025, 04:43:06 pm
content: Quote from: mzzj on September 21, 2025, 09:19:04 pmThose experiments don't look like they are Safe Operating Area tests. They clearly look like Dangerous Area of Operation (DOA) tests. I believe those two (SOA and DOA) are not close to each other. There must be quite large Area of Uncertain Operation between those two. So, you'll need some methodology and possibly some another kind of experiments how to extract SOA from DOA

author: mzzj
author_rank: Super Contributor
timestamp: September 30, 2025, 05:03:03 pm
content: Quote from: Vovk_Z on September 30, 2025, 04:33:20 pmAnother thing is a current. 1000 A / 30 = 33 A/leg. 33 A looks like too much for usual PCB but may be easier done with wired design. Actually, I don' know how to handle such large kilo-ampere currents right.If money is not issue you can get heavy copper or "extreme" copper from 20oz up to 200oz. 20oz is 0.7mm or so.Price scales up exponentially and you can forget cheap prototypes.My plan is to solder 50mm wide and maybe 0.5...1.0mm thick copper strip on one side of the pcb. This should handle 300A per module no problemo. Combine 4 modules and there you go.On one motor controller project I used "pcb" made of FR4 board sandwidched between two 0.7mm copper plates. Coppper plates were connected along entire lenght to 10x50mm copper busbars. Tedious but like I mentioned heavy copper prototypes are not cheap even from china.

author: mzzj
author_rank: Super Contributor
timestamp: September 30, 2025, 05:12:53 pm
content: Quote from: Vovk_Z on September 30, 2025, 04:43:06 pmYes, you can call those DOA numbers. But it gives very clear indication with smoke signal when the SOA limit has been permanently exceeded.For actual use I'll just take best quess how much safety factor to apply to instant DOA. Ideally or on larger scale production you'd set up some sort of statistical testing, possibly with some sort of destructive testing for each batch of mosfets.

author: dietert1
author_rank: Super Contributor
timestamp: September 30, 2025, 11:57:13 pm
content: I reported some test results here:https://www.eevblog.com/forum/projects/mosfet-rdson-test/msg5869247/#msg5869247That IRLB3034 should sustain 100 A at 3 V with sufficient cooling. I think 10 or 20 of those will be a very low fraction of the system price. A 3 KW cooler alone will be a lot more expensive.Regards, Dieter

author: voltsandjolts
author_rank: Unknown
timestamp: October 01, 2025, 06:37:06 pm
content: Quote from: dietert1 on September 30, 2025, 11:57:13 pmInfineon sayIRLB3034is obsolete, recommendIPP015N04NF2Sinstead.

author: mzzj
author_rank: Super Contributor
timestamp: October 01, 2025, 07:20:39 pm
content: Quote from: voltsandjolts on October 01, 2025, 06:37:06 pmIf the datasheets are correct the IPP has itsy bitsy SOA compared to IRLB. Kind of expected with the more modern process.60W vs 375W @3 voltsAnd IRLB SOA looks realistic compared to something like FQA140N100 that probably skips Spirito effect in datasheet DC SOA curve.https://www.onsemi.com/pdf/datasheet/fqa140n10-d.pdfJust for giggles I included one FQA140N100 on my latest TME order. IF datasheet is even half correct that would be great low resistance part that works also at higher voltages.

author: dietert1
author_rank: Super Contributor
timestamp: October 01, 2025, 07:36:19 pm
content: Digikey have some UMWIRLB3034 that appear to be the same, with more on order for November. Those are the ones i tested.Regards, Dieter

author: mzzj
author_rank: Super Contributor
timestamp: October 01, 2025, 08:29:15 pm
content: I upgraded the cooling to old heatpipe CPU monster cooler (Skythe-something)Brand new IRFP260M 42A 8.0V = "ok"42A 9.0V ok for couple of seconds42A 10V destruction in ~1 secondPretty good but RDSon is bit high for this particular project

author: PCB.Wiz
author_rank: Super Contributor
timestamp: October 01, 2025, 08:53:59 pm
content: Nexperia have new MOSFET family designed for parallel operationhttps://efficiencywins.nexperia.com/efficient-products/simplifying-dynamic-current-sharing-with-asfetsThat shows a Zero TC point ~ 10A, 3V Vgs and a negative TC from 3V to 6.3V and it has a good looking SOARCurrently parts seem to only come in LFPAK88 SMD packages - nicer for linear use would be the top-cooling CCPAK1212i package.The PSMNR90-80CSF which does come in CCPAK1212i, has SOAR not nearly as good as the new parts, even at low voltages.

author: Smokey
author_rank: Super Contributor
timestamp: October 01, 2025, 09:40:55 pm
content: What does Rdson really have to do with a FET used in the linear region in a load application? If you drive a FET into saturation in this case, then something has gone terribly wrong with your load.If you are using the FET to make a constant current and you know the voltage, then you know what "resistance" the FET is creating and you know the power dissipation in the FET. From the data sheet you can get the junction to case thermal resistance. With two thermocouples you can measure the case temp and the heatsink temp and you can get an idea of your case-to-ambient thermal resistance.If you are blowing up FETs with DC current you are just exceeding the total power capabilities of that system (junction-case-heatsink-ambient). Get a bigger FET (realistically just a bigger case). Or a bigger/more-efficient heatsink. Or parallel a bunch so they share the power load. None of those things really have to do with transient SOA ratings or internal FET construction (trench, Superjunction, Lateral, etc).

author: mzzj
author_rank: Super Contributor
timestamp: October 01, 2025, 09:48:08 pm
content: Quote from: Smokey on October 01, 2025, 09:40:55 pmI'd suggest reading on Spirito effect and take a look at DC SOA ratings of some examples listed on this thread. Might learn something new.IRFP4768 datasheet is good example. Is it 500W or 20W ?

author: thm_w
author_rank: Super Contributor
timestamp: October 01, 2025, 11:12:21 pm
content: Quote from: dietert1 on October 01, 2025, 07:36:19 pmIt looks like UMW clones a lot of stuff, TI, microchip, even A&O parts:https://www.umw-ic.com/en/productThe last clone brand we had was total garbage though (EVVO), and digikey still has not removed it from their websitehttps://www.digikey.ca/en/products/detail/evvo/LM317K/25188516

author: dmendesf
author_rank: Frequent Contributor
timestamp: October 01, 2025, 11:18:28 pm
content: I'm reading this thread with attention but somehow got confused: all this DC problematic with FETs apply only when trying to use it as a controlled resistors or also when using it as a static switch/ smart fuse (either fully driven on or off, but continuously)?

author: PCB.Wiz
author_rank: Super Contributor
timestamp: October 01, 2025, 11:27:08 pm
content: Quote from: dmendesf on October 01, 2025, 11:18:28 pmIf you look at #42 above, a FET that is OFF has 0A, so is below the bottom axis, and a FET that is ON, follows the 45' RDS slope on the top left.If the FET is switching, it is only the time spent in the active region that needs to stay below those lines.Usually that is fast enough to be OK, but for a fixed drop of say 20V the static/DC current cannot exceed 1A, in order to stay under the curves.

author: mzzj
author_rank: Super Contributor
timestamp: October 02, 2025, 03:43:05 am
content: Quote from: PCB.Wiz on October 01, 2025, 11:27:08 pmI guess Hot Swap controllers and soft-start(soft-stop) circuits would be where this is commonly problem.NASA also found out this in the hard wayhttps://ntrs.nasa.gov/api/citations/20100014777/downloads/20100014777.pdfOLD mosfets seem to be mostly immune.Semi-modern mosfets suffer from problem but the datasheets can give you wrong information. Or DC curves are missing altogether.Latest or updated datasheets seem to give you realistic information.IIRC Art of Electronics also misses the vital part and goes to bluntly recommend mosfets for linear use because they don't suffer secondary breakdown.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: October 02, 2025, 04:44:59 am
content: Quote from: mzzj on October 02, 2025, 03:43:05 amYes, if a designer wants to slow down inrush slew rates, they need to spend more time in that SOAR region.Thus new Hot Swap FETS start to address this with improvements, this from 2021https://www.nexperia.com/about/news-events/press-releases/Nexperia-s-new-Application-Specific-MOSFETs--ASFETs--for-hot-swap-increase-SOA-by-166--and-slash-PCB-footprint-by-80--It is interesting to see that the newestPSMN1R9-80SSJ N-channel 80 V, 1.9 mOhm ASFET with enhanced dynamic current sharing in LFPAK88, has an even better SOAR than the hot swap parts.It also has a lower tempco on VGS and a positive tempco region lower on the VGS curve, which suggests better on-die sharing and fewer hot spot effects.Sadly, none of these exotic parts can be called 'low cost'

author: Vovk_Z
author_rank: Super Contributor
timestamp: October 02, 2025, 06:36:23 am
content: IRFP260 is a great part for linear mode of operation. It is so because it has almost constant power SOA with any voltage.As I know, a pair of the most popular 240/9240 were highly used in mosfet power amplifiers.And 260 has wider SOA then 240. And, IRFP260 will happily work with up to 25 Amps (not sure about more current).When I need TO247 transistor for DC elctronic load I use mostly either IRFP260 or IRFP460. I alwais was happy with them

author: rikkitikkitavi
author_rank: Contributor
timestamp: October 02, 2025, 08:59:06 am
content: Probably so, indicated by its frequent use in audio amplifer which is a highly linear application but reading through datasheets from Vishay, Infineon and Ixys for IRFP260 neither have anything longer than a 10 ms SOA curve indicated. This is a straight slope at around 500W for 10ms.One have to test or use some of the methods to derate the capacity.

author: dietert1
author_rank: Super Contributor
timestamp: October 02, 2025, 07:59:16 pm
content: Quote from: mzzj on October 01, 2025, 09:48:08 pmIn the discussion i linked above i learned that DC SOA often depends on voltage, where 10 V or so seems to be a typical voltage limit for derating. In the IRFP4768 example the DC SOA curve has points at 10 V x 10 A = 100 W and 3 V x 150 A = 450 W. That means the internal heatsinking of the part is a limit at 3V with about 0.2 °C/W while another limitations apply above 3 V. Yes, it should be the "Spirito effect", as any power mosfet is made as lots of small mosfets in parallel. So there is a tendency for "current bunching" like in bipolar power transistors.The IRFP260M seems to have weaker connection structures (pins, bonding) so it is limited to 50 A DC.I think we should be aware how the typical linear application needs isolation pads, which ruins thermal resistance for small TO-220 and TO-247 parts. Maybe better to look for those nice industrial mosfets with larger packages and builtin isolation. One can also try to mount a set of smaller mosfets to a copper bar without thermal pads and then use one larger pad for isolation from the heatsink.Regards, Dieter

author: mzzj
author_rank: Super Contributor
timestamp: October 02, 2025, 08:38:05 pm
content: Quote from: dietert1 on October 02, 2025, 07:59:16 pmI'd say that it is rdson-limited.0,04ohm rdson and at max rated temperature 3x that or 0.12ohms.50A to 0.12 ohms is 300W and with specified 0.5K/W we end up with 175Cel junction temperature. (with the slightly optimistic assumption that Tc is 25Cel)

author: David Hess
author_rank: Super Contributor
timestamp: October 02, 2025, 10:08:02 pm
content: Quote from: rikkitikkitavi on October 02, 2025, 08:59:06 amI also checked the datasheets and that is *not* a part I would recommend for any linear application because there are much better parts.

author: mzzj
author_rank: Super Contributor
timestamp: October 04, 2025, 01:46:19 pm
content: Quote from: David Hess on October 02, 2025, 10:08:02 pmJust curious, what better parts you would recommend?

author: dietert1
author_rank: Super Contributor
timestamp: October 04, 2025, 09:36:16 pm
content: I just looked at the IXTN240N075L2 datasheet. It says: "Designed for linear operation", includes DC SOA curves at 25 and 75 °C and handles about 450 W continuous power at 75 °C. It comes in the isolated SOT-227B package i mentioned above.Regards, Dieter

author: David Hess
author_rank: Super Contributor
timestamp: October 05, 2025, 07:43:24 am
content: Quote from: mzzj on October 04, 2025, 01:46:19 pmI do not have any current favorites. The IXYS linear MOSFETs are the obvious choice, but there are other parts with at least a DC SOA specification.

author: mzzj
author_rank: Super Contributor
timestamp: October 05, 2025, 08:02:58 am
content: Quote from: dietert1 on October 04, 2025, 09:36:16 pmCertainly nice looking if not bit pricy option. (Price is lot more reasonable if you need isolation from heat sink as aluminium nitride used in IXTN package is hard to beat.)Easy also to see why many commercial DC loads use something like IRFP260: IXTN240N075L2 can handle approx 3 to 6 times more power at 35x cost if you don't count for isolation.

author: voltsandjolts
author_rank: Unknown
timestamp: October 05, 2025, 11:19:49 am
content: Infineon claim their family of planar mosfets have good SOA and are suitable for linear operation. Mostly small packages, although there is IRFP3710 which is TO247.https://www.infineon.com/dgdl/Infineon-PlanarMOSFET_IRMOSFET_30V_100V-ProductBrief-v01_00-EN.pdf

author: dietert1
author_rank: Super Contributor
timestamp: October 05, 2025, 01:26:44 pm
content: Another consideration that comes to mind is about how safe paralleling mosfets is in linear mode. Even if a mosfet part is specified and safe for linear operation, paralleling some of the same may not be safe as the device differences between different mosfet chips are certainly larger than within a mosfet chip. Probably one should either select those mosfets for similar input voltages or control each mosfet separately measuring its current with a shunt. I'd guess paralleling more than 10 mosfets for linear operation can become quite complicated.Regards, Dieter

author: mzzj
author_rank: Super Contributor
timestamp: October 05, 2025, 01:28:50 pm
content: Quote from: voltsandjolts on October 05, 2025, 11:19:49 amI remember eyeballing IRFP3710 but datasheet doesn't give DC SOA at all and 10ms SOA graph is suspiciously straight line.IRFP2907 from the same product family on the other hand seems to show DC SOA and it includes Spirito effect.IRFP2907 DC SOA actually looks pretty good for low-ish voltage load use. 20V @ 20A is one of the best I have seen from(cheap) low resistance parts.

author: mzzj
author_rank: Super Contributor
timestamp: October 05, 2025, 01:32:56 pm
content: Quote from: dietert1 on October 05, 2025, 01:26:44 pmWith most mosfets separate current control loop for each mosfet is a must.Nexperias asfet-series (mentioned in post by PCB.Wiz) could be the exception but even those might need selection based on vgsth.

author: voltsandjolts
author_rank: Unknown
timestamp: October 05, 2025, 02:10:48 pm
content: Quote from: dietert1 on October 05, 2025, 01:26:44 pmBut can also be quite simple, depends on particular fet I guess. For example, the commercial dc resistance welder I linked above has 40 parallel IXYS fets with drain, source, gate all connected together (except each gate has it's own series 10R). 1000A - few tens of ms pulse operation though, repeatable every few seconds, 3ish IIRC.