Operation in This Area is Limited by R DS(on) - Page 1
Forum section: electronics / beginners

author: Jwillis
author_rank: Super Contributor
timestamp: December 31, 2019, 07:33:46 am
content: I purchased an inexpensive electronic load off EBay.They're not much different than the one Dave made other than the built in monitoring . It works fine and fairly accurate as long as it's doesn't get over loaded. I burned out the original MOSFET (IRFP260) but I had no exact replacement so I put in an IGBT (G60N100) and it worked great and could handle a bit more than the original. I also changed the heat sink to a much larger 6 pipe double fan. It doesn't seem to go much higher than 40oC with the heat sink.I'm testing a 0-40V 0-20A power supply I'm building and made some changes to the biasing on the power transistors and wanted to do a retest . But I turned the control on the load maybe a bit to high and of course destroyed the IGBT. I thought I would order some MOSFETS that can handle the voltages and currents that would fall within the parameters that I need.The IXFH50N60P3https://media.digikey.com/pdf/Data%20Sheets/IXYS%20PDFs/IXFx50N60P3.pdfor The FDL100N50Fhttps://www.mouser.com/datasheet/2/149/FDL100N50F-1007460.pdfMy question is in the Datasheets where the Maximum Safe Operating Area graph is theirs a note "Operation in This Area is Limited by R DS(on)" I've looked on the net for some explanation but I'm probably not searching right. Can anyone tell me what that means .Am I going to have problems turning these MOSFETs on at lower voltage and high current?

author: Ice-Tea
author_rank: Super Contributor
timestamp: December 31, 2019, 07:52:24 am
content: The SOA maps the current in function of the Vds. However, the Vds can not be higher than the product of the current and Rds. Imagine a 1 ohm Rds and a current of 10A. Obviously, Vds can't be higher than 10V

author: magic
author_rank: Super Contributor
timestamp: December 31, 2019, 09:04:12 am
content: Quote from: Jwillis on December 31, 2019, 07:33:46 amYou will, because of their internal drain-source resistance, RDS(on)There are bigger problems with MOSFET SOA, though. Read the following thread and in particular this post by David Hess and the links he posted.https://www.eevblog.com/forum/chat/darlingtons-used-as-audio-output-devices/msg1388118/#msg1388118You have two MOSFET vendors who pretty much admit that their SOA ratings at DC are plain bullshit. International Rectifier even blew one of their parts at 16V / 4A, which was presumably well withing the part's documented dissipation and temperature limits.And of course, the big "2.5kW power dissipation" spec applies only if you keep the heatspreader of the device at 25°C. Otherwise you need to multiply power dissipation by junction-to-case thermal resistance (from the datasheet) plus case-to-heatsink resistance, typical values scrapped from various datasheets below.Code:TO264 .15 K/WTO247 .21 K/WTO220 .40 K/WIGBTs just suck for linear, don't use them

author: magic
author_rank: Super Contributor
timestamp: December 31, 2019, 08:00:50 pm
content: I apologize for misinformation, the device destroyed by IR actually has very limited SOA spec and is not supposed to withstand 16V / 4A operation. The appnote also shows that IRHM9260 (presumably related to IRF9260, the P-complement of IRF260) can withstand over 100W for 400ms. I observed a tendency for newer, low-RDS(on) parts to be rated much worse in terms of SOA than dinosaurs like IRF240/260/640/etc. BTW, these old HEXFETs are also widely abused for audio by hobbyists and even some commercial vendors and they seem to tolerate it.Not sure what to think about Fairchild; on one hand AN-4161 clearly says that SOA curves are typically specified under assumption of uniform die temperature which may not always be true, on the other hand I cannot imagine how they would attempt to defend themselves if a part with guaranteed DC SOA rating of 10A/100V/Tc=25 turns out to blow up in such conditions.I noticed the FDL100N50F myself and it certainly looks interesting. If you don't mind risking blowing it up I think we all would love to hear if it's any good

author: Jwillis
author_rank: Super Contributor
timestamp: December 31, 2019, 09:25:18 pm
content: Quote from: magic on December 31, 2019, 09:04:12 amI think I'm starting to understand the limitations when dealing with the RDS(on). Probably explains the voltage drops I was getting.The IGBT G60N100 I used worked very well even over extended periods of time within its stated SOA parameters .The newest test is what killed it .At 40 V 20A it clearly went into second breakdown.https://www.mouser.com/datasheet/2/149/fairchild%20semiconductor_fgl60n100bntd-320282.pdfI screwed up by not watching the load while I was checking the temperature of the power transistors on the power supply. The biggest problem was when the IGBT burned out it shorted closed. I panicked when copious amounts of smoke started pouring out under the heatsink. I could see the electronic load had reached the 20A limit I had set, but neglected to see what voltage on the other meter. IGBTs are fine to use but like BJTs you really have to watch that second breakdown.Quote from: magic on December 31, 2019, 08:00:50 pmI ordered several of both IRFP260 and FDL100N50F. Destroying the IRFP260 could have been a power spike because I did find one in my scrap pile but its leads were broken off. I attempted to fix it by craving back the casing enough to weld leads on but the solder melts before I can get an accurate reading. That's really hot. So I think the IRFP260 should be able to take 40V at 20A but its going to be pushing it hard to the edge of the SOA. I just have to be more careful when approaching the top end. I have another electronic load coming so I can experiment with this one more. I think I should be able to parallel mosfets off board.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 31, 2019, 09:43:14 pm
content: Last time I cooked off a Fairchild, think it was a something 6N60, newer generation, it failed about 10% over the published thermal limit. Have used a QFET before in a commercial design [as linear device], was fine.Don't try paralleling MOSFETs naively, the Vgs(th) won't match so neither will the currents. You can use source degeneration resistance to help address this, but better is to just add more current sinks in parallel to reach the desired total.Tim

author: Jwillis
author_rank: Super Contributor
timestamp: December 31, 2019, 11:03:57 pm
content: Quote from: T3sl4co1l on December 31, 2019, 09:43:14 pmI suspected that mosfets would current hog like BJTs do, but not as bad.I figure 3 or 4 should be sufficient.to keep them well within the SOA My biggest concern is the board traces for the drain and source .I may have to beef those up as well with heavy gauge wire.But since paralleling mosfets do I also have to consider the RDSonin parallel as well?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 01, 2020, 12:32:19 am
content: No, that's the whole point of linear operation, you aren't running at Rds(on). Rds(on)'s are safe to parallel, so parallel transistors are often used in switching circuits.Linear operation is not safe to parallel, so source resistors or individual controllers are used in linear circuits.By the way, you should probably consider using a fuse when discharging unlimited sources like batteries.Tim

author: magic
author_rank: Super Contributor
timestamp: January 01, 2020, 09:06:57 am
content: I'm not entirely sure if you have read and understood the thing about thermal resistance I wroteGo ahead and ask if you have any questions or google it yourself.To pull 800W of heat out of IRF260 you would need to cool its case down to about -190°C, which is liquid nitrogen territory.To pull 800W from FGLG60N100 you would cool it below absolute zero. It wasn't second breakdown (or maybe it was), you simply cooked it.A single FDL100N50F maybe could pull it off on air cooling with some heroic effort. No way that the heatsink you showed would suffice. It seems about the size of CPU sinks, so good for maybe 100~200W.And perhaps I was too quick do dismiss IGBTs. IXYS apparently makes a few with decent DC SOA and 1.5kW+ power rating at Tc=25. An advantage over FETs is very high current capability at low voltage (the question that started this thread) simultaneously with benign second breakdown characteristics, in one device. FET technology covers both ends of the spectrum too, but I don't think there is any individual FET which does so.

author: Jwillis
author_rank: Super Contributor
timestamp: January 02, 2020, 02:45:12 am
content: I havn't used the original heat sink at all. I replaced that with a very large 6 pipe dual 90mm fan copper radiator from the start because I knew the stock one was worthless.I doubt I'll be using a single chip anyway .I'm modifying the the unit with Mosfets off board on a very large heat sink with active cooling .Theirs a couple other things to deal with like the onboard thermister , a couple diodes ,and one other FET. I'm still reverse engineering it to create a diagram.The rest of the board is monitoring and display.

author: Jwillis
author_rank: Super Contributor
timestamp: January 09, 2020, 03:46:56 am
content: Quote from: T3sl4co1l on January 01, 2020, 12:32:19 amIs this the kind of circuit to control parallel Linear Mosfets. Since I need at least 40V I should have at least a 40V supply to each op amp .Is that right?

author: Circlotron
author_rank: Super Contributor
timestamp: January 09, 2020, 04:12:54 am
content: No, a 10-15V supply for the opamps would be plenty. The drain of the mosfets with 40V or whatever on them is completely separate from the opamps. Just make sure you use an op amp that has an input common mode range that extends down to zero volts, assuming that the negative supply rail of the opamp is also connected to zero volts.

author: Jwillis
author_rank: Super Contributor
timestamp: January 09, 2020, 04:29:17 am
content: Quote from: Circlotron on January 09, 2020, 04:12:54 amI have TL081 with CM voltage range of -12V to 15V and OPA604 with CM voltage range of ±13V .I was leaning towards the OPA604 because it won't require any offset.http://www.ti.com/lit/ds/symlink/tl082.pdfhttps://www.ti.com/lit/ds/symlink/opa604.pdf

author: Circlotron
author_rank: Super Contributor
timestamp: January 09, 2020, 10:55:45 am
content: ^^ Nup, no good. Gotta have input CM range down to ground. A humble LM358 or LM324 would probably be good enough.pdf.datasheetcatalog.com/datasheet/stmicroelectronics/2163.pdf

author: tszaboo
author_rank: Super Contributor
timestamp: January 09, 2020, 02:08:36 pm
content: Quote from: magic on December 31, 2019, 09:04:12 amThe DC rating is typically 10-100ms rating. Otherwise you get hotspotting. You gotta understand, that a modern FET is a lot of smaller FETs in parallel, sometimes hexagons shaped.If you have a tiny FET which gets hotter, then the VGS of that cell will decrease, so that cell will conduct more current. And it gets even hotter, leading to thermal runaway. Same reason, why you cannot put two FETs in parallel, one gets hotter.The SOA should be replaced with FBSOA, which is forward bias safe operating area. Most FETs dont specify this, some from IXYS or Microsemi do. Or de-rate the FET by a lot.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 09, 2020, 03:18:47 pm
content: Or TLV2372, a good general purpose jellybean without the quirks of LM358. Mind the lower voltage rating, still fine here.Regarding SOA, please stop repeating misinformation. If hotspotting happens, it happens. If the SOA says full DC operation, it means full DC operation (at the plotted voltage and current, and rated Tc). If you don't trust them, you think they're lying, go ahead and perform the actual test. Generalizations are not useful here: there are even IGBTs available with DC ratings. SOA primarily depends upon power density, which increases with tech node (MOSFET --> BJT --> IGBT) and voltage, but current sharing depends on design just as much, and it seems SuperJunction devices are usually optimized that way, and some IGBTs are as well.So, you got legacy (lateral?) MOSFETs that were "free from 2nd breakdown", in contrast to contemporary BJTs which often were subject to that limitation (even the power amp rated ones, which usually have a small corner still subject to the limit). I'm guessing that's when all the classic books were written, from which everyone took as unchanging fact that these were the limits. Meanwhile designs continued to evolve, MOSFETs got terrible SOAs, IGBTs were introduced, SOAs improved. Finally today, parts from several generations and optimization goals are currently available.Tim

author: tszaboo
author_rank: Super Contributor
timestamp: January 09, 2020, 03:25:09 pm
content: Quote from: T3sl4co1l on January 09, 2020, 03:18:47 pmI did, it was part of my job years ago. They blow up.

author: magic
author_rank: Super Contributor
timestamp: January 09, 2020, 04:22:56 pm
content: Quote from: NANDBlog on January 09, 2020, 02:08:36 pmIMO the DC rating should be valid for DC. The rating for 10-100ms ought to be labeled 10-100msBut I did link two appnotes which are frankly quite confusing, by saying things along the lines of "typical plots seen in the datasheets don't tell the full story". Oh well, so why are they even included?And by the way, I believe I have already corrected myself regarding the part that International Rectifier blew up. It was not rated with high DC SOA; in fact, the datasheet clearly shows second breakdown behavior, both at DC and in timed pulses.I went for more information to DIYAudio because that's where lots of people who use and abuse MOSFETs hang out. The impression I got is as follows:Lateral FETs are to the first order indestructible, because their Vth tempco is weaker than Rds(on) tempco for almost any reasonable drain current. But barely anyone makes them anymore, I think Renesas still has one complementary pair which shows up as "special order" rather than "obsolete" at distributors and there are some boutique vendors for audio.Old-school planar verticals like the classic IR HEXFETs (IRFx(9)yz0) or Fairchild Stripe FETs are prone to thermal runaway when biased with constant Vgs because their combined Vth/RDS tempco remains positive up to many amps of current, but generally free of second breakdown. Somebody tested IRFP240 with 2kW pulses at 100V for milliseconds at a time, others are fanboys of Fairchild and so on.Fancy low-RDS(on) parts, particularly those using trench gate technology, have serious second breakdown issues even in short impulses and are not recommended.I know it's just a bunch of unsourced claims, but hopefully they may serve at least as search keywords.Finally, for everyone's considerationhttps://www.diyaudio.com/forums/solid-state/101745-bob-cordell-interview-bjt-vs-mosfet-95.html#post1206694Quote from: Nelson PassKnowing Pass, it's probably mostly class A amplifiers. But I don't know how much he derates, so there is that.

author: Jwillis
author_rank: Super Contributor
timestamp: January 09, 2020, 07:44:40 pm
content: Quote from: Circlotron on January 09, 2020, 10:55:45 amOh Ok . So that would explain why the outputs jump up to 8.3V on the OPA604 and 7.75v on the 741 with a 9V supply .I thought something was wrong but couldn't pin it down. What if I ran them with a negative rail instead of ground ? I don't have any LM324.Quote from: NANDBlog on January 09, 2020, 03:25:09 pmYes running Mosfets in parallel in the linear region can be tricky but it can be done.http://www.digikey.com.au/Web%20Export/Supplier%20Content/microsemi_278/pdf/microsemi-power-an-make-linear-mode-work.pdf?redirected=1. You are correct in many respects as the test in this report indicate . I don't want to have to destroy a whole bunch of transistors just to find out the FBSOA . So if I can keep them in a except-able temperature range things should go Ok. I even considered starting from scratch and using PWM to control parallel Mosfets .I have to admit that using IGBTs was a lot easier.

author: Vovk_Z
author_rank: Super Contributor
timestamp: January 10, 2020, 02:38:51 am
content: If in short: two FDL100N50F must withstand 40 V 20 A (with good enough cooling) . Only one can withstand too, but it will be on the edge.IRFP260 is very good for linear mode, but 100N50F is on the very other level of power capability - it is 10 times more capable.IRFP260 (IRFP360, 460 etc) can work with 50-100 W per case.100N50F can withstand 500-1000W per case.

author: Vovk_Z
author_rank: Super Contributor
timestamp: January 10, 2020, 02:48:31 am
content: Don't parallel mosfets for DC in electronik load, parallel them with opamps.Use LM358 (lm2904) - it's very popular and cheap.Here is an example how to parallel mosfets (it's part of my electronic load circuit):