Linear MOSFETS for Kunkin KP184 electronic load - Page 1
Forum section: electronics / projects

author: Everbrave
author_rank: Regular Contributor
timestamp: July 28, 2023, 02:51:53 pm
content: I am investigating replacing the factory mosfets in Kunkin kp184. My PCB is version 6, equipped with IRFP 264 which is slightly better specified than the former 250M. This MOSFET type is designed for switching application and probably employed for cost reasons.For linear operation modes, LITTLEFUSE (IXYS) offers specially built mosfets with extended SOA (avoiding hotspots inside the chip of multiple mosfets inside).My investigations led to IXTH75N1OL2https://eu.mouser.com/datasheet/2/240/ixyss04257_1-2272176.pdfAnother candidate from the HIPERFET family, not specially declared as for linear operation but specified DC SOA; is IXFH44N50Phttps://www.mouser.de/datasheet/2/240/Littelfuse_Discrete_MOSFETs_N-Channel_HiPerFETs_IX-1856253.pdfhas anyone considered these alternatives or even tried it?Thanks

author: PCB.Wiz
author_rank: Super Contributor
timestamp: July 29, 2023, 05:19:52 am
content: Is this because you have failures ?The parts you want to replace are 250V rated, whilst the parts you link are 500V and 100V, which are fairly large differences.

author: Everbrave
author_rank: Regular Contributor
timestamp: July 29, 2023, 08:22:57 am
content: The KP184 was delivered with one shorted MOSFET which I removed and the unit works again (since all mosfets are in parallel), with reduced max. dissipation of course. In the next few days, I will replace the defective MOSFET with the same type.The voltage rating is not critical to me since I work only with touch-safe voltages (up to 50VDC).The reason for my selection is mainly due to SOA and the characteristics in the current range up to 7-10A, primarily Id vs. Vg and S-Id (in the lower current range). The Thermal resistance was also considered. Other parameters such as Rds(on), Qg, Cin were regarded with lower priority assuming less stringent requirements on the dynamic gate drive capability.Any recommendations?

author: PCB.Wiz
author_rank: Super Contributor
timestamp: July 29, 2023, 07:24:35 pm
content: If you are running low voltage, things are unlikely to be stressed, so I’d suggest replace the single failed part and see if you get more failures.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 29, 2023, 07:53:21 pm
content: Note that the MOSFETs may be matched, i.e. selected for similar Vgs(th) @ Id, or y_fs @ Id, or multiple points of Vgs vs. Id. Might be worth checking a few to see how closely they track, or if the service manual (if available) says anything to this effect.It's possible that you could get a bogey part from a completely different process that has similar characteristics, but it is less likely than drawing from the same family.HEXFETs are not normally rated for DC SOA, but they are of the age where power density was low enough to be largely free of 2nd breakdown. Occasionally you can find older 2nd-source datasheets which do in fact plot a DC SOA; mind, this is no guarantee that all makers of a given part (say IRFP264) will survive such, or that modern versions of the same part (by any make) will.The safest procedure is to test the devices in question, at maximum Vds and Id, with adequate heatsinking, to verify freedom from failure. When you don't have the SOA in the datasheet, that is.It may also be that the source resistors are generous enough not to need matching, or that local bias/control circuitry is integrated which fully compensates for mismatch. I don't know what design that amp uses offhand.Tim

author: Everbrave
author_rank: Regular Contributor
timestamp: July 30, 2023, 09:21:43 am
content: Quote from: PCB.Wiz on July 29, 2023, 07:24:35 pmI run , mostly, low-voltage high-current tests, say 50V 10-15A; means around 2.5A at 50V (125W) per MOSFET.Practically, I will replace the faulty MOSFET, check current balance between all 6 MOSFET and see

author: Everbrave
author_rank: Regular Contributor
timestamp: July 30, 2023, 09:27:31 am
content: Quote from: T3sl4co1l on July 29, 2023, 07:53:21 pmTim,Please note that Kunkin kp184 is an electronic load with 6 MOSFETs in parallel (not an amplifier), so I doubt all 6 are matched. I bought 10 replacement MOSFETs but to test the matching with all 10 is quite tedious, let alone the stress of soldering/unsoldering on the board. So, I am left with try and error

author: Everbrave
author_rank: Regular Contributor
timestamp: July 30, 2023, 09:36:51 am
content: Thanks for all contributions. Please remember that it is not about fixing the unit but employing linear instead of logic-level MOSFETs, which is more suitable for the application in electronic loads. I know, it is costly, but one buys reliability through design robustness.I hope some have tried or at least thought of it!Thanks

author: PCB.Wiz
author_rank: Super Contributor
timestamp: July 31, 2023, 04:33:56 am
content: Quote from: Everbrave on July 30, 2023, 09:36:51 amThe web suggests that design uses 0.05 ohms source matching/ballast resistors, so you are looking at 0.333v at the 6.66 A top per-fet rating.You should measure the variations across the existing fets matching resistors under some useful load, to get an idea of the degree of matching they used.then compare the middle example of those, with your new fets. You would expect same date code parts to have better match within the batch.Quote from: Everbrave on July 30, 2023, 09:21:43 am125W per mosfet is well outside the web banner rating for that product, if it even allows you to do that, it will be a brief rating.They spec 400W which is 67W per mosfetAddit : I did find this, that shows the heatsinks are all plus-rail live. (In this example, a dropped unit deformed a smaller transformer to hit a heatsink bolt )The author did not twig that was done to lower the thermal resistance.Live heatsinks is a common way to drop thermal resistance to the lowest possible.

author: Konkedout
author_rank: Frequent Contributor
timestamp: July 31, 2023, 05:12:24 am
content: Do you have a schematic of the unit? I would expect that it might make more sense to have the MOSFET gates driven by separate amplifiers so that they do not need to be closely matched. I doubt whether it would have been practical for the manufacturer to tie the gates to the same drive voltage, thus using a single amplifier and requiring close MOSFET matching. Op amps are not generally so expensive. I do think that your idea to use "linear" MOSFETs is probably on the right track. I think that IXYS and Infineon and NXP (or Nexperia?) make linear mode MOSFETs.I occasionally use an e-load that I designed and hand-wired maybe 30 years ago. Each current range uses one modestly rated 50V MOSFET driving the bases of two or four bipolar NPN power transistors. Each of those has its own emitter resistor. This is sort of a "FETlington" arrangement. In the time since I built this, with occasional use, nothing has ever failed.

author: Vovk_Z
author_rank: Super Contributor
timestamp: July 31, 2023, 08:52:46 am
content: Quote from: Everbrave on July 28, 2023, 02:51:53 pmI'm not sure you have to replace them. Despite being Chinese-made, it is an original factory transistor, so I am still almost sure it'll meet its power dissipation specification and it must safely dissipate up to 80 W continuously or 100-120 W peak for a short time (as it is TO-247 case).80 W x 6 is 480 W continuously. And you don't want to expect to dissipate more than 480 W into that not very large case.You may consider IRFP250, IRFP260, IRFP350, IRFP360, IRFP450, and IRFP460 - all of them are totally fine for electronic load jobs.

author: Everbrave
author_rank: Regular Contributor
timestamp: July 31, 2023, 02:53:38 pm
content: Thanks all for the good responses:)May be I should say that I’m not a beginner, so to say that obvious measurements come in due of the replacement.Then, I absolutely agree that the thermal design of the Devis-Under-Modification (DUMod) is a, if not the; limiting factor!A good article to explain my motivation for this project is this one, among many others,https://eepower.com/technical-articles/understanding-linear-mosfets-and-their-applications/Summarizing,I will replace the one defective MOSFET (10 arrived today) with one from the same manufacturer (Siliconix) and accept it as it is, relying on source resistance and the assumption that every mosfet is driven separately; since I don’t have the schematics of the Kunkin KP184. Of course I will be very glad if somebody can pint me to it.I will continue searching for a suitable linear MOSFET (probably from LITTLEFUSE) and replace all of factory -HEXFET- installed, realizing that a new calibration will be necessary.I remain hoping that somebody had a similar intention or even has done it lready to shar his experience.Thanks a lot

author: Vovk_Z
author_rank: Super Contributor
timestamp: July 31, 2023, 05:13:39 pm
content: Quote from: Everbrave on July 31, 2023, 02:53:38 pmI guess this Device-Under-Repair is quite new so there isn't much info about it yet. PCB is in your hands so you may look at it carefully and tell us if gates are connected together or if they are driven separately from their own opamp. Chinese designers tend to use LM324 quad opamp.I guess they used a separate drive.

author: Everbrave
author_rank: Regular Contributor
timestamp: July 31, 2023, 05:27:02 pm
content: Quote from: Vovk_Z on July 31, 2023, 05:13:39 pmThe device is not under repair and is actually not new despite the relatively new PCB version 6 (2022). If you google for Kunkin KP184 you will find many tear-down videos and reviews. Unfortunately, I don’t have the circuit schematic but unlikely LM324-based.

author: Everbrave
author_rank: Regular Contributor
timestamp: July 31, 2023, 05:29:57 pm
content: Quote from: Vovk_Z on July 31, 2023, 08:52:46 amThanks for the MOSFET suggestions, but these are not linear MOSFETs. I’m considering the IXTH75N10L2 (see first post).

author: Vovk_Z
author_rank: Super Contributor
timestamp: July 31, 2023, 05:51:31 pm
content: Quote from: Everbrave on July 31, 2023, 05:29:57 pmThey are really good for 'linear' (??) circuits like amplifiers or electronic load. I'm not sure IXTH75N10L215 € ($17)will help itself much if there are some flaws in the design.You don't really need 'Linear L2 (TM) ' MOSFET for an electronic load. Typically you need one with appropriate and well-known DC SOA and appropriate Rds.Quote from: Everbrave on July 31, 2023, 05:27:02 pmYes, I'll find but why?(I designed several different DIY electronic loads for my own DIY laboratory. They are simple but reliable. I quite well know what I can see inside the factory devices. I know well a typical 'DC electronic load sell').

author: Everbrave
author_rank: Regular Contributor
timestamp: July 31, 2023, 09:03:38 pm
content: Quote from: Vovk_Z on July 31, 2023, 05:51:31 pmAgree regarding the DC defined SOA but why is Rds(on) is relevant for the linear operation mode (where you want to dissipate the power, in the case of electronic loads)? My understand is that it is irrelevant.Usually, the switching mosfets don’t have that DC SOA defined at all. Some types have it, e.g. my second selection IXFH44N50 (not a L2;)As for the why google? Of course it is up to you, but you asked questions about the circuit design which I’m unable to answer

author: Everbrave
author_rank: Regular Contributor
timestamp: July 31, 2023, 09:17:59 pm
content: Thinking further about this modification; it is neither fixing nor building; the more I think I need to consider the calibration after replacement the MOSFETs!The factory HEXFETs posses rather a linear characteristic for Id-Vgs in the current range used which makes the two-point calibration procedure (by SW) feasible. The linear MOSFETs show rather -the normal- parabolic Id- Vgs relationship which requires at least three-point calibration!I need to further investigate and be prepared for many experimentsThanks for all the voluble responses 😊

author: Vovk_Z
author_rank: Super Contributor
timestamp: July 31, 2023, 09:34:26 pm
content: Quote from: Everbrave on July 31, 2023, 09:03:38 pmRds is relevant when you want to work with low voltages, like 3.3 VDC or 1.5 VDC, etc., and high currents. If your use case is dealing with voltages 12 VDC and higher - then it becomes almost irrelevant. You may calculate the voltage drop budget to see which Rds suits you.Quote from: Everbrave on July 31, 2023, 09:03:38 pmYou may consider IRFP250, IRFP260, IRFP350, IRFP360, IRFP450, and IRFP460 (and similar don't mentioned their brothers/sisters as IRFP240, IRFP264 etc.) as 'general use' MOSFETs (they are).

author: Vovk_Z
author_rank: Super Contributor
timestamp: July 31, 2023, 09:59:38 pm
content: Quote from: Everbrave on July 31, 2023, 09:03:38 pmIXFH44N50 can dissipate up to 600 W itself in an optimum (or rather nearly ideal) environment. So it is a bit kind of an overperformer too. But it's up to you.I would look at IRFP260, for example. It is a bit more powerful than the original IRFP250 and if it burns again - then there is some design flaw (or user flaw).

author: Vovk_Z
author_rank: Super Contributor
timestamp: July 31, 2023, 10:04:53 pm
content: Quote from: Everbrave on July 31, 2023, 09:17:59 pmIt's ok to check but I am almost sure you don't need any calibration after the power transistor replacement. Calibration handles with current and voltage measurement circuits only. So if you'll replace transistors carefully there won't be any essential change in measurements.

author: Everbrave
author_rank: Regular Contributor
timestamp: July 31, 2023, 10:10:26 pm
content: Quote from: Vovk_Z on July 31, 2023, 09:59:38 pmYes, this is a very practical suggestion which I also considered, especially the IRFP26N or the IRFP264N which are specified for a max. Junction temp of 175Deg.Celsius. The downside is that it would take me the project fun awayI’m aware that I will not be able to dissipate the max. of IXFH44N50P since the thermal design is given. It would rather call for smaller PD but the prices are close.Thanks for your good contributions

author: Vovk_Z
author_rank: Super Contributor
timestamp: July 31, 2023, 10:12:33 pm
content: Quote from: Everbrave on July 31, 2023, 09:17:59 pmMOSFET works deeply inside the opamp feedback loop (in these DC load sells). So you may use either linear, parabolic, logarithmic or exponential, hyperbolic, square, triangle, round, or any else existing and non-existing Id-Vgs characteristic of a MOSFET - it doesn't matter. Just don't look at it. The accuracy of such DC load sell depends on current shunt accuracy, opamp accuracy, and PCB design. Voltage divider resistors matter for voltage and power measurement accuracy of cause too. But not Id-Vgs characteristic.The only thing that matters here is well known DC SOA.We may not see the DC line at SOA of IRFP250 actual datasheet (for example) but it is well known (it's an old well-known MOSFET). And there may exist different datasheets (e.g. old ones) that have this needed DC SOA line.The properties of TO-247 IRFP2xx, IRFP3xx, and IRFP4xx series are well known that's why I recommend them (as cheap and powerful enough).

author: Everbrave
author_rank: Regular Contributor
timestamp: August 01, 2023, 09:08:30 am
content: Quote from: Vovk_Z on July 31, 2023, 10:12:33 pmThanks for the hint; I think you would be right if the feedback controller is of the PI type otherwise there would remain an offset between the actual value and the set-value! In addition, a nonlinear characteristic can introduce instability. In fact; this can be a reason (together with the price), the designers chose a HEXFET such as the IRFP264 for the Id-Vgs operation range. I guess some experiments are dueBTW: I have already replaced the faulty MOSFET with one of the same type and manufacturerEDIT: in case someone is interested, after replacing the shorted MOSFET with one, randomly taken out of 10 new ones, the imbalance measured by the voltage drop over the 0.05Ohm source resistor was max. 20% between all 6 MOSFETs. This, naturally, includes the tolerance of the source resistors.

author: oz2cpu
author_rank: Frequent Contributor
timestamp: October 14, 2023, 07:31:03 pm
content: hi everbrave, great you made it work again,I am sure you can fit another mosfet better designed for analog powerversus the original switch types,you said your unit was deliverd with a broken mosfet ??you ordered it from new ?how about the unstable regulation ?did you also find that ? fixed that ?-i explain about that in this videohttps://youtu.be/1CFb5oo-P0I?si=GnW22lHhI3Or9FqP

author: MathWizard
author_rank: Super Contributor
timestamp: October 15, 2023, 02:00:42 am
content: Makes me wonder whats upgradable on my Siglent PSU. It's probably not under warranty. I have no complaints about it, but I'd spend a few bucks if there was something easy to upgrade on the output or it's own internal PSU section. Assuming it can be calibrated still tho.

author: Ashwin619
author_rank: Regular Contributor
timestamp: October 16, 2023, 09:27:44 am
content: Hello all,I recently found out this series of Linear Mosfet's with enhanced DC SOA ->https://www.infineon.com/cms/en/product/power/mosfet/n-channel/optimos-and-strongirfet-latest-family-selection-guide/optimos-linear-fet-100v-150v-200v-/I am planning to use the IPB033N10N5LFATM in one of my current load design which will be having a max Vds of around 16V and Id of 5A (4 channels). The design will be a classical opamp based with a Shunt of 100m Ohms. Do you guys think this is a overkill or will it do its work just fine.The SOA considers the Tc to be at 25C so Ill have to derate the curves (unfortunately there doesn't seem to be a way to verify this). Please enlighten me if there is...Apologies if I have gatecrashed this thread

author: wraper
author_rank: Unknown
timestamp: October 16, 2023, 09:38:03 am
content: Quote from: T3sl4co1l on July 29, 2023, 07:53:21 pmUnless this is an utter trash, each MOSFET should have it's own current feedback. Quite easy to figure out by looking on PCB.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 16, 2023, 12:48:49 pm
content: Quote from: Ashwin619 on October 16, 2023, 09:27:44 amFor rather low voltage operation the SOA is not that critical and many FET types are OK and I would prefer a classic THT case like TO220 for the low power or TO247 for higher power. They are easier to mount on a heatsink.The problem with the SOA is usually the break point that for the SOA shown is at some 35 V - where it excatly depends on the type and unit. The tendendy is to get it lower with lower voltage and more modern parts. A nasty point is that many SOA curves shown are derived from the dynamic heat capacity. This may be OK for the fast part, but it fails to include the break point and thus often gives false DC curves. Usually be careful if the transition to a faster drop is not there. This often is a bad datasheet and not a good FET.

author: Vovk_Z
author_rank: Super Contributor
timestamp: October 16, 2023, 05:35:55 pm
content: Quote from: Ashwin619 on October 16, 2023, 09:27:44 amAm I wrong or is it D2PAK-3 case MOSFET? Typically you need TO220, TO247, TO264, etc case types that suit well mounting to the heatsink. You definitely don't need state-of-the-art optimized for low Rds and low switching losses MOSFET.16*5 = 80 W - this means you need two TO220 cases or one TO247 case to safely dissipate this power.Look at some popular jelly-bean TO220 mosfets (like, IRFZ44, IRF530, IRF540, IRF640, IRF3205). But those must be used at least two in parallel or even three in parallel.As for me, I would use TO247-case transistors because it is much easier to dissipate needed power with larger cases and you need only one or two of them for up to 100W. Look at IRFP240 or similar.

author: Ashwin619
author_rank: Regular Contributor
timestamp: October 17, 2023, 05:04:57 am
content: Quote from: Kleinstein on October 16, 2023, 12:48:49 pmHello Kleinstein,I agree with your point regarding TO220 and TO247 being easier to use due easier heatsink mounting. Although I do not fully follow you wrt the later part regarding the breakdown point. Will try to understand more about this breakdown or thermal instability region.Thanks.

author: Ashwin619
author_rank: Regular Contributor
timestamp: October 17, 2023, 05:17:45 am
content: Quote from: Vovk_Z on October 16, 2023, 05:35:55 pmHello Vovk_Z,I was planning to go with a TO47 earlier due to its easier mounting options but then I saw some posts on how the regular switching Fet's like the Hexfet and other's are not suitable for such an application and can suffer from hot spot regions due to a probable breach of the thermal instability region in the SOA.I just checked the datasheet of the IRFP240 and although it doesn't specify the DC region in the SOA, I have concerns wrt the hot spot regions mentions in this article ->https://eepower.com/technical-articles/understanding-linear-mosfets-and-their-applications/There are some TO247 Linear optimised fets from IXYS, but they are quite expensive. My plan with the above D2PAK package is to have a Big copper plane for the TAB with lots of thermal vias and a heatskink mounted on the bottom with some good thermal paste + silpad. Ill check the possibility of placing a footprint of a TO247 below the D2PAK (in parallel) to test both the options.Thanks,Ashwin619.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 17, 2023, 06:29:26 am
content: You have to look at the original databooks to find DC SOA on HEXFETs for some reason. Mind, that doesn't mean anything about modern production; the designs may've changed since then. But of what I've tested, they haven't changed much it seems, and they do still have large DC SOA.So far as I've seen,every single articlethat mentions Spirito effect, either misses the point, or misunderstands the effect. I'm sure there are good articles from deeper within the industry (i.e. semiconductor physics), but that is of course the catch, I haven't seen them yet (implied: seen referenced in discussions on forums or [non-academic] articles).The closest truth I've seen is: while we can calculate stability, we must know the thermal conductivity, temperature dependence, and variance per cell, of the process itself. And manufacturers don't publish those data. So it doesn't matter, Spirito is irrelevant, it all boils down to the only thing we have: the manufacturer's published SOA.Tim

author: Vovk_Z
author_rank: Super Contributor
timestamp: October 17, 2023, 06:43:09 am
content: Quote from: Ashwin619 on October 17, 2023, 05:17:45 amIRFP240-260 and similar types (360, 460) are used in linear circuits tens of years, I guess. They are very often used in electronic loads too, because they suite well.I want to see how you are going to dissipate 80 W of power with bare PCB without a heatsink. It must be hard even with active cooling. That must be very huge PCB

author: Kleinstein
author_rank: Super Contributor
timestamp: October 17, 2023, 07:36:47 am
content: Hot spots and thermal instablity are a problem with MOSFETs for linear operation. The IRFP250 and similar are old types that are regularly used for this purpose, though today one usually gets the newer IRFP250N, whicht is better for switching and not as good for linear use. Still it is a well tried solution, though usually with no DC SOA in the data-sheet.If TO220 size is enough the IRF840 and similar would be an option. At least the Fairchild datasheet says the DC SOA is simply power limited and thus no hot spot problem.Using 200 or 500 V Mosfets looks like overkill, but the higher voltage ones are usually better in handling power.The modern IPB033N10N5LF at least has a DC SOA in the data-sheet, but this SOA is showing a massive problem with thermal instability already at some 10 V. So this is a known bad choice.

author: Smokey
author_rank: Super Contributor
timestamp: October 17, 2023, 08:44:51 am
content: Quote from: Everbrave on July 30, 2023, 09:27:31 amThat's even more reason to have them matched. Unless they have active matching, which is unlikely, they probably are either matched or seriously derated.First pass matching isn't too hard. Sweep the gate voltage and pick one point to match Vgs vs Current. Don't assume devices from the same batch will match. They never do in my experience.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 17, 2023, 09:12:12 am
content: It usually makes sense to have 1 OP-amp per FET and the Kunkin PCB seems to have plenty of parts around. Besides the matching / load sharing, there is also the problem that with increasing gate capacity the regulation gets slower. Already one of the large FETs is on the high side to be driven by a single OP-amp. So direct parallel FETs are not that attractive alone from the capacity.The FETs are also mounted quite a bit apart - with direct parallel connection one also wants tight thermal coupling, just matching is not enough.

author: Smokey
author_rank: Super Contributor
timestamp: October 18, 2023, 07:46:59 pm
content: Quote from: Kleinstein on October 17, 2023, 09:12:12 amIdeally the multiple gates are driven by a high current capacity buffer amp or equivalent not just a normal opamp.

author: Vovk_Z
author_rank: Super Contributor
timestamp: October 19, 2023, 08:23:56 am
content: Quote from: Smokey on October 18, 2023, 07:46:59 pmThat is a way if you make DC/DC where you want transistors to act like switches either fully on or fully off. Here we have electronic load where transistor works in linear mode and you typically want to share current (and power) equally across several transistors. So driving each transistor from its own opamp (with it's own shunt) is a good idea.

author: Smokey
author_rank: Super Contributor
timestamp: October 19, 2023, 08:50:50 am
content: Quote from: Vovk_Z on October 19, 2023, 08:23:56 amWe are both talking about linear operation. Yes, as I said above, independent active sharing is good. But that means separate control loops for each transistor which increases cost and complexity. If you just match them in sets (and add a degeneration resistor if you are really paranoid) and drive the gates together with a single capable buffer, then you can get pretty high bandwidth as an active load and they will all share the current.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 19, 2023, 09:07:48 am
content: The actual amount of drive required in linear mode is quite modest, on the order of 100 ohms for 100s kHz bandwidth say, for typical 100-300W size transistors. It'll be worse of course if you choose excessively large dies (large capacitance for dissipation rating, excess Vds rating for required load range); or very old types with poorly / unoptimized Crss, but that doesn't even really say much because HEXFETs weren't terrible to begin with (still useful for modest switching purposes today; mostly inferior in the higher voltage ratings), and nothing else archaic survives today. Or better for low-Crss types (NEXFETs among others; SJ above depletion Vds).Tim

author: Kleinstein
author_rank: Super Contributor
timestamp: October 19, 2023, 11:18:40 am
content: An OP is OK to drive some 5 nF of gate capacitance to get a reasonable BW of a few 100 kHz. The modern types with reduced R_on and capacitance tend to be those with a poor SOA. For linear operation the old types (e.g. IRFP250, IRF840) and also a relatively high voltage rating make totally sense to get a good SOA even withput special types for linear operation.The extra effort for a seprate control loop is quite modest: 1 OP amp, 2-3 resistors, some 1-10 nF and a separate shunt / source resistor. For the OP-amp the LM358 is not ideal, though single supply can be handy. It may be worth using low noise / low dirft types (possibly even OP27 or similar, but an OPA202 would be a good start) to get away with smaller shunts and thus less heat there.The simple parallel connection would need higher power source resistors, a lower noise OP-amp with good drive power and still has to add extra reserve for uneven load sharing - so maybe 3 instead of 2 FETs.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 19, 2023, 01:35:20 pm
content: Or if you can afford a relatively high dropout voltage (compared to Rds(on) limit alone), a generous source resistor (dropping a handful of volts at full current) enforces sharing, allowing a single common gate voltage to be used.I'm fond of TLV2372 for generic purposes: similar to LM358, but with RRIO, and without the dumb class C operation.A diode-biased complementary emitter follower, or diamond buffer if you're feeling fancy, can afford stronger drive, or just use a somewhat beefier amp, but yeah, such speed is rarely necessary for an electronic load.Tim

author: Wolfgang
author_rank: Super Contributor
timestamp: October 19, 2023, 10:52:58 pm
content: Probably a smart decision to get IXYS parts that are specified for electronic load/linear use.A lot of manufacturers try to cut corners and use switching FETs because they are cheap.Downside is that they may fail in DC mode and then they cook off (and sometimes theDUT with it).Some things to consider:- each FET should have its own current sensing.- Watch out for much increased gate capacitance of linear parts compared to switching FETsInput capacitance can be in the nanoFarads for larger parts. You need a "stiff" drive toavoid instabilities.

author: Ashwin619
author_rank: Regular Contributor
timestamp: October 23, 2023, 05:46:09 am
content: Hello,I just made a brief comparison of the SOA's of the different fet's that were suggested above as an alternative for the IPB033N10.[ Specified attachment is not available ]The IRFP264 and 260 as suggested by Vovk_Z and Kleinstein seems to be cut out for this specific job. They are also quite cheap compared to the newer Fet's. As per this application note ->https://www.infineon.com/dgdl/Infineon-ApplicationNote_Linear_Mode_Operation_Safe_Operation_Diagram_MOSFETs-ApplicationNotes-v01_00-EN.pdf?fileId=db3a30433e30e4bf013e3646e9381200for continuous linear mode operation the ability to dissipate the heat effectively and having a Low ZTC point (to avoid thermal instability) are of high importance, so I believe a TO-247 would be much better than a D2PAK considering low Rth and easier mounting options.Here's a snap of the PCB that I am trying to come up with->[ Specified attachment is not available ][ Specified attachment is not available ]I had planned to keep an option for both the D2PAK and the TO247, but now I think I might as well the remove the former and optimize the layout for the latter.Note:- The PCB layout is still incomplete. Will share it again once it's done for review.Regards,Ashwin 619.

author: Vovk_Z
author_rank: Super Contributor
timestamp: October 23, 2023, 10:05:53 am
content: Quote from: Ashwin619 on October 23, 2023, 05:46:09 amFor continuous linear mode you definitely should consider not only the SOA of MOSFETs but the whole cooling system first. I gave some examples of one of my own designs, just for example. It uses active cooling. I guess you have to consider active cooling too.I'm not sure you need separate potentiometers for separate channels. It may be convenient to have separate channels if you want to make an electronic load to test the multi-channel power supply like an ATX power supply (which has different channels +3.3VDC, +5VDC, +12VDC, +5VSB). Otherwise, you may use the same potentiometer for all MOSFETs.

author: Ashwin619
author_rank: Regular Contributor
timestamp: October 23, 2023, 10:29:56 am
content: Hello Vovk_Z,Thanks for sharing your project. I am also planning to use a heatsink with forced air cooling and if possible also add an over temperature protection using some ntc's, tl431 and some other passives. I'll be needing the 4 pots as all the channels are independent and will be having separate loads.Regards,Ashwin619.

author: p.larner
author_rank: Frequent Contributor
timestamp: October 23, 2023, 01:17:41 pm
content: i always regarded bfets as beeing poor performers for linear modes,have things changed?.