IGBT Linear operation - Page 1
Forum section: electronics / projects

author: Phil1977
author_rank: Frequent Contributor
timestamp: September 06, 2024, 05:24:13 pm
content: Hi,following situation: I have an ATORCH DL24 electronic load and today its MOSFET (CS50N20) blew up when it reached 200W for a few seconds.I had no appropriate replacement available, but a box of original Infineon IRGP4068. I thought why not give it a try, the gate threshold voltage is similar and in an electronic load it really doesn't matter if they have some higher forward voltage.The regulation seemed to work, I could start the electronic load with low load settings like 5V 1A and checked with the scope that nothing is oscillating.But already at 100W and with around 40°C heatsink temperature the IGBT was killed again. Acc. to absolute max ratings it is allowed to dissipate 330W at 25°C and 170W at 100°C - so plenty of margin there.The power source was a current limited power supply, and I didn't notice any peak in its current. It seems the IGBT died just dissipating around 100W in linear operation.My questions: Is this to be expected? Are IGBT not suited for linear operation? Can a circuit that´s designed for a MOSFET somehow kill an IGBT without exceeding its SOA?What MOSFET can be recommended for linear use up to 100V / 20A ?

author: Andy Chee
author_rank: Super Contributor
timestamp: September 06, 2024, 05:43:30 pm
content: IGBTs can be used linearly but not very well, and somewhat derated. The junction isn't designed for linear operation.For linear MOSFETs, try this:https://www.littelfuse.com/products/power-semiconductors/discrete-mosfets/n-channel-linear/standard_linearmode.aspx

author: schmitt trigger
author_rank: Super Contributor
timestamp: September 06, 2024, 06:42:57 pm
content: Please thoroughly read AND understand the following white paper, there are many more:https://www.nexperia.com/applications/interactive-app-notes/IAN50006_Power_MOSFETs_in_linear_mode

author: wraper
author_rank: Unknown
timestamp: September 06, 2024, 07:01:15 pm
content: 100W from a single mosfet is wishful thinking. Even more so for IGBT in linear mode.QuoteNot in linear mode and good luck trying to keep such case temperature at this much power dissipation.

author: Andy Chee
author_rank: Super Contributor
timestamp: September 06, 2024, 07:16:08 pm
content: Quote from: Phil1977 on September 06, 2024, 05:24:13 pmBy the way, just in case you were not aware, the absolute max ratings is NOT the same as SOA.For SOA, you need to refer to the graph in the datasheet.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 06, 2024, 09:51:03 pm
content: Quote from: schmitt trigger on September 06, 2024, 06:42:57 pmUnfortunately, misinformation on this subject is rampant --QuoteThis is blatantly false -- but in a subtle way that is easily missed.The NTC isa necessary BUT NOT SUFFICIENTfactor for instability.The realization is simple. If two thermally-independent transistors are wired in parallel, operated in the NTC region, there will be some temperature difference between them -- not necessarily fatal (total power could be less than individual ratings), but also not wholly disparate, i.e. they will still sharesomecurrent, if just not all of it.But that temperature difference depends on thermal resistance. Namely, the differential thermal resistance between devices (or, between different points on a given die).It's perfectly possible, acceptable, and normal, to operate in the NTC region, at power levels that are low in relation to the thermal resistance within the part.And since a die (and the copper backing) are quite conductive, it's easily possible to have a full SOA free of 2nd breakdown, in the NTC region. It's also possible to have very little SOA, where the NTC is just that strong, or thermal resistance is poorer.Which is also why small devices simply don't have 2nd breakdown; they overheat completely, not enough temperature drop across the thing to activate the instability.Themost correct(but still ultimately absolutely inconclusive -- but necessarily so) appnote I know of ishttps://www.infineon.com/dgdl/Infineon-ApplicationNote_Linear_Mode_Operation_Safe_Operation_Diagram_MOSFETs-AN-v01_00-EN.pdf?fileId=db3a30433e30e4bf013e3646e9381200which concludes in the lamest possible way at the top of page 11: "This means that the temperature coefficient must be known. The latter cannot be easily calculated." In other words, because manufacturers don't publish mechanical construction of their devices (for obvious reasons?),we can only know about stability by the SOA.---Regarding IGBTs, generally you would assume they are worse, as current density is much higher, and the minority carrier mechanism should tend to have even worse tempco. Indeed, many don't have a SOA plotat all, and of those that do, many only give very short time scales (that might be relevant for overload conditions but not DC).That said, I've seen more and more lately, it seems like, that show a wide SOA --https://www.onsemi.com/pdf/datasheet/ngtb40n120fl3w-d.pdffor example.I might not trust that SOA very much, and would want to run a spot test on a sample device before putting something into production that depends on the behavior.Tim

author: DavidAlfa
author_rank: Super Contributor
timestamp: September 06, 2024, 09:57:53 pm
content: MOSFET works with voltage, IGBT with current, so they can't replace each other.Not that they can't work in linear mode, but the circuit must be designed to handle them.Your question is like asking "Is diesel a bad fuel? My gasoline car runs badly with it".- That's what happens after a very long week + bad sleepingQuote from: wraper on September 06, 2024, 07:01:15 pmWhat? 330w continuous is 330w continuous - period.330w loses in switching is basically linear operation anyways (Rising/falling edges).Though that power is a bit crazy for a single transistor in "small" package.200W is just too much anyways, thats why it blowed in first place!Chinese crap... to handle 300W properly you would need at least 3 or 4 mosfet working in parallel.

author: wraper
author_rank: Unknown
timestamp: September 06, 2024, 10:06:26 pm
content: Quote from: DavidAlfa on September 06, 2024, 09:57:53 pmHave you ever heard about SOA? The issue with linear region is hotspots on the die. Not to say you'd need liquid nitrogen cooling to reach conditions for 330W.

author: Phil1977
author_rank: Frequent Contributor
timestamp: September 07, 2024, 07:41:39 am
content: Thanks for all the answers - it seems this topic is not really easy if there are several long and exhausting application notes..Just a few comments:Quote from: DavidAlfa on September 06, 2024, 09:57:53 pmYou can say bad things about China as much as you want, but this time it was my fault. This unit was limited to 150W, I deliberately increased the limit due to a simple misunderstanding that is related to the thermal capabilities:Quote from: wraper on September 06, 2024, 07:01:15 pmOf course, to cool 330W from a TO247 component is difficult. But I didn't want to operate it continuously with 200W. And that´s where usually the dissipation value from the absolute max ratings gets relevant. If there is no pulsed operation specified, but if the pulse power stays below the max dissipation then for many semiconductors the world is okay. But obviously not for FETs / IGBTs in linear operation.Quote from: T3sl4co1l on September 06, 2024, 09:51:03 pmThat´s what´s really confusing me. In my naive sight on the electronic world I usually think that if you follow all the limits in the datasheet then the component should expect a long life. But there obviously are SOA limitations that are not content of the basic datasheet! In case of the IRGP4068D I can find only a SOA diagram for the forward diode and this is just a rectangleQuote from: DavidAlfa on September 06, 2024, 09:57:53 pmI thought the same before replacing the part. But in linear mode and controlled by a heavily dampened opamp circuit I think they may be exchangeable. Roughly simplified they are both kind of a voltage controlled current source. Of course the regulation behaviour will differ but that´s why the first thing I checked was if there are any oscillations - and there were none.In conclusion: I´ll get one of these expensive FETs intended for linear operation from little fuse and I´ll keep the power limit at 150W. But thanks again for all the insights, it´s really nice here that you get so much expert knowledge

author: Slh
author_rank: Regular Contributor
timestamp: September 07, 2024, 08:42:06 am
content: It might be worth looking at superjunction MOSFETs. I've noticed that a lot of those have good DC SOA. I haven't tested any to the limits but I do have a to-247 superjunction that's done a few seconds at 50W and minutes at 15W.Superjunction FETs are typically the 650V ones.I haven't looked into the physics of why superjunctions have such good DC SOA (or why they bother to test them as they're mostly for switching applications) but someone on here might know why.

author: wraper
author_rank: Unknown
timestamp: September 07, 2024, 10:56:08 am
content: Quote from: Phil1977 on September 07, 2024, 07:41:39 amWhich is still a deception. A half decent electronic load would have 4x the number of MOSFETs for this power rating. 150W is an extreme abuse which won't last long, even if done for a few seconds at a time. Especially with a piss-poor cooler it has. It's would need high-end heat-pipe cooler with liquid metal thermal interface to keep case temperature reasonable.

author: Phil1977
author_rank: Frequent Contributor
timestamp: September 07, 2024, 11:24:14 am
content: No one forces you to buy this thing - if you have something better then no one wants to talk bad about it.But please lay down that hubris that everything needs to be thick and heavy and oversized. 150W is a common TDP for CPU coolers, and the T_j of a FET may be significantly higher than of a CPU.And the effect of "liquid-metal" thermal paste is more a hocus pocus esoteric of the gamer scene. A well applied thin conventional paste is well capable of transferring the heat.Let´s please come back on topic about FET and IGBT linear operation. I´m absolutely sure that a well defined SOA can tell you how to reliably operate a FET.

author: wraper
author_rank: Unknown
timestamp: September 07, 2024, 11:34:01 am
content: ^You forgot several things: 1. cooler used in this electronic load is something that would come with 65W TDP CPU at best. CPU silicon die and heat spreader size are much larger than this MOSFET die/package, so there isn't anywhere as much power density in CPU. 3. In linear mode some spots on the die heat up way more than the die on average.

author: Andy Chee
author_rank: Super Contributor
timestamp: September 07, 2024, 11:53:36 am
content: Quote from: Phil1977 on September 07, 2024, 11:24:14 amHere's the SOA for your CS50N20Note the Tj specification. There is no way your current cooling arrangements are capable of maintaining that specification.

author: Phil1977
author_rank: Frequent Contributor
timestamp: September 07, 2024, 12:38:40 pm
content: The IXTH80N20L specifies the SOA for 75°C Heatsink temperature:This still looks like 300W of allowed dissipation at 30V/10A. Of course the PC cooler that was delivered with the cheap electronic load can not perform that well, but I can try and monitor the temperature where the FET is mounted.If this component still gets fried with 150°C I´ll give up and get an other electronic load.

author: Andy Chee
author_rank: Super Contributor
timestamp: September 07, 2024, 02:15:40 pm
content: remember to subtract your room temperature from that 75 degree allowance.For example, if your room temperature is 25 deg C, then your heatsink is only permitted to rise 50 additional degrees before MOSFET failure.There's no way to dissipate 300W with only 50 degrees rise from your tiny cooling setup! That explains the commentary re: exotic cooling techniques.

author: moffy
author_rank: Super Contributor
timestamp: September 07, 2024, 02:20:40 pm
content: Monitoring the static temp of the heatsink in such a situation doesn't take into account the impulse response of the heatsink or how quickly a thermal impulse is spread from a point. This is the most likely cause of a failure, excessive junction temperature, pity it doesn't have an integral diode for temperature monitoring.

author: Andy Chee
author_rank: Super Contributor
timestamp: September 07, 2024, 02:28:31 pm
content: A lesson in thermal calculations might be in order here (courtesy of TI):https://www.ti.com/lit/an/slva462/slva462.pdf

author: wraper
author_rank: Unknown
timestamp: September 07, 2024, 03:42:18 pm
content: Quote from: Andy Chee on September 07, 2024, 02:15:40 pmNot heatsink but case temperature that will be significantly higher than heatsink temperature. Not to say heatsink surface under the package by itself will be much hotter than the rest of the heatsink.

author: Phil1977
author_rank: Frequent Contributor
timestamp: September 07, 2024, 05:28:48 pm
content: That´s exactly why I trust the CPU coolers to be quite capable there. They are designed to keep a CPU under 80°C in a case that´s already 45°C or more. As far as I remember typical CPU fans have around 0.2K/W or better from the heat spreader to ambient. That means that 150W dissipation (I never told I want to use the 300W that are allowed in the SOA. It´s just good to have some margin) heat up the heat spreader of the CPU cooler to 30K over ambient.And the electronic load is not running in a closed housing but freely on the bench with not much more than 25°C. So 55°C at the heat spreader seem feasible.Please lets stop this discussion about heat management here. As soon as I get the new FET I will insert a thermocouple into its housing. The TO-247 package easily allows to drill a small sinkhole next to the mounting hole. A thermocouple with some thermal paste inside the package should give quite a reliable temperature measurement. I´m quite sure I can keep the temperature of the tab itself below 75°C - if the standard CPU cooler is not enough I still have a quite serious water cooler available.

author: schmitt trigger
author_rank: Super Contributor
timestamp: September 07, 2024, 05:33:28 pm
content: Quote from: T3sl4co1l on September 06, 2024, 09:51:03 pmLearned something additional today. That is the reason I mentioned in my original post that there are several other app notes related to this topic. Each carries some kernels of useful information.The conclusion: don’t exceed the data sheet’s SOA. With adequate margin.

author: Phil1977
author_rank: Frequent Contributor
timestamp: September 07, 2024, 05:48:27 pm
content: Quote from: schmitt trigger on September 07, 2024, 05:33:28 pmThat´s what is still confusing me: In the datasheet of the mentioned IGBT is no SOA is given but it was killed at around 30% of it´s max dissipation.The FET that originally was in the electronic load was probably overloaded by my 200W-excursion. And it was already running for many hours with 150W before, but that of course doesn't mean it was safe (luckily it didn't blow up when connected to unprotected Makita batteries before. This would probably have created some fireworks)

author: wraper
author_rank: Unknown
timestamp: September 07, 2024, 05:55:51 pm
content: Quote from: Phil1977 on September 07, 2024, 05:28:48 pmType of the heatsink used in this electronic load is the worst that are barely able to cool low end CPUs under load. There is nothing to trust, it's the simplest shit with no heat pipes.QuoteOnly in your dreams. Even high end water cooler will struggle to do it with such heat dissipation over area this small.

author: Jeroen3
author_rank: Super Contributor
timestamp: September 07, 2024, 06:16:35 pm
content: If you look at commercially made dc loads they limit the dissipation per transistor to sub 50 W.Eg thisArrayone has 6 mosfets for 300W.Cooling solution quickly becomes incredibly tricky if you go higher. Try to limit your load to 50W, it will save you mosfets.The limits in the datasheet are measured with aninfinity heatsink, taking it to the package limits (Rthj-case).If the heatsink temperature goes up, the delta between junction and case shrinks, limiting your overall power budget.An IGBT is physically incapable of being used in linear mode. They are highly tuned switching devices so their structure is optimized for turn on and off.In linear mode you will burn it out because only a portion of the device will conduct and that is a guarantee to overheat and fail-short.There is a name for this effect, but I can't remember! (positive feedback loop in transistors that increases gain with current/temperature)->Spirito effect!Some IGBT also have a maximum switching time specified in their datasheet. It's hard to make it switch slower, which can be real challange in EMC tests.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 07, 2024, 07:00:37 pm
content: Quote from: Slh on September 07, 2024, 08:42:06 amAgain, like IGBTs, their current density is higher than ever; that would seem to be a problem. My guess is, because the die can be thinner and smaller for the same voltage rating, the thermal resistance across it is smaller. There may be other tweaks that are possible, that just happen to give better SOA as well (maybe quirks of the design make it more amenable to full SOA, so why not adjust it a little further to get that too? etc.).Classic HEXFETs are generally fine -- IRFP460 for instance, or friends, if ratings are adequate for your operating range. They were originally published with full DC SOA curves. Most/all manufacturers have dropped that for some reason, so, no design assurance sadly, but so far I haven't heard of one that didn't pass the test (and was legitimately sourced). Later generation HEXFETs ("advanced" if at all; they mostly dropped the number after 5th gen) tend to have high enough density not to offer full SOA anymore -- but those are mostly in lower voltages I think. CompareIRFP4768for instance.Incidentally -- or, at least AFAIK -- SJ MOSFETs have a lateral structure on top, just like HEXFETs and their striped cousins; this is different from the vertical channel of trench MOS (used for lower voltage parts), but I don't think it has any important consequences like for tempco, as can be seen from the overall curves.Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 07, 2024, 07:04:47 pm
content: Quote from: Phil1977 on September 07, 2024, 05:48:27 pmYeah they're telling you the SOA is "don't".Quote from: Jeroen3 on September 07, 2024, 06:16:35 pmThis is false -- IGBTs are continuous devices like any other transistor. Please do not spread misinformation; there is already dangerously much in this subject already, as this thread plainly shows.IGBTs *can* exhibit latchup, in which case it fucking shorts out (controlled switch on, uncontrolled switch off) -- it becomes an SCR. IGBTs have been largely latchup-free since the 80s or 90s, so much so that datasheets don't even bother to proclaim their freedom of it.IGBTs typically have small (or N/A!) SOAs, but some are showing full SOA.IGBTs have been used in audio amplifiers; I can't say this is a very wise or effective choice, there's often no accounting for that among audiophile types, y'know, but more to say it's possible, and has been done. I believe there was even a line of Toshiba parts intended for linear operation.QuoteI don't even know where one comes up with this--?MOSFETs may have max time as well, it's a characteristic not a rating. Max is just the worst case parameter spread, not a "you must obey".Tim

author: David Hess
author_rank: Super Contributor
timestamp: September 07, 2024, 09:37:02 pm
content: Quote from: Phil1977 on September 06, 2024, 05:24:13 pmModern IGBTs have poor SOA so are not suited to high power linear operation. Substitution between MOSFETs and IGBTs is possible depending on the matching of the specifications, but for equal voltage and current specifications, the IGBT will be smaller and have a lower power rating. This suggests looking for a replacement with an equal or higher power rating, even if this means a higher voltage and current rating.Quote from: T3sl4co1l on September 07, 2024, 07:04:47 pmToshiba made n-channel and p-channel IGBTs for complementary class-AB linear audio power amplifiers and they would have been well characterized for linear operation. I think it was more of a marketing thing rather than because of better performance.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 07, 2024, 11:46:42 pm
content: The datasheet shows a full SOA for the CS50N20 MOSFET. However chances are that this graph is derived from transisent thermal resistance and this way ignoring possible thermal instability. It is unfortunate that such false SOA graphs are sometimes shown.There are special, but quite expensive MOSFETs specified for linear operation. If more standard FETs are wanted, one should look for either rather old types with relatively large chip and large R_on or types for a rather high voltage, like the superjunction types. An old time favorite when MOSFET audio amplifiers came up was the IRFP250, that could still be not too bad, even though the IRFP250N one often gets now is not as good for linear operation.The IRFP460 mentioned in another answer before would be more powerful.A problem may be that those fets that are better suited for linear operation tend to have a higher gate capacitance, which can make the replacement a bit tricky. One should also derate the power to something like 50% or maybe < 100 W for a TO247 case. A direct parallel connection is not practical - so spreading the heat would need a change to the circuit.

author: Phil1977
author_rank: Frequent Contributor
timestamp: September 08, 2024, 07:49:31 am
content: Thanks for the well qualified last posts!The IXTH80N20L is explicitly specified for linear operation and IXYS (now littlefuse) gives a so called "Forward Bias Safe Operating Area (FBSOA) - Conveniently they specify it for 25°C heat sink and 75°C heat sink temperature. This part is not crazily expensive (11€ at mouser). The datasheet explicitly states its suited for DC loads and current regulators.I well know what it means to cool parts with crazy power densities. I worked on industrial projects with high power LEDs, and these have forward currents up to 7A/mm2. That means up to 10W/mm2heat dissipation, that´s 1kW/cm2- much more than a standard CPU. The LED dies really needed to stay below 80°C to stay efficient, we did lots of FEM-simulations and did experiments with diamond heat spreaders. And I´m quite sure you don't need a FEM simulation to get rid of 150W of roughly one cm2of the TO247 tab. A decent copper heatspreader with a standard 0.2K/W CPU fan will do it.PS: The IXTH80N20L is not in stock - I´ll try the IRFP460 as proposed by Kleinstein.

author: Phil1977
author_rank: Frequent Contributor
timestamp: September 08, 2024, 12:23:22 pm
content: Just found this on the Infineon homepage:Quotehttps://community.infineon.com/t5/Knowledge-Base-Articles/Is-linear-mode-allowed-i-e-Desaturated-IGBT/ta-p/491055#.

author: Phil1977
author_rank: Frequent Contributor
timestamp: September 10, 2024, 12:12:21 pm
content: As an interim solution I exchanged the blown FET by a 20N60C3 in a TO220-package.Together with a CPU cooler with a copper heat spreader it runs at 120W with a T_case of 75°C - measured with a thermocouple at the upper side of the tab and confirmed by a thermal imager.The 20N60C3 is also not explicitly qualified for linear operation but it seems the FETs for higher voltage with a bit higher RDS_on have less problem with that.For sure there are better electronic loads available - but so far the DL24 is working fine for me.

author: Jeroen3
author_rank: Super Contributor
timestamp: September 10, 2024, 12:23:58 pm
content: Quote from: T3sl4co1l on September 07, 2024, 07:04:47 pmOkay, I will tell Semikron to update their application manual.Quote from: SEMIKRON_Application-Manual-Power-Semiconductors_English-EN_2015.pdfAnd they mention it several other times as well.

author: Phil1977
author_rank: Frequent Contributor
timestamp: September 10, 2024, 12:32:35 pm
content: Quote from: Jeroen3 on September 10, 2024, 12:23:58 pmI think it´s a misunderstanding: You can bring an IGBT into linear operation (what T3SL has mentioned and why I tried to use it as the FET-replacement...), but the manufacturers do not allow it because it quickly leads to islanding effects with following destruction (as mentioned by Infineon and proofed by the destruction at comparatively low dissipation).But anyhow it´s different compared to e.g. a SCR, which has no linear operation mode from the beginning.

author: Jeroen3
author_rank: Super Contributor
timestamp: September 10, 2024, 01:32:45 pm
content: If you can get an SCR into linear mode I'll buy you a beer, haha.

author: wraper
author_rank: Unknown
timestamp: September 10, 2024, 01:39:50 pm
content: Quote from: Jeroen3 on September 10, 2024, 01:32:45 pmAlthough I did not try, I suspect it may be possible if current is kept below latching current.

author: Zero999
author_rank: Super Contributor
timestamp: September 10, 2024, 02:35:03 pm
content: Quote from: wraper on September 10, 2024, 01:39:50 pmToo lower trigger curent can cause an SCR to operate in linear mode, when it behaves like a BJT. This can kill it, if the SOA is exceeded.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 10, 2024, 08:07:25 pm
content: Quote from: Jeroen3 on September 10, 2024, 12:23:58 pmRead more closely:1. Apparently this is a multi-chip module.2. The dies are probably matched for Vce(sat), and maybe tempco thereof (if it's also subject to variation), but not also Vge(th) and its tempco (which may be independent of the other parameters).3. Theyare separate dies, no different than paralleling discrete transistors, which isn't what we're talking about here.4. It can still turn out well, if the thermal resistance is good enough -- modules generally are quite good at thermal, so there's still a possibility, perhaps if they had used (linear mode-) matched dies; but5. They don't specify this mode of operation, and don't intend to support it, just nip it in the bud, discourage operation regardless of the considerations (specifically because, the above considerations are not something they want to bother supporting).Tim

author: Jeroen3
author_rank: Super Contributor
timestamp: September 11, 2024, 10:01:00 am
content: They also explain their dies are a hexagonal pattern of junctions. With multiple chips it is obvious a problem.Can you provide an example of an IGBT suitable for linear usage, I can't seem to find any.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 11, 2024, 10:30:41 am
content: I mean hex doesn't matter, HEXFETs have had wide SOA for a long time, only relatively recently (early 2000s?) they've achieved such density (and other optimizations) to affect SOA. Over SOA-relevant time scales, there's not nearly enough curvature in the temperature field to matter, it's just the overall power density and tempco that matters; it's treatable as a continuous medium.The Toshiba mentioned earlier was,https://www.alldatasheet.com/datasheet-pdf/view/30904/TOSHIBA/GT20D201.htmlA newer one,https://www.onsemi.com/pdf/datasheet/fga50t65shd-d.pdf(I have some of these, I might test one or two)Another,https://www.onsemi.com/pdf/datasheet/ngtb40n120fl3w-d.pdf"Suitable" might be overstating it. I don't really know why they show a DC SOA. Or how seriously they mean it. But there it is.Tim

author: Kleinstein
author_rank: Super Contributor
timestamp: September 11, 2024, 10:58:50 am
content: The toshiba part seems to be made for linear operation.The SOA in the Onsemi parts is likely just calculated from the thermal transients.The same problem happens with some MOSFET SOA curves. Derive the maximum power for a given pulse length from transient thermal resistance andthan get a useless SOA curve that misses the important part (possible thermal instability / 2nd break-down).Even the Toshiba part shows the break at high voltage where the SOA is no longer limited by power or pulse energy, but lower for the higher voltages.With a more normal IGBT a SOA curve without that extra limitation is more like a red flag indicating non trustworthy SOA curve.So I consider the 2 onsemi part no better than the typical IGBTs, just a worse data-sheet.

author: Phil1977
author_rank: Frequent Contributor
timestamp: September 11, 2024, 01:28:10 pm
content: Is it a stupid idea to replace an IGBT by a small FET + large BJT for linear applications? Like in its equivalent circuit?

author: Andy Chee
author_rank: Super Contributor
timestamp: September 11, 2024, 01:59:17 pm
content: Quote from: Phil1977 on September 11, 2024, 01:28:10 pmI'd imagine the linearity of the darlington-like FET/BJT combo, would be worse than a single IGBT device alone, even if the IGBT isn't specified to operate linearly.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 11, 2024, 09:46:03 pm
content: Quote from: Kleinstein on September 11, 2024, 10:58:50 amI've heard this paranoia before, but never any justification for it.Do you know of any evidence to support the claim that an SOA might be calculated without taking instability into account?How can you tell when one is calculated, and another is legitimate?Or put another way -- should one put blind trust in the marketing blurb that e.g. "Linear L2" MOSFETs are, in fact, suitable for linear operation? How much veracity does that claim carry, versus a Superjunction MOSFET, or IGBT, that doesn't make such a statement but carries an identical(ly shaped) SOA?I've certainly been burned by marketing claims before (granted, mostly on much more complicated things -- ICs), and a lack of reading the fine print, the data tables, the characteristic curves. I've not seen a scenario yet where the inverse is true -- but that doesn't mean they don't exist, and I would be very interested to see counterexamples.QuoteSo you categorically deny that they might have wide SOA? Why do you know this with such certainty---you must have proof to this effect? Is it a fundamental semiconductor issue, are there journal articles discussing this? (I admit I haven't searched journals for IGBT SOA before.) Is it proven that hotspotting can occur from the resistance of the die thickness itself, thus independent of mounting plate resistance? Could it occur at multiple points simultaneously, or even individual cells, rather than a mass effect biased towards the center of the die (as is widely documented to happen with BJTs and MOSFETs)?Or perhaps it's much more mundane than this -- you simply asked an FAE and they said yeah it's full of shit. Could you quote the e-mail so we can name-and-shame?Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 11, 2024, 09:56:03 pm
content: Quote from: Phil1977 on September 11, 2024, 01:28:10 pmYes, but for interesting reasons: one of the ways they vary speed grades (and consequently optimize voltage drop and other parameters), is basically by varying the hFE of the bipolar effect. The gate-switched (majority carrier) current might be as much as half the bipolar-switched (minority carrier) current. This allows significant lifetime-killing (reduces hFE, storage time) but reduces overall current density (increases cost).Since low-hFE parts aren't available, you can't really do this; let alone high voltage PNPs, that just plain don't exist (I think?).Basically, imagine you had an upside-down line-output transistor: even then, hFE might be in the 10-20 range at modest currents, though dropping to just a few at high current in saturation.There's also the JFET effect, that I keep forgetting what role it plays in IGBT (and MOSFET to some extent) behavior... but hey, I use IGBTs so rarely, that would happen.You'd expect IGBT SOA to be generally comparable to BJT SOA for such reasons, but you never know if they're doing things that have a strong impact on that, or what. Field stop seems to have some benefit, positive Vce(sat) tempco seems correlated, and they could always be doing something gross/stupid but effective like emitter degeneration resistance -- unlikely given the pressure to low Vce(sat), but who knows, lots of tweaks are possible.Tim

author: Kleinstein
author_rank: Super Contributor
timestamp: September 12, 2024, 09:33:58 am
content: I don't claim that the SOA for the onsemi IGBTs is for sure false, but there is a large chance that it is. False SOA curve are not so rare with MOSFETs. So for me the claimed SOA curve has zero credebility.An only power limited SOA would be extraordinary for a high voltage BJT or IGBT. The current concentration / thermal instability can happen aready with only the thermal resistance of the silicon die. The countemeasures (emitter or base resistance) should comes with a higher saturation voltage and thus not so good switching performance. In a BJT there may be addition (e.g. hot electrons) unstable effects.So it is unlikely that a IGBT with an ideal SOA would not specially mark this as a special feature.The linear marked MOSFETs may internally not be different from superjunction MOSFETs, at least not much. The manufacturers should have a reasonable good idea on which types are well suitetable for linear operation and they may have slight differences in the parameters, like a thicker substrate. One difference should be in stricter testing, e.g. for a flaw-less die attach and ideally an individual testing of at least the pulsed SOA. At least for audio BJTs they do (or did) offer parts that are individually SOA tested. The need for stricter testing can explain the extra costs for linear rated MOSFETs.

author: mtwieg
author_rank: Frequent Contributor
timestamp: September 12, 2024, 11:29:59 am
content: Quote from: Slh on September 07, 2024, 08:42:06 amI had been using a superjunction FET (FCA35N60) in a linear amplifier with pretty good results, but unfortunately they're now obsolete with no form-fit-equivalent available.I'm not sure superjunction FETs have higher SOA specs in general. They're certainly not marketed that way AFAIK.Quote from: T3sl4co1l on September 07, 2024, 07:00:37 pmIndeed, older HEXFETs have excellent SOA specs compared to modern parts. For example, theIRF630 from Vishaycan handle Ids=12A at Vds=50V for 1ms. But theIRF630 from STMcan only handle Ids=6A under the same conditions! So even with the exact same part number you have to be careful.Those are "third generation" HEXFETs. TheIRF630NPBF from International Rectifier(now Infineon) is a "fifth generation" HEXFET, and can only handle 4A under those conditions.I always check the curves carefully in every datasheet if I think SOA is important.

author: David Hess
author_rank: Super Contributor
timestamp: September 12, 2024, 01:22:00 pm
content: The oldest lateral and vertical power MOSFETs had a square SOA limited only by power and temperature, and this was a big feature for marketing, along with no secondary breakdown.Lateral power MOSFETs used to exist which had all kinds of advantages like square SOA making them useful in linear applications and low capacitance for higher frequency operation, but vertical power MOSFETs had smaller dies making them more economical and quickly displaced them.Quote from: Phil1977 on September 11, 2024, 01:28:10 pmI doubt there will be any advantage. Both parts have to sustain the full voltage, and discrete PNP transistors are not the highest voltage.In some applications using a low voltage n-channel MOSFET in combination with a high voltage NPN cascode transistor can have some performance advantages over a single high voltage n-channel MOSFET or single high voltage NPN, but MOSFETs are always getting better and IGBTs exist now so this configuration is less common than it used to be.

author: Phil1977
author_rank: Frequent Contributor
timestamp: September 12, 2024, 02:51:45 pm
content: Quote from: David Hess on September 12, 2024, 01:22:00 pmIt was just loudly thinking about enhancing the power of the electronic load. But if I would convert it to BJTs I could probably completely omit the FET and control the BJTs in a Darlington circuit.But as I said, it was only loud thinking. There are a lot of possibilities and ideas for DIY electronic loads or even better DC-DC-converters with controllable input power.Anyhow, it´s really nice to learn about the basics and obviously the operation of modern power semiconductors in linear mode is not trivial.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 12, 2024, 03:55:33 pm
content: As a replacement in position a cascode circuit with 2 MOSFETs could be possibility. A lower voltage one for low capacitance and the high voltage one with the gate at a fixed voltage of some 5 to 15 V. There may still be issues turning off fast with a low votlage.

author: David Hess
author_rank: Super Contributor
timestamp: September 12, 2024, 05:26:40 pm
content: Quote from: Phil1977 on September 12, 2024, 02:51:45 pmI looked at the economics of that problem a couple years ago. Using bipolar transistors is a competitive solution, with big ring emitter bipolar transistors up to 200 watts available for linear audio and servo applications at reasonable prices. Power MOSFETs have the advantage of being available in larger sizes, up to about 500 watts, so fewer packages are required, but costs are higher.

author: Randy222
author_rank: Frequent Contributor
timestamp: September 16, 2024, 02:30:05 pm
content: This is more of a "specs" comment than a "solution" comment.Specs of the items are almost always come from modelingorof the package. It's rare to see a manufacture do actual testing when it comes to thermal controls."watts" almost always means it's a temp related issue. But we don't see maker mounting the package to a 1ton block of aluminum (or ideal finned sink) at room temp to sink heat out during the "max" test.IIRC, few if any show graphs for "max". It's temp that kills the device, but you could exceed a max number if the time there is very small, because it takes times for temp changes, etc.We also don't really see spec sheets calling out heatsink specification numbers, they kinda leave the thermal design up to you.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 16, 2024, 06:33:29 pm
content: Quote from: Randy222 on September 16, 2024, 02:30:05 pmDo you have a citation for this statement of fact? Example, have you worked at one and have read process documents to that effect? Or you've seen appnotes which document it?QuoteWell... no... why would they? There are much better ways anyway.The typical standard is nucleated boiling liquid, a bath of Freon stirred aggressively and held at a pressure such that it's just boiling at ambient temperature. This holds Tc, very literally every exposed point of the case, at 25°C for the test.This itself is not well documented, to the consternation of many; but IR did at least release an appnote during the peak of the specsmanship era when this started really getting pushed (and, AFAIK, is still the accepted standard outside of SMT devices; though many tabbed SMTs still also have a useless Tc=25°C spec anyway).Namely, AN-1140:https://www.infineon.com/dgdl/Infineon-ApplicationNote_MOSFET_Continuous_Current_Rating-AN-v01_00-EN.pdf?fileId=5546d462689a790c0169166671e9454cQuoteI...guess? Not sure what max you mean. SOA plots don't change much with temperature. (Tempco and thermal resistance change a little with temperature, which can change size of the instability region.) There's transient thermal resistance for pulses. Naming a particular heatsink wouldn't be very useful, though one might argue SMT footprint data (minimal, thermal pours of given area, inner plane, etc.; see JESD51 series) is somewhat analogous -- but also more flexible, and (nearly) free of cost in comparison, so is worth documenting.Tim

author: PCB.Wiz
author_rank: Super Contributor
timestamp: September 16, 2024, 08:51:37 pm
content: A problem with linear-anything, is power dissipation.The new SMD packages are great for low loss switching and have very low on losses, but are not so great at heat removal.I did see a new isolated power package from IXYS that could be used for more modern Linear MOSFET designs. It just needs the market demand.This isolated package is designed for heatsink clip/pressure mount, not soldering.

author: Randy222
author_rank: Frequent Contributor
timestamp: September 18, 2024, 03:50:01 pm
content: Quote from: T3sl4co1l on September 16, 2024, 06:33:29 pmI don't know of any manufacturer that places every new item into a lab test jig.Packages, various silicon junctions, etc are well-known things these days. Most of them are already modeled. We know in good terms how heat will move through the packaging along with max junction temp of the design.We don't really see an abundance of new packages being used, that would require new lab testing for each new item.The post behind this one shows some "new" ceramic bottom to the package. A model already exists that accounts for that type of configuration, we know how heat moves through the ceramic.When the heat density goes up (watts per area) it's harder to sink to keep junction temps stable. Why not make the rated 20A 100W IGBT (that comes in TO220) in a package that is 1/2 the surface area of the TO220? It can be made, but controlling the temp inside will be an issue. The makers would love to make things smaller, but heat density becomes an issue even when the heatsink is very large.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 18, 2024, 04:55:46 pm
content: Those are all very nice assumptions -- but I can extrapolate from physical principles as well; notice I asked for citations, so, I guess I should assume you don't have any? (In other words: "a 'no' would suffice".)I'd love to have such references myself; they're dreadfully sparse on the internet as it is. Oh well, there's always hope.Quote from: Randy222 on September 18, 2024, 03:50:01 pmWell, packing the die into such a small part is the first challenge.It's not impossible; maybe not very practical outside of pulsed application, but a few products exist. DPAK is much smaller than TO-220/D2PAK:https://www.st.com/content/ccc/resource/technical/document/datasheet/bb/1b/ca/f3/e3/b6/4e/02/CD00058414.pdf/files/CD00058414.pdf/jcr:content/translations/en.CD00058414.pdfThe other part of that is, the heat density doesn't at all go up; it's limited by die thickness and attachment. DPAKs can be rated fairly generously, in the most generous cases at least, but at 62W maximum, this certainly isn't one of them. Doesn't help that it's a co-pack, so (a bit less than) half the bond area is committed to a diode.Tim

author: DavidAlfa
author_rank: Super Contributor
timestamp: September 18, 2024, 07:04:05 pm
content: But - Do we agree that continuous power dissipation is just that, no matter what?Yes, I perfectly understand surge/peak power, meant to be a small, low duty occurrence, let's say 1000% of the of max continuous power, lasting 20ms every 1s.But "100W continuous power", to me it means "This device can dissipate 100W 24/7 below a defined Tj limit".No matter if it's a small array of IGBTs or whatever, continous power should account for hotspots and manufacturing tolerances, some parts migh be extremely well bonded and tolerate 200W, while the worst only 130W.Apply a safe gap and certify for 100W.

author: Randy222
author_rank: Frequent Contributor
timestamp: September 18, 2024, 10:49:11 pm
content: Quote from: DavidAlfa on September 18, 2024, 07:04:05 pmDissipating joules per sec means just that, but it in no way describes the temps.Mount a TO220 high power metal film resistor to a huge heatsink and dissipate 100w, mount the same TO220 to a much smaller heatsink and dissipate 100w. The smaller one will be much hotter, including the TO220 package.Watts just gives you an energy rate, how you sink out that energy is a bigger discussion/problem. As long as the sink is big enough to keep junction temp below max, for any application environment, then the package should survive.The package itself is a thermal resistor, so for any given heatsink there will come a watts number where package temp gets too high. As example, TO220 in open air vs same in open air with a fan vs same mounted to large hunk of aluminum that is water chilled.If the heatsink is very good (big water cooled, big fan cooled, etc) you can likely have the package dissipate more than rated watts, as long as the package temp does not go high enough to damage it. In real world there are limitations when it comes to physical heatsinks.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 19, 2024, 10:28:33 am
content: The rated power (P_tot) for power transistors is for the limit of a very good heat sink, that can hold the case interface to 25 C.With a more realistic, no so perfect heat sink one would need to reduce the power from the P_tot parameter.It would need cooling the case to less than 25 C to safely allow for more than the nominal power.With plastic cases P_tot is usually calculated for 150° C junction temperature, while for a metal case (e.g. TO3) they usually allow 200° C. A higher temperature will cause a reduced lifetime. With a plastic case presumably from degrading plastics. It can still work for some time.Especially modern MOSFETs in a TO220 case give power ratings that are not practical to be actually used.

author: Phil1977
author_rank: Frequent Contributor
timestamp: September 19, 2024, 11:07:53 am
content: That´s exactly the issue with the Tc=25°C values.They sometimes are tested by connecting the package to an active cooler that pumps a cooling agent through the mount. Then a thermal sensor is attached to the package and the cooler is controlled by a PID-loop so that the package temperature stays exactly at 25°C, though the mount may need to be actively cooled to 10-20K below ambient.In this system you can test how much wattage P_tot can be dissipated in the semiconductor so that it´s junction reaches e.g. Tj=150°C.This value is valuable for calculating thermal resistances and setting up thermal simulation models. But it´s quite far from a safe dissipation value in real world usage. For this purpose it´s much better to have e.g. a Tc=75°C value that allows the real heat sink that is connected to the component to heat up during usage.PS: You can anyhow get quite good estimations of the possible real world dissipation. In a first approximation you can just interpolate between Tc=25°C and Tj=150°C - that means if the component is specified for P_tot=250W at Tc=25°C then it probably survives a dissipation of half of P_tot = 125W for a case temperature half between 25°C and 150°C - which is 87,5°C.