mosfet running at 170 degC in prototype - Page 1
Forum section: electronics / projects

author: ocset
author_rank: Super Contributor
timestamp: July 07, 2018, 08:08:15 am
content: Hello,In our 180W LED driver, the SPB07N60C3 mosfet was running with 170 degree measured on its tab.We wonder how long it could survive at this temperature and whether much internal damage will be done?SPB07N60C3 DATASHEEThttps://www.mouser.co.uk/datasheet/2/196/SPB07N60C3_Rev.2.5-72639.pdfThe datasheet says it can operate at up to 150degC…so presumably there is no wearout mechanism up to 150degC?

author: hans
author_rank: Super Contributor
timestamp: July 07, 2018, 08:25:15 am
content: 150C is the absolute limit, not sure about this part but most differentiate spec into 150C storage, 125C operation.If you run the device above the recommended (or even absolute max) spec, you'll likely see failures on relatively short term. The internal junction temperature will also be even hotter, as you said you were measuring the case.You could calculate it if you know the case temperature and how many Watts of power is dissipated in the device. It says the temperature resistance of juction to case is 1.5K/C or 3.9K/C (fullpak), so I guess junction temp is 5-10C hotter if you dissipate about 3W.If you want to build a converter that still works after many years, don't do this. Especially not considering you can get way better MOSFETs than 0.6 ohm Rds(on).

author: dazz1
author_rank: Frequent Contributor
timestamp: July 07, 2018, 08:29:59 am
content: HiIf you are operating this device outside its limits your circuit and design are not right.Parts fail because electrons and atoms move around. Heat accelerates movement therefore accelerates failure.

author: ocset
author_rank: Super Contributor
timestamp: July 07, 2018, 09:05:19 am
content: ThanksMay i add, We didnt measure the temperature on the case of the dpak, we measured it on the metal tab.....the DPAK was dissipating about 20W altogether. The datasheet doesnt give a thermal resistance from junction to metal tab unfortunately .

author: hans
author_rank: Super Contributor
timestamp: July 07, 2018, 09:22:57 am
content: You can assume that the tab is the case temperature. Thermal calculations can be complex, so overapproximations are very common and probably the best you can do with the data supplied by a typical manufacturer.With 20W and 1.5K/W junction-case thermal resistance, the internal junction is baking at 200C. That's 50C over the max rating!Or perhaps better put; 60C below the reflow soldering temperature you should hold for 10seconds. (I know that temperature is specified at the case again, but it shows the extent of pushing the device limits)You should use a better MOSFET or power design architecture (e.g. multiple phases).

author: AndyC_772
author_rank: Super Contributor
timestamp: July 07, 2018, 09:47:05 am
content: Quote from: treez on July 07, 2018, 08:08:15 amThat's not a valid assumption. Many components which are rated to 150C include a lifetime-temperature graph in the data sheet, with lifetime plotted on a log axis, and it's a straight line.Component life at 150C can be as short as 1000 hours, typically limited by wire bond failure or electromigration. The variation in life vs temperature is typically something like 2x the life for every 10C decrease.On a simple, coarse geometry component like a power MOSFET, lifetime may well be greater at any given temperature - but despite this, you're a *long* way over the temperature at which your MOSFET will work reliably for any length of time. Your design *desperately* needs changing.A competent power supply designer will be able to examine your design and establish why the MOSFET is getting so hot. The two main causes tend to be:a) RDS(on) is too high, so the I2R heating effect when the device is turned on is too great. Choose a MOSFET with lower RDS(on).b) Gate charge is too high, so the turn-on and turn-off is too slow, and the MOSFET spends too long in a state where it is neither fully on nor fully off. In this state, the power dissipation is incredibly high, so it's always best to spend as little time in this state as possible. Fixes for this problem are to choose a MOSFET with lower gate charge, or to drive it with a gate driver that can charge and discharge the gate faster.There's a trade-off here. Smaller MOSFETs have higher RDS(on) and lower gate charge, and vice versa. One with a higher VDS(max) will have worse gate charge and RDS(on) than an otherwise similar device with lower VDS(max), so choose one with as high a voltage rating as you need, but no higher.Faster turn-on and turn-off is good for efficiency, but bad for EMC.If the turn-on/turn-off spikes are the main source of heating in your MOSFET, then consider running the whole PSU at a lower switching frequency, and accept that your L and C may need to be bigger.Your MOSFET may be too small (RDS(on) is too high but the gate charge is OK), or one which is too large (RDS(on) nice and low but requiring too much gate charge for your circuit to turn it on and off quickly enough). It may just be one which is an old, poor performing design, and there's an alternative out there which is just plain better.

author: jbb
author_rank: Super Contributor
timestamp: July 07, 2018, 10:24:14 am
content: The MOSFET and the PCB itself will age _really_ fast.I did some testing in years gone by, and had MOSFETs operating (linear mode) while floating on molten solder. They lasted a couple of minutes...Just because it works for a brief period doesn’t mean it’ll be reliable.

author: digsys
author_rank: Unknown
timestamp: July 07, 2018, 10:39:02 am
content: As suggested by others - irrespective of specs, this is bad situation. My ageing / failure tests I carried out with linear p/supply design showed a "knee"point temperature of ~75-85oC -for extended periods. Degradation climbed slowly to ~ 90-95oC, then went up rapidly (we're talking over 100s hrs).I agree with all the suggestions as well. Either lower Rds on or a larger package.

author: daqq
author_rank: Super Contributor
timestamp: July 07, 2018, 03:00:38 pm
content: QuoteIt is certainly not a happy transistor. It's impossible to say how long it has, but it's certainly going to die sooner than it would under normal conditions.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: July 07, 2018, 03:45:06 pm
content: Seems like this led driver is engineered to fail a week after the warranty expires.

author: SiliconWizard
author_rank: Super Contributor
timestamp: July 07, 2018, 04:04:20 pm
content: 170°C, now that sounds hot. Fine if you intend to switch your business to cooking ovens maybe?Is it a switching driver? If so, a transistor reaching this temperature is a probable sign of very poor efficiency.

author: chemelec
author_rank: Contributor
timestamp: July 07, 2018, 04:35:40 pm
content: Do you have a Heat Sink on it?

author: boffin
author_rank: Unknown
timestamp: July 07, 2018, 04:43:36 pm
content: Quote from: treez on July 07, 2018, 08:08:15 amSounds like you need something with a much lower RdsON. For that package it's 0.6, whereas it's fairly simple to find things with 1/10th or less of thatPerhaps:https://www.digikey.ca/product-detail/en/on-semiconductor/FDD86380-F085/FDD86380-F085CT-ND/6009705

author: Ice-Tea
author_rank: Super Contributor
timestamp: July 07, 2018, 04:49:39 pm
content: Would be a race between the FET and the FR4 to see who gives up first...

author: james_s
author_rank: Super Contributor
timestamp: July 07, 2018, 05:21:13 pm
content: Something is *very* wrong. Either the mosfet is not properly heatsinked or it is being used improperly. I would consider anything hotter than about 60C sustained under normal ambient conditions to be unacceptable.

author: Howardlong
author_rank: Super Contributor
timestamp: July 07, 2018, 06:03:21 pm
content: In addition the thermal failure mode of a mosfet is typically a Drain/Source short, which may well be not what anyone wants.I did some work on this some years ago for ESA which explained the failure of an in-orbit satellite. The team involved in the failure unit’s design read that 150 celsius on the data sheet and (bizarrely to me) designed to that. For unexplained reasons no peer review ever discovered or questioned this. In addition, thermal testing was done in a vacuum, and with a solar simulator ourside of vacuum, but not both at the same time. Last time I checked, there’s no convection in a vacuum.The rule of thumb I’ve used for my entire EE career is that if it’s too hot to touch, it needs fixing.I don’t know much about your design, but 0.6 ohm RDS is, as someone else said, pretty high. Either way, it might be worth checking also it’s not working in the linear region, i.e., Vgs both on and off are right, and it’s not sitting midway for any period of time.

author: sokoloff
author_rank: Super Contributor
timestamp: July 07, 2018, 06:12:22 pm
content: Quote from: hans on July 07, 2018, 08:25:15 amI always understood those type of limitations to beambienttemperature limitations, not operating temperature limits.Is that incorrect?

author: james_s
author_rank: Super Contributor
timestamp: July 07, 2018, 06:21:54 pm
content: It's typically the maximum allowable die temperature, which can be considerably hotter than case temperature. If you had a 125C ambient temperature you wouldn't have any room for temperature rise of the part.

author: Ice-Tea
author_rank: Super Contributor
timestamp: July 07, 2018, 06:22:27 pm
content: Yes, it is. It's either case temp or more likely junction temp. Regardless of ambient temp.

author: Gyro
author_rank: Super Contributor
timestamp: July 07, 2018, 06:33:13 pm
content: It's normally explicitly stated in the datasheet. In the case of Treez's part it's Tj (junction) of 150'C max.... So he'swayover. [Edit: as hans worked out, with the device dissipation and thermal resistance, it's running at about 200'C!]

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 07, 2018, 06:42:00 pm
content: Quote from: Doctorandus_P on July 07, 2018, 03:45:06 pmYou're giving the OP a lot more credit than deserved. If you want a good read some time, check out their other threads...Tim

author: james_s
author_rank: Super Contributor
timestamp: July 07, 2018, 09:07:30 pm
content: I try not to pick on anyone personally but I kind of wonder if they're an actual engineer or not, the exploits remind me of the sort of thing I'd expect from a young intern. I would expect an average hobbyist tinkerer to have a more thorough understanding of these sorts of designs.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 07, 2018, 09:56:16 pm
content: Ask me questions once, you're a student.Ask me questions your whole career, well...Tim

author: Zero999
author_rank: Super Contributor
timestamp: July 07, 2018, 09:56:36 pm
content: To copy and paste from another post. . .Even if it's exceeding the maximum rated base current, it doesn't mean it's automatically toast. Components can often withstand way over their maximum ratings, with no damage,. Obviously it's bad design practise to build anything which will stress the components beyond their maximum ratings, even if they tolerate it.

author: james_s
author_rank: Super Contributor
timestamp: July 07, 2018, 10:04:07 pm
content: Well yeah, I mean if you exceed the redline in your car it doesn't automatically mean the engine is going to explode, but make a habit of it and something will break sooner or later.

author: ocset
author_rank: Super Contributor
timestamp: July 07, 2018, 11:55:25 pm
content: Thanks,This is a prototype linear led driver. Its a product which we are trying to find out how high we can get the power without the mosfet overheating.In the current use_case, the fet is around 70 degc.What we wish to know, is how high can we have the junction of the fet and still have a 10 year lifetime based on 4 hours of use per day.The D2PAK fet sits on a pcb, which has thermal vias (16 x 0.3mm thermal vias in the d2pak pad itself, the rest are 0.6mm thermal vias outside the d2pak pad) down to bottom layer copper, and then there's a rubbery thermal pad, which is on a big heatsink.We did the test at 25 degc ambeint. The fet is however, totally enclosed over the top in a plastic casing which screws into the heatsink.

author: james_s
author_rank: Super Contributor
timestamp: July 08, 2018, 03:33:04 am
content: Somebody who can't figure out how to design a dependable LED driver without asking on a forum at every step is probably going to have difficulty finding another job, at least in that field. There are dozens of reference designs out there that could be used with very few changes.

author: boB
author_rank: Frequent Contributor
timestamp: July 08, 2018, 04:05:58 am
content: I would check the tab temperature against the heatsink temperature and get a feel for how well your heat transfer is doing.RsdOn won't matter so much as Safe Operating Area of the device. (SOA) This particular part has a fairly small die which meansit won't have a very good SOA. You may need a D-Squared package rather than a D-Pak to get any better Junction to Case resistance.Higher dissipation rating is typically what you want for linear regulators OR reduce the input voltage.boB

author: TJ232
author_rank: Frequent Contributor
timestamp: July 08, 2018, 07:09:25 am
content: Quote from: treez on July 07, 2018, 08:08:15 am1. Survival time at 170 deg C: 54.78 seconds. If you don't believe me, run proper simulation process or even better for you take a proto test board and run some tests. Best result can be obtained with a mercury thermometer in a Petri dish combined with a high precision Cuckoo watch. Even a standard alcohol thermometer is working but precision is reduced by alcohol proof and percent of ingested quantity.2. If the datasheet say that, you need to believe. Don't search, believe. Is written even in the Bible that, so must be true. As you seems to know already, semiconductors wearing out mechanism is not something linear with temperature increase, no way. In your specific case under 150C is 0 all the time and from 150 to 170C is going quick upto 100%.PS: are these above real questions related with a real product development or are you just bored to death by the nice UK weather these days?

author: ocset
author_rank: Super Contributor
timestamp: July 08, 2018, 10:31:56 am
content: Thanks i agree we deffo woudlnt have done it this way if we were starting from scratch. However, money is very very tight so no new product development is allowed, and we just have to squeeze as much as possible out of the existing products, which , in their current use, involve fet temperatures well below 100 degc.We have searched for lifetime equations for FETs at certain temps, but nothing exists on the web

author: SeanB
author_rank: Super Contributor
timestamp: July 08, 2018, 11:25:39 am
content: I worked on avionics where the original designers at Thompson, either because of having had Ergot that week in the bread, or because they were stuck in a tight corner, made a lamp driver board that used a 2N3773 transistor, in a darlington configuration with a 2N2219 driver, that had to handle 10A on the lamp drive. PWM dimming and such, using a lovely combination of design decisions, like using a TIL100 optocoupler to drive the darlington, TIS43 unijunction transistors thoughout, no pull down resistors anywhere on the darlington drive ( thus leakage played merry hell), and they placed this small board, with it's small heatsink ( you fitted a TO3 package on it, and there was room for another TO3 device on it, nothing much more space wise, with only 2 1cm by 1cm blocks as heatsink to the big main chassis) right under the 100W lamp. Most common failure mode was the poor 2N 3773 unsoldering itself, the emitter lead springing off the lead and then no light, or it would be run in a linear mode ( remember there is no base turn off, relied on leakage being small enough to keep the darn thing off) and get to the point where the transistor finally went short circuit, blowing the lamp. Then pilot switches to second spare lamp, and this instantly blows, along with the 2 parallel 6A3 Littlefuse fuses that power this thing.First case fix was check transistor for shorts, and then replace driver, because it was leaky. Could put a pull down resistor ( but not on flight units, because you need to fill in that 500 page change document first, and it takes years to do so) and it would run perfectly, even running at boiling water temperatures, but transistor was at risk after 100 hours anyway. Never had one run over 200 flight hours anyway without coming in for a fault, and these were transistors with a pedigree page per unit, in a massive Kovar and copper TO3 case, and a die big enough to actually see the features with the naked eye.Your Dpak does not stand a chance at 150C case temperature, either bigger die in a bigger package, a bigger heatsink, or double up on driver boards in the case and have redundancy, because those boards will fail in weeks in operation, even if you are running them in a freezer at -20C. Putting them up a pole where the chassis will reach 70C in summer will have them popping ( just like Photonic induction "I Popped It!") in only 2 days or so.

author: Howardlong
author_rank: Super Contributor
timestamp: July 08, 2018, 03:45:11 pm
content: It’s not just the static temperature that affects lifetime, it’s also thermal cycling stress.

author: SeanB
author_rank: Super Contributor
timestamp: July 08, 2018, 04:01:32 pm
content: Quote from: Howardlong on July 08, 2018, 03:45:11 pmIf they are at 170C case when in best case conditions, somehow I think cycling will only make them fall off the board faster when they are up a pole and being lovingly toasted.

author: james_s
author_rank: Super Contributor
timestamp: July 08, 2018, 04:19:32 pm
content: I wish I knew what company this was so I'd know to never buy anything made by them.

author: CopperCone
author_rank: Super Contributor
timestamp: July 08, 2018, 04:25:44 pm
content: Lol, 170C is something you would run seismic deep boring equipment at with special brazed SiC transistors and stuff.I wonder if corporate pressure from organizations like treez company will lead to interesting research.

author: Gyro
author_rank: Super Contributor
timestamp: July 08, 2018, 06:03:46 pm
content: Corporate pressure? treez's employer?

author: katzohki
author_rank: Frequent Contributor
timestamp: July 09, 2018, 09:25:23 pm
content: Quote from: james_s on July 08, 2018, 04:19:32 pmI suspect you could find their products being sold on eBay or Alibaba

author: Ice-Tea
author_rank: Super Contributor
timestamp: July 10, 2018, 10:20:56 am
content: UK outfit. So probably not.

author: JPortici
author_rank: Super Contributor
timestamp: July 10, 2018, 10:40:27 am
content: Quote from: jbb on July 07, 2018, 10:24:14 amcool. i always wondered if something like this would possible... i mean, if you could run a semiconductor at temperatures so high that you could melt solder.

author: AndyC_772
author_rank: Super Contributor
timestamp: July 10, 2018, 10:46:06 am
content: Sure you can, there are components out there rated to 210C or even higher.They're fearsomely expensive, though, and are designed both electrically and mechanically with high temperature operation as a main priority. They're used with high temperature solder which is close to being pure lead (and yes, there is, or at least was, a specific RoHS exemption for this very purpose).Fun fact: I was once involved in debugging a voice telephony card which had a timing problem that meant there were pops and crackles on the line whenever the codec was cold. Warming it up with a heat gun made the problem go away, and I still remember holding a conversation which was perfectly loud and clear, right up to the point where the codec slid away across the PCB, having reached the melting point of the solder joints.

author: SiliconWizard
author_rank: Super Contributor
timestamp: July 10, 2018, 05:26:29 pm
content: Quote from: JPortici on July 10, 2018, 10:40:27 amIt is:http://www.cissoid.com/high-temperature-electronics/transistors-diodes/sic-power-devices/neptune-1210-high-temperature-high-voltage-1200v-10a-power-mosfet/

author: ConKbot
author_rank: Super Contributor
timestamp: July 11, 2018, 06:02:31 am
content: Quote from: SiliconWizard on July 10, 2018, 05:26:29 pmEven with plain silicon!http://www.irf.com/product-info/datasheets/data/irf1704.pdfrun it upside down, and it'll fall of the board before it even hits TJ max if you aren't hitting it with huge power pulse. Plastic case too, not even a swanky metal hermetic can.

author: SiliconWizard
author_rank: Super Contributor
timestamp: July 11, 2018, 08:48:10 pm
content: I would not connect those transistors if meant to run at those temps with soldering.

author: xani
author_rank: Frequent Contributor
timestamp: July 12, 2018, 02:09:51 am
content: Please, do tell us what company you work for so I can never buy its productsEven considering running elements so far out of spec in consumer product is insane. Hell, that could potentially be a fire hazard when tranny goes up in smoke.Also "just ignore the smoke on first start, that's just residual flux burning" is not good look