Mosfet selection & thermal management. - Page 1
Forum section: electronics / general

author: zanzeoo
author_rank: Contributor
timestamp: February 11, 2023, 12:51:50 pm
content: Hello everybody.I guess that point has been frenquently asked but i struggle to understand it very well without a practical exemple.(with the maths)Let say i want to use a mosfet in the linear mode to sendONEpulse of50msin a3 Ohm load(resistor).The input voltage is 28V and the mosfet Gate is controlled by a hotswapp controller so the current is set to be5A.The power dissipated in the mosfet is about 65W during the pulse.What is the good method to determine if a mosfet is suitable to whistand such a pulse ? (Suppose there is no heatsink )I 'm aware about the SOA curve in the datasheet , but they are for TA =25°C , What if TA is 55°C. ?I'm a little bit confuse about zth which i belive is negligeable because the pulse si well above 10msMany Thx in advance.

author: Psi
author_rank: Super Contributor
timestamp: February 11, 2023, 01:23:57 pm
content: I've always assumed if you're under the max power and max amps rating of the FET, then you just have to worry about heat.And if you are doing a single pulse followed by plenty of time to cool back down then all is good heat wise.But i guess you need to know the max power rating of the FET without any heatsinking (as you state), so that might be a bit more tricky to work out. The datasheet usually states the power ratings at some level of heatsinking via PCB copper area.EDIT: Ignore me, i forgot how different it is when running in analog region.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 11, 2023, 02:44:54 pm
content: Quote from: zanzeoo on February 11, 2023, 12:51:50 pmThere is no easy answer - you are running the part in operating mode manufacturer does not consider important enough to properly characterize their device at. All you can do is to estimate, do your own tests, and so on. If you are going to buy millions of parts, you can probably get the manufacturer to characterize their part better.You can estimate by temperature differences. Destruction is by die temperature exceeding around 150°C. Therefore, for first order approximation, one would linearly derate the SOA curve to zero at 150°C; i.e. for your operating Ta, that would be (55-25)/(150-25) = 24% derating. But reality could be worse than that, so beware.SOA curve is "typical" and not "minimum"/worst case anyway, so all bets are off.

author: zanzeoo
author_rank: Contributor
timestamp: February 11, 2023, 06:11:28 pm
content: What about this one IPT007N06NThings is my desgin...works ! even at TA = 55°CI managed to dissipate 65W during more than 50ms...(single pulse with 5sec pause) but to be honest by reading the datasheet i belive this mosfet should not whistant such.Ironic question ...why ? (i guess may be enough PCB copper ?)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 11, 2023, 06:18:29 pm
content: Are we to understand 65W * 50ms is the worst-case dissipation, then?Simply plot that point on the SOA plot, and if it lands within the respective curve, and the initial temperature matches, you're done, it's safe.Note that SOA shrinks with initial temperature; datasheets don't often publish SOA at multiple temps. You might use transient thermal impedance instead, but do mind if there are steeper slopes on the SOA curve (2nd breakdown), which are not reflected by thermal impedance.On 2nd breakdown: you may find mentions of "conventional wisdom" that MOSFETs are "free from" 2nd breakdown; or that BJTs are better but only particular ones (power amp types). This is all nonsense anymore, as in the intervening decades since such words were passed down, MOSFETs have been optimized to power densities comparable to BJTs, and so all types are susceptible. Among both types, it comes down to design and ratings.Also, all types have always been susceptible anyway (transistors always have a negative tempco over most of their transfer function), it's just a matter of degree: thermal runaway only occurs above some power density, and if the package burns up before reaching that point, it can be said to be "free of 2nd breakdown". Note that an identical die chip could be used in e.g. a SOT-223 package and be called this (because it can only handle a couple watts), or TO-220 (capable of 100W+) and be susceptible.Curiously, the latest generations of HV MOSFETs (SuperJunction) offer higher power density than ever, but often sport full SOA. Not sure how they do it, but at least the couple I've tested, do indeed offer what they claim.

author: zanzeoo
author_rank: Contributor
timestamp: February 11, 2023, 06:52:47 pm
content: Quote from: T3sl4co1l on February 11, 2023, 06:18:29 pmthank you for your answer butSeem that 65w is in the millisecond zone on the plot no ?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 11, 2023, 07:43:49 pm
content: Right, that part is almost all 2nd breakdown, and not very large die/tab.Often, big old parts like IRFZ46N (and various xx50N06 equivalents), IRFP3710, etc. are better because of their large, less optimized dies and relatively low cost. Shop around, check lots of datasheets.Tim

author: zanzeoo
author_rank: Contributor
timestamp: February 11, 2023, 07:55:23 pm
content: Well...Again thx you...But my question now is why this is working ? am i just lucky with this part ?Quote from: zanzeoo on February 11, 2023, 06:11:28 pm

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 11, 2023, 08:03:32 pm
content: Beats me. Did you measure the power correctly? What are the voltage and current waveforms?Tim

author: zanzeoo
author_rank: Contributor
timestamp: February 11, 2023, 08:15:13 pm
content: I measured the current and the voltage of theLOADwith a scope with differential probe and current probe, then deduce the VDS of the mosfet.It's a single pulse ( square ).If the load is 3 Ohm , the current 5A and the input voltage 28V, I may be wrong but there is approx 13volts in the VDS.My board is approx 40mm x 120mm, 2 layers PCB, mosfet has thermal vias, i don't remeber how much copper area is on the mosfet but do you belive is it what it help the mosfet to survive ?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 11, 2023, 08:45:42 pm
content: Oh, 13V? That's below the 10ms curve. Heatsinking may be relevant too.Tim

author: Someone
author_rank: Super Contributor
timestamp: February 11, 2023, 11:23:15 pm
content: Quote from: zanzeoo on February 11, 2023, 07:55:23 pmFrom your last post it sounds like you are right around the edge of the SOA, is that a guaranteed or typical specification?

author: nctnico
author_rank: Super Contributor
timestamp: February 12, 2023, 12:38:27 am
content: Looking at the SOA graph, a pulse duration 50ms is considered DC. I like to stay at half the power as indicated by the SOA graph to have some margin (50% derating).

author: Faringdon
author_rank: Super Contributor
timestamp: February 12, 2023, 01:47:17 am
content: You havent yet put your FET on a test jig like that, and left it doing that for weeks and weeks.When you do it may fail.FETs can often take loads of grief out of the datasheet for short ish times.I worked on a 300W buck converter which worked in an ambient temp of 180degC...heatsink was not big as it was for MWD.Talking about "outside of datasheet"I worked on a 150W flyback which survived for years in the field in volumes of 1000's with 810v on the 650V fet drain. (but most of time was in light load where was under 650V).One company thought theyd gotten away with it with a 1210 , 47r inrush resistor inrushing off EU mains into a 330nF cap......then after a year of being switched on once each day, virtually the entire batch failed.When you go outside of datasheet, you are on your own...it may work for x weeks, then fails.Repeated Expansion/contraction is very bad for fets...so your 50ms ON, 5 seconds off thing, repeated, is very bad.

author: zanzeoo
author_rank: Contributor
timestamp: February 12, 2023, 10:10:56 am
content: Found some usful information about that :From Art of electronics X chapter page 239 :The analogy is made with an IRF1405

author: nctnico
author_rank: Super Contributor
timestamp: February 13, 2023, 07:43:37 pm
content: Keep in mind that these formulas assume even heating across the entire die. So you'll need to include a healthy safety margin in your design.

author: zanzeoo
author_rank: Contributor
timestamp: February 13, 2023, 09:07:47 pm
content: Does it mean that in the formula TC = TA ?suppose my initials condition in my first post. : One sigle pulse , intial TA is 55° , no heatsinkIPT007N06NIs is true to say that if (zth x Pdiss) + TC = TJ . So zth = 0.4 C/W , Pdiss = 65W , TC = 55°C . then Tj = 81°C well below 175 (Tjmax)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 14, 2023, 03:35:10 am
content: Correct.But again, assuming even heating. 2nd breakdown means a hotspot is forming, and dissipating most of the heat, therefore less of the die is available for dissipation (and as the process develops, even less is available, until failure occurs quite suddenly). This is expressed by the steep lines on the SOA curve. Lines which are below the strict power limit (or transient thermal using Zth for the respective shorter duration curves). So it matters how the power is dissipated, not just power but also the voltage and current.50ms is also long enough that the heatsink (if applicable) may be just barely relevant.Tim