Rule of thumb for current derating of inductors? - Page 1
Forum section: electronics / projects

author: Pack34
author_rank: Frequent Contributor
timestamp: January 12, 2018, 07:32:02 pm
content: For capacitors I've always used the rule-of-thumb of derating the voltage rating by 50%.What about inductors? If I can expect a current of up to 3A going through it, should I size for 6A? So far, I've just been giving myself an amp or two of head-room but I'm questioning myself if this is enough.Thoughts?

author: oldway
author_rank: Super Contributor
timestamp: January 12, 2018, 08:17:06 pm
content: For an inductor, there are two different limitations:1) heating of the copper wire .... do not exceed a current density of 5A / mm². Measure the wire diameter, calculate the section and you will have the maximum current.2) the saturation of the magnetic core if there is one .... If it is possible, calculate the induction in the magnetic core .... one must not exceed 2000 Gauss (0.2T) for ferrite or 10KGauss (1T) for a ferromagnetic core.If induction can not be calculated, use 80% of the manufacturer's specified current as the maximum peak current acceptable.For coreless inductances, the only limits are those of copper heating.

author: james_s
author_rank: Super Contributor
timestamp: January 13, 2018, 12:16:20 am
content: I typically don't derate inductors by much, the main thing you have to watch out for is saturation, the heating due to DC resistance is is relatively easy to calculate. I typically give them the finger test in a prototype, if it's not too hot to hold my finger on then it's probably ok.

author: wraper
author_rank: Unknown
timestamp: January 13, 2018, 12:20:42 am
content: Quote from: Pack34 on January 12, 2018, 07:32:02 pmThis is not a rule of thumb but completely inadequate point of view. It's highly capacitor type dependent, some don't need derating, others need 65% derating for high reliability.

author: David Hess
author_rank: Super Contributor
timestamp: January 13, 2018, 12:41:38 am
content: Quote from: oldway on January 12, 2018, 08:17:06 pmThis number also decreases with frequency due to increasing loss from hysteresis. Manufacturers provide a graph showing peak flux versus frequency.

author: Pack34
author_rank: Frequent Contributor
timestamp: January 15, 2018, 08:34:33 am
content: Quote from: wraper on January 13, 2018, 12:20:42 amIs it detrimental though? Besides the added cost.

author: tszaboo
author_rank: Super Contributor
timestamp: January 15, 2018, 09:27:43 am
content: Quote from: Pack34 on January 15, 2018, 08:34:33 amThere is two capacitors from the same manufacturer. Both of them is 100nF, X7R, 0603, one is 10V rated, the other is 50V. The 10V has 20mOhm ESR at 10MHz, the other has 120mOhm.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 15, 2018, 12:43:35 pm
content: Quick refresher:Electrolytics should be run near ratings (at, say, 50-90%, leaving adequate headroom for high-line and surge conditions to fall within the surge rating), lest they decay over time (oxide thins --> C drifts up).Ceramics break down at some large (but highly variable, so don't depend upon it) multiple of rated voltage, but often become uselessly small (due to saturation) at much less than that. Check mfg data.Films are fine at rated voltage, and are usually generously rated for surge (1.5x or so). Beware that very high density, low cost, self healing parts (often X1/X2 EMI and general purpose types) may have a lower ceiling and will suffer wear (C decreases and ESR increases, until some day there's nothing left) under surge conditions.Aluminum polymer are very much like film (in more ways than this), but at low voltage.Tantalums are the only kind that need significant derating, due to their incendiary tendency.As for inductors:Inductors have only two kinds of loss: copper and core. They are rated by DC current causing copper losses, which brings part temperature up to the limit (usually a 40C temp rise). They are occasionally rated for core loss (but core loss is never factored into the DC current rating, of course). They are usually rated for saturation current, which is the current where inductance falls by some nominal amount (beware, this varies from 10 to 50%). Your application may be limited by the lowest of either parameter.Voltage ratings are even less talked about (much as current ratings for capacitors), and apply to voltage between wires (relevant to applied voltage, particularly for peaky / surge applications) as well as wire-to-core voltage (both ferrite and powdered iron core materials are a little conductive).Tim

author: BravoV
author_rank: Super Contributor
timestamp: January 15, 2018, 12:46:06 pm
content: Quote from: T3sl4co1l on January 15, 2018, 12:43:35 pmTim, how about core's temperature, for example like its too hot to touch like PC motherboard or those graphic board VRMs ?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 15, 2018, 12:58:11 pm
content: Max temperature is in the datasheet.Core materials cease to function at high temperatures (Tc usually over 200C, but depends on material), but beware that saturation current drops rapidly as T approaches Tc.Core losses are usually fairly stable with temperature (some ferrites have a negative tempco; powdered iron is stable, but core loss goes up with age, which is accelerated by high temperature), while copper's tempco is pretty strong.Squishy meat bags are poor judges of temperature, with respect to inanimate plastic, metal and semiconductor parts.Tim

author: BravoV
author_rank: Super Contributor
timestamp: January 15, 2018, 02:39:39 pm
content: Quote from: T3sl4co1l on January 15, 2018, 12:58:11 pmWhat if around say 100C , half of the critical point at the common 200C ?Will the saturation point decrease "significantly" ?Quote from: T3sl4co1l on January 15, 2018, 12:58:11 pmSure, no argument bout that, its just I often noticed on motherboard/gfx card, when overclocking, the inductors were way too hot to touch, I guess they're overrated.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 15, 2018, 03:14:35 pm
content: Quote from: BravoV on January 15, 2018, 02:39:39 pmNot usually.Ferrite is down maybe 10%, typically. Powdered iron hardly at all (it's literally powdered iron, so Tc is something ridiculous like red hot).QuoteYeah, touch is pretty useless above 40 or 50C. Once it's "this burns" hot, you can't really tell. Next step at least is >100C, where a licked finger will sizzle on the component...Got a thermistor on leads, or something like that? Hold that against the part in question (with the help of some thermal grease), and measure a number.It's not science until you measure something.Tim

author: MikeH
author_rank: Newbie
timestamp: January 17, 2018, 08:25:29 am
content: NASA and the Air Force did a lot of studies to figure out how they wanted to derate things for high reliability. In a previous aerospace job, I usedEEE-INST-002andSSP30312RH. I think it worked pretty well, since I don't remember having any inductor failures in orbit...

author: mikerj
author_rank: Super Contributor
timestamp: January 17, 2018, 08:59:45 am
content: Quote from: NANDBlog on January 15, 2018, 09:27:43 amAnd what is the actual capacitance of each one when working at 10v?

author: tszaboo
author_rank: Super Contributor
timestamp: January 17, 2018, 11:08:54 am
content: Quote from: mikerj on January 17, 2018, 08:59:45 am10V has -5% the 50V negligible.You are free to check it for yourself, I've used WE 885012206020 and 885012206120. Just to make a point.

author: oldway
author_rank: Super Contributor
timestamp: January 17, 2018, 12:36:16 pm
content: Pack34 had asked a question that, in reality, has no answer .... an inductor, it is calculated and sized according to many parameters of the circuit, the value of the inductance, the tolerance on this value , the ambient temperature, the rms current, the peak current, the maximum possible overload, the frequency, the type of core, the possible gap, the coeficient of security, ... etc ....So, in reality, there is no rule of thumb for inductors ....That's why, I answered with only very simple elements .... For example, the current density of the copper wire can be deduced from the rules applied to the transformers .... for the rest, I think that this is not in a few lines on a forum that one can explain the dimensioning of an inductor.

author: Siwastaja
author_rank: Super Contributor
timestamp: January 17, 2018, 01:25:16 pm
content: No rule of thumb. You should consider the specs valid, i.e., no derating needed. Sometimes you can even overrate: for example, many inductors are specified for 40 degC temperature rise, and in some applications, 50 degC rise is just fine.However, it's quite challenging to actually analyze:(1) real losses in the inductor, especially AC (copper: skin/proximity effect. core: hysteresis losses)(2) thermal performance of your system, i.e., how the heat is removed.The AC part of the losses is often hard, since almost no inductor manufacturer provides sufficient data to model it. If you just do the DC calculation in a switching converter, the actual losses may be double, even triple that!Thermal side needs analyzing the PCB layout (in case of SMD inductors), heatsinking, air flow, worst case ambient temperature, etc.You should analyze these effects as well as you can, given your skills and design time available, then approximate how well you were able to do that, and derate based on that confidence number. Prototyping, and measuring actual temperatures in the worst-case emulated environment is highly recommended. BTW, the same is true for most types of components.Only tantalums are a rare expection to the general rule of "part is (barely) usable within its specifications - just add some safety margin for your own part of the design". For some reason, the datasheet specifications for tantalums give ridiculously bad reliability, so you need to apply a significantextrafactor (typically 0.4 to 0.6) in addition to your standard safety margin for surges, worst case conditions, voltage regulation tolerances, etc.

author: wraper
author_rank: Unknown
timestamp: January 18, 2018, 11:14:26 am
content: Quote from: Siwastaja on January 17, 2018, 01:25:16 pmIt's not "some reason", it's the damage they receive during reflow. They have completely valid ratings when not soldered yet. However to guarantee all of them will self heal successfully and not fail short, you need to derate them quiet a lot. Basically you could "train" them at lower voltage so they self heal, then increase the voltage.

author: David Hess
author_rank: Super Contributor
timestamp: January 18, 2018, 12:27:07 pm
content: Quote from: wraper on January 18, 2018, 11:14:26 amIt is not only reflow though because leaded parts appear to suffer from the same problem. I suspect the mismatch in coefficient of expansion with the epoxy is also a cause whether temperature or humidity induced. Hermetically sealed solid tantalums do not have the same problem and are rated to a higher voltage. NASA also mentions long term "field crystallization" in high voltage parts.

author: duak
author_rank: Super Contributor
timestamp: January 18, 2018, 06:12:04 pm
content: There's an effect with iron core toroids that I didn't see mentioned here. If the core is conductive, as most are, circulating currents can be induced in it by AC current in the coil. I had a possible EMI problem with a 1 KW PWM driver and whipped up a filter using some 50 uH iron core toroids I had on hand. After a few minutes I could smell something hotter than it should have been. The cores were hot enough to burn skin. I was surprised because they were from a 30 amp DC supply and were rated for more current than I was running thru them. Turns out there was enough ripple current at the 20 KHz switching frequency and the 60 Hz drive frequency to heat up the cores. Didn't seem to materially affect their characteristics but the wire looked a bit darker than before.Just sayin'

author: Someone
author_rank: Super Contributor
timestamp: January 18, 2018, 11:12:56 pm
content: Quote from: BravoV on January 15, 2018, 02:39:39 pmThe permeability might not crash until close to 200 degrees but other effects come into play before then, as you increase temperature you can enter positive feedback and thermal runaway so there are many parameters to assess when designing magnetics. Most only specify operating conditions to 120 degrees in the core (as below), which has to be calculated against the expected power dissipation and thermal environment as Siwastaja mentions.