Inductor rated current confusion - Page 1
Forum section: electronics / beginners

author: Felicitus
author_rank: Regular Contributor
timestamp: March 15, 2014, 10:38:21 pm
content: Hi,I'm in the progress of building a MPPT solar charge controller. While selecting inductors, I came across the term "Amps at rated current". The datasheet for the inductor I've chosen lists it as 10A. I did a test with a smaller inductor which was rated for about 2 amps, and it got pretty warm, so I am a bit confused.My questions are:- Will the inductor get warm if running at the rated amps, or should it mostly stay at ambient temperature? The wires look pretty big, similar to what one would typically use for mains wiring- Can I safely run DC through the inductor? (the algorithm also allows for pass-through mode, in that case DC runs through the inductor)The inductor in question is this one:http://www.reichelt.de/TLC-10A-47-/3/index.html?&ACTION=3&LA=446&ARTICLE=105603&artnr=TLC+10A-47%C2%B5&SEARCH=47%B5ThanksFeli

author: johansen
author_rank: Super Contributor
timestamp: March 15, 2014, 11:12:41 pm
content: That toroid you see in the link is a mix 26, I know that because its a white/yellow core, which is traditionally a micrometals mix-26.there are parts coming out of china and asia that don't follow the color code, but that's a separate topic.Basically, Iron powder cores have a permeability. lets say its 35, for mix 26. that basically means that the core is about 1% air, and 99% metal. the ratio doesn't matter but for instructional purposes. the lower permeability cores have more air, and less metal. furthermore the metal is in a different form, different alloys, etc, but none of that matters at the moment.Ferrite cores without an air gap have a permeability of, for instructional purposes only, infinity.This means that they store almost no energy. one amp turn of dc and you saturate the core. (not actually true, but for instructional purposes only)since yours is an energy storage application, you don't care what the inductance actually is, what you care about is how much magnetic energy the core can hold, and what the static winding losses are to hold that much energy in the core. this is independent of inductance and instead depends on three things:1) how big the core is(bigger= better),2) what the ratio of copper area to core area is (typically they are between 1 and 3)3) what the permeability is. (higher permeability requires fewer amp turns to saturate the core) higher permeability = less energy stored.4) what the max flux density is (energy stored is proportional to flux Squared)But all cores have hysteresis and eddy current losses.All wires have proximity and skin effect losses.Often times the cores are selected such that the dc resistance losses match the eddy current and hysteresis losses, but this isn't always true.Mix 26 which is a high-medium permeability iron powder core that is often intended for low ripple applications.90% dc, 10% ac current ripple for example.you'll find them on the secondary of many cheap power supplies, typically they are a T-130 or T-105 core.higher ripple currents might use the blue-green cores, which are lower permeability.If you stuff one of those yellow-white T105 cores full, and i do mean full of copper, you can shove about 250 watts through it as a buck converter, at 50-60 khz and it will get very warm, probably wasting 5 watts or so. the blue-green cores are less lossy.if you want a better than 99% efficient system you will be forced to switch to a ferrite inductor.

author: Felicitus
author_rank: Regular Contributor
timestamp: March 15, 2014, 11:23:02 pm
content: Thanks for the extensive reply. In the design where my inductor got warm, I used this inductor:http://www.reichelt.de/Power-Induktivitaeten-SMD/L-PISR-10-/3/index.html?&ACTION=3&LA=2&ARTICLE=73064&GROUPID=3709&artnr=L-PISR+10%C2%B5, and this is a ferrit based inductor.I might need to do extensive re-checking again, as I believe that peak current from the switching heated up the inductor. Also a possibility is that my PSU didn't display overcurrent spikes, as I was driving the input with about 200mA.

author: mos6502
author_rank: Frequent Contributor
timestamp: March 15, 2014, 11:23:52 pm
content: I would highly recommend getting the Micrometals design software:http://www.micrometals.com/software_index.htmlThis will tell you all you need to know.Iron powder cores don't really have a fixed inductance. As the flux density increases, the inductance decreases linearly. The current the inductor can take depends on the frequency, the acceptable losses, available airflow, acceptable lifetime ...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 15, 2014, 11:29:28 pm
content: So this is a switching application, not just DC filtering?Beware, mix 26 cores are extremely lossy and only suitable for low frequency, low ripple applications. This means a much larger choke is needed than otherwise. The only reason they exist is, the material is massively cheap (e.g., a 3" core is around $10 -- good for a kilowatt or a few).To save space, choose a higher current ripple, frequency, and a ferrite cored choke. (Kool-mu toroids are nearly as good, if you can verify that's what they used. I think the Bourns 2100 series are for example, but check with the manufacturer if you can.)Tim

author: Felicitus
author_rank: Regular Contributor
timestamp: March 15, 2014, 11:31:53 pm
content: Quote from: T3sl4co1l on March 15, 2014, 11:29:28 pmYes, that's correct.QuoteI was hoping that the SMD PISR/PISM series from coilcraft was sufficient, but that one heated up. As I mentioned previously, I might need to do some more extensive tests what actually happens at the switching stage. Didn't realize that inductors are kind of rocket science

author: johansen
author_rank: Super Contributor
timestamp: March 16, 2014, 12:02:05 am
content: • DCR max.: 29 mOhm• Rated DC I: 10 A10 amps is 2.9 watts, but i doubt you are running a high enough frequency to make significant core loss.but proximity effect is significant too, so the actual dc resistance will be higher.how much current you running this at, and at what frequency?

author: Felicitus
author_rank: Regular Contributor
timestamp: March 16, 2014, 12:55:39 am
content: Quote from: johansen on March 16, 2014, 12:02:05 amSwitching frequency is 50 kHz with PWM. Input current for the step-down converter was about 200mA, output was about 2A, but from memory I'm not certain about those numbers. I have yet to re-create the setup to do further measurements, as I can't tell for certain which PWM setting the MPPT algorithm was using at the time the inductor got warm (however, it was not hot, I could still touch it).

author: IanB
author_rank: Super Contributor
timestamp: March 16, 2014, 01:13:44 am
content: Quote from: johansen on March 15, 2014, 11:12:41 pmYou wrote lots of very interesting things there, but I lacked for a conclusion to tie it all together.Let me ask some questions in an attempt to draw out appropriate conclusions:Firstly, I think an idealized inductor has only two properties:1. An inductance, measured in henries2. A DC resistance, measured in ohms.Secondly, I think a practical inductor has a third property:3. The maximum energy it can store before the core becomes saturated.Is it therefore possible to relate properties 1-3 to typical circuit design decisions?Are there any other independent properties I have not listed?If I know properties 1-3, do I need to know anything about core size, permeability, number of turns, etc?(I understand that I may need to know how the more fundamental properties like core material, size and number of turns relate to properties 1-3 if I wish to create or wind my own inductor.)(If there are magnetic losses in the core from AC currents, what fundamental property describes that?)

author: johansen
author_rank: Super Contributor
timestamp: March 16, 2014, 02:03:01 am
content: there are no conclusions, only specific judgements empirically observed related to the properties of the inductor core.i have no interest in inductance, that's like saying my 22R toyota engine can produce .025 foot pounds of torque per rpm, when we haven't decided if we're pulling an elevator up a mineshaft or driving a go-cart.and like an inductor, the torque per rpm will drop with rpm, dropping 25% when the first rod blows at 5,000 rpm, then the next at 6,000 rpm when the torque is only half what it was (Tim could work out a better analogy lol!)energy stored is interesting, but only when expressed in millijoules per dollar or joules per kilogram (yes, it is that low)or, more importantly, when we express it in energy stored per dollar per watt multiplied by switching frequency.which Mix-26 is really good at, when we ignore eddy current losses.the example i gave for instance, running 250 watts through a T-105 core.kilowatts per dollar divided by temperature rise is the most interesting metric but its the hardest to figure out, and the manufacturer is least likely to give you.as far as helping the inexperienced out, the best thing to do is give ball park figures for what you can push though a commonly available core at reasonable frequencies and loadings.most people here know that a 4mH common mode choke won't work as a buck inductor, even though its rated at say, 6 amps.but many have tried, have they not?btw, the core of a common mode choke stores only microjoules.. the cores are often in the 20,000 permeability figure.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 16, 2014, 02:37:46 am
content: Well.. what's a core? Powdered iron, well, that's made of... iron. Which is conductive. So you can guess it will carry eddy currents -- not much, that's why it's powdered -- but always some, it doesn't simply go away. You can effectively model this as an inductor in series with a capacitor; this model goes in parallel with the magnetizing inductance (which, mind, is itself only a model). The capacitor represents the cutoff frequency due to skin effect, and the resistor represents the primary reflected resistance of the core material itself.Skin effect actually goes as 1/sqrt(f) or so, whereas a capacitor's impedance goes as 1/f. It's not a very good model, especially over a wide range of frequencies. Eliminating the capacitor entirely (so the resistance is equivalent parallel resistance, EPR) goes as f^0, which is literally just as good -- the error factor is merely inverted. So, most times you might as well model the losses as EPR. In either case, mind that it's an approximation for a given frequency range.Laminated iron cores fit this model very well. In a prior project, I had chosen a toroidal transformer, 10kVA capacity, and suitable for square waves in the mid audio range. (It was a neat project, worked just as it should; and yes, it was loud...) When we got the parts in, I did a small signal test:- The inductance was relatively low, as expected: a quirk of iron is it's "sticky", i.e., it doesn't start being a good magnet (mu ~ 20k) until you put some field through it. The initial permeability might be under 1000.- At low frequencies, applying a square wave voltage results in a triangle wave current. So it looks like an inductor. The current varies inversely with frequency (I = V / (2*pi*F*L), of course), from very low frequencies up to maybe 500Hz.- If you look closely at the triangle waveform, it's got a bit of a step when it flips direction, not just a simple direction reversal. At low frequencies, it's not very noticeable, but at medium frequencies, because the triangular component shrinks so much, it becomes very obvious, and at high frequencies, it even dominates. An R || L model exhibits exactly this behavior: the resistor delivers a square wave current, while the inductor delivers a triangular current waveform. The sum is measured externally.- At high frequencies (2kHz or so), something different happens. The triangle wave component is now so small that the current waveform looks like a square wave with only a slight tilt on it. But now the current actually rises slightly with increasing frequency -- maybe 10% per octave. Indeed, the parallel resistance is dropping in this range. I think an explanation for this effect is, as the skin depth becomes more shallow, the magnetic field is squeezed out of the core, and any time the volume occupied by a field drops, so does the inductance; thus the impedance drops. True, according to the scope, we're not talking inductance anymore, but in the presence of phase shift, you end up with loss, or something.Now, all these effects are characteristic of laminated steel only, but the general form remains applicable even to ferrites. Which, by the way, are still conductive; try probing a bare MnZn core (most common type) some time. (The other main material is NiZn ferrite, which is much less conductive, and also has a brown streak ('streak' in the mineralogical sense). It finds use in the MHz+ range.) Hysteresis rather than eddy current loss is dominant in this material, but losses and phase shifts still work out the same. (By the way, ferrite cores over a few inches thickness -- for only the biggest applications of course -- also exhibit skin effect, so it's useless to make single cores bigger than this.)Ferrites exhibit cutoff frequencies, though it's usually assumed characteristic of the material rather than the geometry. The direct tradeoff is permeability, so a 10k mu material might drop off around 50kHz, while an 800 mu material is usable to nearly a MHz.Tim