Can inductance vary depending on the DC current?  Power Choke Tester - Page 1
Forum section: electronics / beginners

author: Tycho_Brahe
author_rank: Contributor
timestamp: November 05, 2024, 08:05:11 pm
content: I wasn't sure if this topic belonged in 'Beginners' or 'Test Equipment' - anyway.....I received an email from an electronics test eq. store that reads:QuoteThey are promoting a test device called a Power Choke Tester. Which uses pulses, I think.I'm not really interested in the device, but in the theory behind the need. I suppose my questions are, why would the DC current affect the inductance? Is it because at higher currents the iron core can saturate (I'm guessing) or something else? I assume the discrete part is passive.If it has anything to do with saturation, I still don't understand why a typical LCR meter couldn't be used to read the value accurately. Another assumption - that it wouldn't be normal to want to drive the part to saturation. So testing the value at a low current (with an LCR meter) should give a good reading. But apparently I'm wrong all over the place.I realize that a reactive passive component can have a different reactance value (measured in ohms) at different frequencies. But I always thought that the value (whether it's capacitance or inductance) was constant for that specific part (no matter the frequency). Not sure if I can even count on that, but I hope so. But at any rate, now I'm reading that the inductance can change depending on the DC current. And if this isn't some breakdown current type of thing (saturation), then I'm totally lost.I just want to understand it. Thanks.

author: coppice
author_rank: Super Contributor
timestamp: November 05, 2024, 08:14:25 pm
content: Any inductor with a magnetic (e.g. ferrite or soft iron) core is susceptible to magnetic saturation of that core. So, its inductance varies very little across a range of small currents, but as the current rises it can start to push the core into saturation, and the inductor's effective inductance falls.

author: TimFox
author_rank: Super Contributor
timestamp: November 05, 2024, 08:28:55 pm
content: Many power inductors for DC have an air gap in the core, which dominates the “reluctance” of the magnetic path to reduce the dependence of inductance on DC current.However, ungapped toroidal or other geometries have an inductance that depends directly on the core’s permeability. If you look at the magnetization curve (Bin Gauss or Tesla vs.Hin Oersted or amp-turns per meter) you see a medium slope at zero (initial permeability), a maximum slope at a higher current (maximum permeability), and a flat asymptote at high current (saturation).Air-core coils should have constant inductance with respect to current.

author: CaptDon
author_rank: Super Contributor
timestamp: November 05, 2024, 09:24:39 pm
content: Google it. Theory, Charts, Explainations abound.

author: dietert1
author_rank: Super Contributor
timestamp: November 05, 2024, 09:49:00 pm
content: Yes, what the voltage spec is to a capacitor is the current spec to an inductor. Current is limited by heat (wire resistance) and by core saturation. Close to core saturation inductance will be less. There are "hard" and "soft" core materials and the effect can be used to tune inductance within some limits.Regards, Dieter

author: thm_w
author_rank: Super Contributor
timestamp: November 05, 2024, 10:58:16 pm
content: Depends on current and temperature too:https://electronics.stackexchange.com/questions/585379/why-does-inductor-core-saturation-depend-on-the-temperature

author: MrAl
author_rank: Super Contributor
timestamp: November 06, 2024, 02:42:06 am
content: Quote from: Tycho_Brahe on November 05, 2024, 08:05:11 pmHello there,SUMMARY:Power inductors are complex devices that have a variability that make it impossible to test with a device that does not allow us to set the needed parameters in order to get complete test results for that inductor.Inductors made for power applications have a core material that is very magnetically active. However, there is a limited range of use for a given chunk of material. When you have that core and a coil of wire wound around it, the current in the wire excites the core and the core reacts to the field that is produced. The higher the current, the higher the field, but the character of the core is such that it can only handle so much field excitation. In the theory of magnetic domains, there are a lot of domains and each domain acts like a tiny magnet. The number of domains depends on the size of the core cross section. As the current increases the field increases, and as the field increases more and more, domains flip to align themselves with the direction of the field. If the current reverses (AC current) then the process starts over again and the domains flip the other way. For a DC current, the domains flip and stay that way. As the DC current continues to rise more and more domains flip, but eventually we start to run out of domains to flip because almost all of them are already flipped. When that happens, we call that saturation, and any further increase in DC current will have no more domains to flip and that will be the end of the inductance and the coil and core start to look only like a coil of wire with no core. That means the effective resistance to the increase in DC current appears to decrease which means it starts to act like a short circuit.Before that happens however, the inductance just decreases little by little, so that any further increase in DC current makes the construction look like an inductor with lower inductance, and that means the current will go up even more in most applications, which leads to a further loss of inductance. This is a very important character to understand in DC to DC converters because there we see at least some DC current all the time.For your questions, this means there is no way we can test this kind of inductor with a typical inductance tester, simply because it will not be able to supply the necessary DC test current to be able to characterize the construction properly. If the inductance is 100uH with 10ma and only 50uH with 10 amps, the typical inductance checker would measure around 100uH and would never be able to tell you the inductance drops to 50uH with a DC current of 10 amps. That's why a regular inductance checker will not work with power inductors. The core characteristics with different DC currents is just too variable meaning the very thing you are trying to measure (the inductance) cannot be measured except at very low currents.There's an even bigger catch here too. Many core materials do not show the maximum permeability with low currents either. That means that the inductance being measured at very low currents may even be less than what it would be with a slightly higher current. That's because as the DC current varies, the inductance can start out low, then creep higher, then creep lower again. That's a sort of horizontally stretched out "S" curve.This leads us to a more descriptive test for power inductors.The best test for a power inductor is actually in the application it is going to be used in. That means if you are going to use it in a Buck converter that runs at 20kHz, then put the inductor in a Buck circuit that runs at 20kHz, and be sure to test it at no load, partial loads, and full load. That's the best test and probably the ONLY test that will be definitive because any other test would have to be able to run the inductor DC current up to the right full load test value as well as produce the pulses at the right frequency. That means that if you did buy a tester for power inductors, it would have to be very variable as so what you can set on it: the frequency, the pulse height, the DC current, etc. Without that it's not going to be a good enough test really. You may be able to test it to some degree, but if the settings are too different than in the actual application it will be used for, it may be a complete waste of time to test it that way.I hope this explains it well enough but feel free to ask more about this if you like.There is one more little interesting side to this. That is, since the inductance varies with DC current by going down with increased DC current, we can use that to our advantage in some applications. This involves understanding how efficiency varies in a converter that has to convert AC to DC, then DC to a different voltage DC, combined with the effect of the inductance going lower as the DC current increases.As the current in a rectifier circuit with an inductor input filter increases, the filter inductor inductance starts to drop, which means it offers less resistance to the DC current flow. Since at higher loads we may not need filtering as good as when there are lower values load currents, having the inductance drop means higher efficiency for the converter circuit. An inductor designed for this application is often referred to as a "swinging choke" and was popular many years ago and maybe still today in some applications.So we have many applications where we do not want the inductance to change much, but other applications where we actually design it to change in order to improve certain aspects of the design of the product. With your question in mind again, there is no way in hell we can test this functionality with a standard, typical run of the mill inductance tester

author: TimFox
author_rank: Super Contributor
timestamp: November 06, 2024, 05:38:45 am
content: Remember that many coils designed to carry DC have an air gap in the core: this makes a huge difference to this question.

author: coppice
author_rank: Super Contributor
timestamp: November 06, 2024, 12:35:39 pm
content: Quote from: TimFox on November 06, 2024, 05:38:45 amIt doesn't affect the question very much. It just shifts how the saturation effects play out. Anything done to enhance the inductance or contain the field of an open air coil has some non-linear behaviour, since it relies on repeated magnetisation and demagnetisation of some material that is saturable.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 06, 2024, 01:21:26 pm
content: ... also,QuoteNo, it really isn't fine with capacitors. High capacitance ceramic capacitors have similar saturation effect, capacitance dropping as a function of applied voltage.For both inductors and capacitors this saturation is often dramatic enough that it really matters. Whether you absolutelyneedan LCR meter at your lab which can do DC-biased measurements is of course a different question.

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: November 06, 2024, 01:28:51 pm
content: Another issue with measuring inductors is that the core behaves differently at different frequencies, as alluded to in another reply, ideally you need to measure the inductor at the intended frequency. A typical low cost LCR meter only has a limited range of frequencies.Who would have thought an inductor could be so complex?SJ

author: coppice
author_rank: Super Contributor
timestamp: November 06, 2024, 01:35:29 pm
content: Quote from: Solder_Junkie on November 06, 2024, 01:28:51 pmI assume anyone with a functioning brain.

author: TimFox
author_rank: Super Contributor
timestamp: November 06, 2024, 02:53:19 pm
content: Quote from: coppice on November 06, 2024, 12:35:39 pmI disagree: the air gap used in, for example, filter chokes reduces the variation of inductance with current by dropping most of the "MMF" across the gap, which has constant permeability = 1.The gap is also used in audio transformers that have substantial DC current in the primary.If you Google "inductor air gap", you will find posts from inductor manufacturers about the use of the gap in modern circuits.

author: mawyatt
author_rank: Super Contributor
timestamp: November 06, 2024, 03:58:01 pm
content: Quote from: Siwastaja on November 06, 2024, 01:21:26 pmActually it's not thecapacitancevalue but thematerialthe capacitor is made from. High density ceramic capacitors utilize a high "K" Dielectric Constant ceramic material to improve capacitance density like Class II types, these usually have a highly dependent "K" wrt to applied voltage and temperature. However some ceramic materials like Class I (C0G/NP0 for example) have excellent voltage (and temperature) performance, but have a low "K", thus lower capacitance density. These are often utilized in highly critical applications such as the integration measurement phase in high resolution DMMs (for example HP34401A), where any voltage dependance would introduce serious errors.Most folks, ourselves included, often refer to ceramic capacitors as X7R or C0G types, same for "Film" type capacitors such as PolyStyrene, Mylar, or PolyPropylene to indicate the Capacitor Dielectric construction material.Having the ability to perform and study DC Voltage Bias effects with Bench Type LCR Meters was the emphasis behind developing the DC Bias Fixture shown here:https://www.eevblog.com/forum/projects/bias-network-for-lcr-meterSome Bench LCR Meters have Built-In DC Bias capability, this is usually limited in applied bias range tho, and the above adapter can provide a higher range of applied DC Bias for testing.Applying DC BiasCurrentfor Bench LCR Meters at levels which would be useful for Inductor DUT evaluations including saturation effects is a different overall scheme and requires a different approach, one we are presently looking into.Best

author: TimFox
author_rank: Super Contributor
timestamp: November 06, 2024, 04:18:11 pm
content: A classic General Radio instrument for measuring inductance at audio frequencies with substantial DC current through the inductor:https://www.ietlabs.com/pdf/Manuals/GR/1633a%20Manual.pdfThat manual is from 1962.

author: MrAl
author_rank: Super Contributor
timestamp: November 07, 2024, 04:53:39 am
content: Quote from: TimFox on November 06, 2024, 05:38:45 amHi Tim,I think the reason you posted this is because of the effect of the air gap on the linearity of the inductance with DC current. I have to agree with that.I do think we still need a way to properly test an inductor with an air gap though in order to get the details of what happens at the higher currents.Air gaps in magnetic cores are interesting. They reduce the initial inductance by a large amount, but then allow a higher DC current before saturation. That's a good quality for different applications.The instrument you linked to in your following post is a good example of what it takes to test an inductor properly. Even with a casual inspection of the device, we can see a LOT of knobs used to vary the operating conditions. That says something profound about the nature of testing inductors.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: November 07, 2024, 11:29:22 am
content: It is very easy to build a high current MOSfet and inductor tester.It is just:Power supplyBunch of capacitors to be able to get high current pulsesInductor + flyback diodeMOS Fet (N-channelShunt resistor between the Fet's source and GNDMOS Fet driver to make sure it switches fast enough.And on top of that a function generator (or some uC with adjustable PWM output) and an oscilloscope.A setup like this is much more useful then an 8 digit accuracy LCR meter.If you are designing an SMPS circuit, you can examine what the real behavior is of your inductor. You can get a good enough estimation of induction, by measuring dI/dt and you can see what the real behavior of your inductor is when it gets near to saturation. For small pulse widths, dI/dt will be a straight line on your oscilloscope, and as the pulsewidth gets wider, the current increases (duh) and your inductor will go into saturation. Even for a closed magnetic field, there is no hard saturation. Usually the straight dI/dt line will start bending upwards and it will get steeper.I have built a test board for this. It is made out of a piece of the plated though matrix board. On the bottom Ive glued a piece of cardboard so I can be sure no solder gets though, and below that I have a capacitor bank of 8 or so 22000uF capacitors (I just filled up the whole size of the PCB). On the top I have soldered a big sturdy GND loop to clip on my oscilloscope, and whatever inductor or FET I want to test I add to the top. But I don't put THT parts into the holes of the matrix board. I just solder them on top in "Manhattan style". This makes it very easy and quick (my new soldering iron is hot in 3s) and the plated though holes in the matrix board keep make sure for good pad adherence.With this setup I can easily put 60+ Ampere pulses though the inductor and Fet, while keeping the average current consumption low (100mA or less). You probably want to swap out the current shunt resistor too. It's a balance between getting a decent resolution on your oscilloscope and not putting too much voltage over it to prevent your circuit from working. (A FET driver with a relatively high gate drive voltage helps here).

author: TimFox
author_rank: Super Contributor
timestamp: November 07, 2024, 05:36:34 pm
content: MrAl: that is right. The original post was about current dependence of inductance in practical inductors.Of course, practical components have some degree of non-linearity.Good resistors can be very close to constant resistance (ohmic), and good capacitors show very low voltage dependence of their capacitance.Carbon resistors can have measurable non-linearity (voltage co-efficient of resistance) and bad ceramic dielectrics have large voltage dependence.The parasitics of practical components (inductance of wire-wound resistors, ESR and self-resonance of capacitors, etc.) are a separate issue, and often can be modeled as extra linear components inside the “box”.

author: MrAl
author_rank: Super Contributor
timestamp: November 08, 2024, 06:01:59 am
content: Quote from: TimFox on November 07, 2024, 05:36:34 pmHi,Yes, very good Tim, and I would have given your post a "like" but I realized now there is no "like" button or anything, which is strange as most other sites have that.So I guess all I can do is give your post an unofficial "like" but consider it equal to a 'like' on any other siteThat applies to your other post with that piece of test equipment too.

author: watchmaker
author_rank: Unknown
timestamp: November 08, 2024, 12:20:09 pm
content: ^^It is the "say thanks" in upper right corner.

author: MrAl
author_rank: Super Contributor
timestamp: November 09, 2024, 08:50:08 am
content: Quote from: watchmaker on November 08, 2024, 12:20:09 pmHi,Yes I know about that, but that's not the same as a 'like'. The OP should use that though.You can like something without saying thanks, and you can say thanks for something you don't likeI did give an unofficial 'like' thoughThe reason I liked it is because this issue comes up a lot on forums like this and it is not easy to explain why the inductor can be so complicated. By seeing an instrument that has so many settings, it may become instantly obvious, at least hopefully.I wasn't really saying 'thanks' because even though I like the post I don't actually need the device myself. I only test inductors in the applications they belong in.

author: iMo
author_rank: Super Contributor
timestamp: November 09, 2024, 08:58:26 am
content: FYI -https://en.wikipedia.org/wiki/Magnetic_amplifier