is an inductor with a lower saturation current than the rms current any good ? - Page 1
Forum section: electronics / beginners

author: Simon
author_rank: Unknown
timestamp: June 01, 2023, 06:32:30 pm
content: I've been on the hunt for inductors. And this seems to be mainly a Wurth thing. They have inductors that will do say 5a RMS but saturate at 4A. What is the point. Some of the inductors will work under saturation at the RMS current with a lower inductance. Is this a good idea? my understanding is that a saturated inductor is turning energy into heat as it has no more storage capacity in it's magnetic field, so while at the higher current I could accept a lower inductance in a switchmode converter do I want to run in saturation?

author: PartialDischarge
author_rank: Super Contributor
timestamp: June 01, 2023, 06:53:31 pm
content: Inductance is heavily dependant on DC current across the inductors, and using inductors over its DC rated current for saturation is a thing, basically an inverter designed over saturation can use smaller and cheaper inductors.Rohde made a seminar about thishttps://www.rohde-schwarz.com/hk/knowledge-center/webinars/fundamentals-of-switched-mode-power-supply-register_255462.html

author: Simon
author_rank: Unknown
timestamp: June 01, 2023, 08:49:44 pm
content: so he starts going on about all sorts of tests. I don't have the time for that sort of thing. I expect the inductor manufacturer to tell me. So if Wurth give a saturation current for -10% and -30% inductance, are they saying that it is safe to use the inductor in this region. Given the higher current the inductance may fall. I needed a 300µH inductor to control 200mA, but once I am into 5A much less inductance is required.

author: TimFox
author_rank: Super Contributor
timestamp: June 01, 2023, 08:58:36 pm
content: When using choke-input filters in vacuum-tube circuits, it was common to use a "swinging choke", whose inductance fell dramatically as the current increased.An example:http://www.vias.org/crowhurstba/crowhurst_basic_audio_vol3_025.html

author: tszaboo
author_rank: Super Contributor
timestamp: June 01, 2023, 09:35:30 pm
content: Quote from: Simon on June 01, 2023, 08:49:44 pmIt's a Würth part, so you can just ask them. Call up your local rep and ask them to explain it.Not only do they help selecting parts, give you a much better price than the catalog distributors (because that's not how they usually sell) but will explain all these questions.I cannot understate how helpful that company is, and you don't need to be a big customer for it.

author: thm_w
author_rank: Super Contributor
timestamp: June 01, 2023, 11:46:25 pm
content: Here is an example:https://www.we-online.com/components/products/datasheet/74439346100.pdfRated current: 5A (dT = 40C)Saturation current (<L = 10%): 5.05ASaturation current (<L = 30%): 9.7AIf you want to be safe and simple, you just use the 5A rating.If you want to cheap out or save space (as PartialDischarge says above), you might be able to get away with the 9.7A rating. Knowing that: your ripple may be higher, inductor temperature rise will be higher than +40C, and converter will have to deal with the change.Some more details here:https://www.we-online.com/components/media/o393343v410%20ANP096a_EN.pdf

author: TimNJ
author_rank: Super Contributor
timestamp: June 02, 2023, 12:38:07 pm
content: Lower saturation current than RMS current rating just means that inductor can thermally handle the RMS current (some acceptable temperature rise), but it may not be a very good inductor (at all) at that point. Generally, you try to avoid saturation because, well, you said you needed an inductor there, not a piece of wire, right?As mentioned, sometimes you can allow “partial saturation” of the core, perhaps for peak load or edge case situations, where you can momentarily accept a higher peak to peak inductor current (and thus higher semiconductor and capacitor currents). I would say it’s still not something you *want* to happen, but if you are space constrained on inductor size, you may have no choice. If you are planning to let the inductor saturate to some degree, it may be beneficial to use an inductor with a soft saturation characteristic curve, e.g. powdered iron, Sendust, MPP, High Flux and so on.

author: Simon
author_rank: Unknown
timestamp: June 02, 2023, 03:39:41 pm
content: my main querry is does this mean that the overall solution is less efficient as the inductor is loosing energy in heat dissipation due to the saturation rather than just current flow. In the case of a buck converter, the more current you want at a given frequency the lower the inductance required. So as the current goes up, inductance comes down which is fine because if I design to work in continuous conduction at 1A when I am at 4A I need 1/4 the inductance anyway, but at this point am I much less efficient?

author: Siwastaja
author_rank: Super Contributor
timestamp: June 02, 2023, 04:29:21 pm
content: Datasheet front page thermal RMS current is useless in switch mode converter design anyway because it omits AC losses. Forget that rating and use the online calculator by the manufacturer. WE especially have had a decent online tool available for years. You get total loss and temperature rise, given the topology (buck/boost/etc.), current ripple %, f_sw, V_in, V_out, I_out.

author: MrAl
author_rank: Super Contributor
timestamp: June 02, 2023, 04:38:51 pm
content: Quote from: Simon on June 01, 2023, 06:32:30 pmHi there,Inductor selection is not as simple as capacitor selection usually, because the type of inductor is highly dependent on the application. In one app the inductor may work really well even be ideal, in another app it won't work at all. The selection will always depend on the application.This is because there are many, many attributes all of which contribute to the operation in a given circuit.These include:max RMS current, min current, Saturation current, max frequency, effective series resistance (ESR), just to name a few.The max RMS current rating is really all about the wire used. If the wire can handle that current, it should be ok, except for a large build which may have problems dissipating the heat and then it may be rated lower.The min current is usually not specified, but that is the current where the inductor will start to work right. It is probably low enough so it does not always have to be specified.The saturation current, although given as a seemingly cutoff point, is not really a cutoff point in most cases but is instead just a reference point where there would be another inductance value specified. For example, 100uH and at 5 amps 80uH. That means that at 5 amps you can expect a decrease of 20 percent. This rating can vary though as to how it is specified. You really have to consult the data sheet to be sure.The max frequency may be given also, which of course refers mostly to the core material. Some will be better than others for higher frequencies.The ESR is a very important spec for power supplies and other power uses. If the ESR is too high for your application, it will drop too much voltage and thus waste a lot of power if not worse. For example, if the ESR is 1 Ohm (high for a power inductor though) and the current is 10 amps, you loose 10 volts right off. It all depends on the application though.As to your question about the sat current and RMS current, again it depends highly on the application. Some inductors are made to decrease in inductance on purpose so that at full load they don't impede the current as much as for lower loads, thus allowing better filtering at low loads and less filtering at higher loads. Someone else also mentioned this as the "swinging choke". They can sometimes be found in frequency converters also.So all questions about inductors really default to what application they are being used in. There are a lot of different basic types and you really have to match them up with the application to get proper operation.Good question though.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 02, 2023, 04:39:27 pm
content: A couple reasons:- It might be chosen as a swinging choke (see above); this doesn't really apply to switching converters though.- It might be used (or even intended?) for lower power dissipation, i.e. Irms is measured at 40C temp rise, so you get maybe 30C at Isat, it's something.- Saturation might not be very aggressive: ferrite cores generally drop off sharply, while powder (ring and molded) cores drop more gently. Also check the threshold condition (usually -30%). Usually there's a L(Idc) plot in the datasheet; for Wurth, check their REDEXPERT thing too.- It might be in a series where they use the same winding and core, and vary the air gap. This gives poor results (core loss is higher in the larger values, and overall Q factor peaks somewhere in the middle of the series)Saturation doesn't cause dissipation. It might cause increased core loss, which might be a problem in a switching converter, but not in an EMI filter. But the varying inductance is likely the bigger problem; how big depends on the control scheme, but it's generally best to avoid.Meta:- I notice you rarely "have the time" for these things. It can be well worth spending weeks, months even, studying calculations, construction techniques, and optimizations, to achieve the best result (performance, cost, size) with magnetics. These are not things that suppliers will tell you (or necessarily know/understand themselves), nor can be found on the internet (well, maybe -- but good luck finding it). There are good books, but you'd have to study for even more time than a few video clips' worth.Sure, we're talking about much simpler matters here than "full stack" optimization, but these occasional roadblocks are well worth pondering for a bit.I assume you have a manager breathing down your neck, forcing the urgency; make it clear to them that good engineering is worth the wait. You have no duty to them beyond your immediate employment; never be afraid to shop around for a new job. Keep your options open.Tim

author: Simon
author_rank: Unknown
timestamp: June 02, 2023, 08:53:26 pm
content: well time is never on my side but this is not a specific work requirement more of a personal endeavor. My first skill on the job is to predict the future and know where things will go so that I can prepare myself technically. I can apply what time I have for these things to anything so may as well be something that increases my ability at work if it interests me anyway. That is what I am doing here. i have in fact been researching the whole SMPS buck converter for some months and know a lot more than I did at the start. I also have at least 3 specific work related things I want to apply my growing skills to. My boss would just let me bey one off the shelf but I want to do better than that.I am thinking of running a converter that simply uses a comparator, this would allow the frequency to shift based on the load. If an inductor does reduce it's value with increasing load then this is not a problem as I would be sizing the inductor to suit a lower load so get a battery converter all round that will not suffer if the inductance drops by 30% at full load as the frequency will have dropped anyway so just won't drop as much. Yes I would need to make sure the minimum load is there to stop the frequency going out of control or provide some other maximum speed mechanism.