Power Magnetics/Transformer Design Resources? - Page 1
Forum section: electronics / projects

author: NorthernWing
author_rank: Contributor
timestamp: February 19, 2021, 04:48:15 pm
content: Just gonna throw this out there and see if anyone has any thoughts. I'm coming from the land of FPGAs and digital logic but have decided to tackle magnetics (specifically switching supplies) as the next big thing I don't know much about. Mainly transformers.Does anyone have any recommended reading that focuses on transformer selection for switching supplies, or just power magnetics in general? Things like:...this push/pull simulation shows that the coefficient of coupling K of a transformer has a huge impact on performance as you'd expect, but most off the shelf parts I've seen list neither this nor leakage inductance to find it, so what am I supposed to do here?...how exactly do you find K given other information?...the Hexapath transformers I'm using from Coilcraft list neither leakage or coupling, so how am I supposed to account for this in a design? Should a robust design just be able to handle any typical part it might encounter?...if so, what's considered a 'typical' coupling factor?...even if this is the case, how do you properly snub a ringing transformer without this info other than trial and error?...why the hell is the primary side current of this thing ringing so badly in the first place?Not really looking for an answer to these, just somewhere to start reading. I'd like to think I understand the math behind flux/collapsing fields/oscillating LC stuff etc, but have been given nothing useful whatsoever to translate that into selecting parts and putting together something useful. That's all great until the voltage spike from diode reverse recovery blows up your primary switches.Any recommendations?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 19, 2021, 10:17:52 pm
content: The old TI (nee Unitrode) appnotes are pretty good. There are plenty of books on the subject, alas I don't have any recommendations handy... I'm largely self-taught on the matter.For a more basic explanation of terms and use, I wrote a page years ago,https://www.seventransistorlabs.com/tmoranwms/Elec_Magnetics.html#magnetwhich is still not too bad I think. I'm a bit less equivocal on the matter of inductor design, though, alas, explaining how will not "fit into the margins here"...Regarding your points, in order--1, 2. k is related to leakage inductance,https://en.wikipedia.org/wiki/Leakage_inductanceMutual inductance as well, but that's almost only used for theory, you'd rarely use it in circuit.3. Those are more or less wound multifilar, so we can approximate leakage as the length of the winding times the inductivity of the transmission line thus formed.Transmission line transformer theory is very useful; at least, if you know transmission lines already.It's not always a great derivation method -- multilayer windings aren't easy to describe as TLs. But the low-frequency equivalent is still useful, in terms of measuring and relating the parameters of the basic 2nd-order model,https://www.seventransistorlabs.com/Images/XfmrEquiv.png(where T1 is a transformer with some turns ratio, and primary/secondary inductance)Namely that Cp (or other C depending on connection) and LL are equivalent to a transmission line of Zo = sqrt(LL / Cp) and τ = pi sqrt(LL * Cp) / 2, or thereabouts.Anyway, single layer windings, particularly with matching turns counts, so that wires are largely on top of each other for the whole length, do model quite nicely as TLs. Imagine parallel-wire TL, wound edgewise onto a bobbin -- that's a two layer (one layer per winding) 1:1 transformer. Doesn't matter if there's tape between layers, that just sets the spacing between the wire pair. (There is some effect from adjacent turns, when the winding pitch is small; and from the inner layer being shorter length. But these are somewhat lower order effects, and the TL equivalent is in the right ballpark.)So, you still have to know the wire size, length, number of turns -- they may not tell you these, in which case you can reverse-design the transformer from what it looks like, assuming standard components (core shapes, materials, airgap..), and arrive at a reasonable guess.Better still, you can sample a few and take 'em apart.4. Good transformers, >0.98 I'd say. You'll find coupled inductors (particularly split-wound toroids -- popular with CMCs as well) that are a bit worse, but if it's much worse than say 0.95 or 0.9, they might just be something special purpose, like for resonant power conversion, or RF tuning. Pulse transformers may be in the >0.998 range.To clarify -- CMCs (common mode chokes) are actually more like transformers than chokes. They're used in a somewhat different way (indeed,sideways); the important part is high magnetizing impedance (hence,chokingoff an AC current).Transformers generally have high impedance, so that they have little effect on the circuit -- also it doesn't much matter what phase that impedance is, so they can be quite lossy (in terms of core loss versus magnetizing reactance), it's just that, the high impedance draws a small fraction of total signal current so you don't care so much.Transformers, designed with significant (i.e., relatively low) magnetizing inductance, could be better described as coupled inductors. Mind, they're still sold as transformers: this is a conceptual distinction, not a marketing distinction.And, inductance stores energy, so for a given flux, less inductance stores more energy.Flux, is integral volts dt, as explained by the link; inductance is the ratio of flux to current. If inductance is lower, it means more current flow for a given flux, and thus more energy storage. It might seem counterintuitive when energy is usually given in terms of current: E = 0.5 L I^2 But this assumes the amount of flux is unlimited, so we can just keep packing more and more energy into an inductor by increasing the current or inductance. But the core size would go up extremely quickly. For a given core (material, size and shape), flux (in the core) is fixed, and all we can do is vary the air gap between core halves (if it's a cut core), and the number of turns (which converts core flux into in-circuit flux, i.e. in terms of terminal voltage; and core magnetization into circuit current). And there's nothing special about turns, you'd store the same energy for the same amp-turns -- just at different flux and current, as you vary the number of turns.5, 6. Well, if nothing else, you measure it once, and calculate the equivalent -- this can be done with a test circuit, or often, in the switching circuit as well. Example, use these formulas,https://www.seventransistorlabs.com/Calc/RLC.html#frqor the last section, to go from a resonant frequency and impedance, to L and C.Specifically, for testing Lp or Ls, leave all windings open-circuit, and connect one winding as shown. The resulting measurement will include Rp or Rs (as a parallel equivalent), and core loss (typically shown as parallel equivalent as well). For an inductor, or coupled inductor (or flyback transformer, etc.), this should be of modest value, and relatively high Q. For a transformer, this should be a relatively high value, and the Q may be poor.For testing LL, short all other windings, and repeat the measurement. You may want to (or need to) change R1 or C to keep the frequency and voltage ratio in reasonable ranges. Also, don't forget to include signal generator impedance: if you measure V1 and V2, simply use their ratio, and the explicit R1 between them; if you measure V1 open-circuit and V2 closed-circuit, add source resistance to R1.For two-winding transformers, this gives you all the magnetic parameters; for multiwinding, you may want to continue testing other windings with respect to the others shorted. In general, the coupling factor is a matrix, giving the coefficient between each pair of windings. There are certain limitations on this (I don't know the general formula offhand), namely that if for example k12 = 0.5 and k23 = 0.5, then k13 >= 0.25 or something like that; and, a passive linear transformer will obey reciprocity, meaning the matrix is symmetrical (k12 = k21, etc.). It'seven rarerthat manufacturers give any hint about this sort of info, so you're on your own for this I guess. I'm not sure if I've ever seen more than one winding's leakage specified.As for pulsed operation, presumably you'll get some kind of ringdown:Here's a typical flyback waveform. Referring to Ch3, at the far left, voltage is low (transistor is on) then jumps up suddenly (turn-off). There is some ringing. In this instant, the transistor turns off, the secondary side diode turns on, and that means the secondary is an AC-short circuit -- the diode and filter capacitor are a low impedance. So the primary side shows leakage inductance, and this resonates with transistor capacitance. LL is small and Coss is modest, hence the fast blip of ringing. Some time later, the flat peak rolls off, as the inductor discharges and the secondary side diode turns off. Now Coss acts in parallel with diode Cjo, and the transformer reflects magnetizing inductance only -- hence the slower ringdown.This is a typical DCM (discontinuous current mode) waveform, i.e., inductor current goes to zero between cycles. In CCM, the free ringdown period is gone, as the transistor turns on during the top peak.Note that it works reciprocally: when the transistor turns on, it presents a very low impedance to the primary, so the secondary impedance reflects leakage inductance. This rings with diode capacitance Cjo.For all three cases, we can identify an inductance and capacitance, and therefore a resonant frequency and impedance.To dampen these resonances, simply connect an R+C across it, where Rsnub = sqrt(L/C) and Csnub >= 2.5 C.When LL isn't particularly nice, you'll find this dissipates way too much power; it's not a particularly efficient method, rather brute-force. It's also poor at controlling peak voltages: C needs to be even larger, and R somewhat smaller.In some cases, you might have enough core loss to do the job already; this is often the case for the lowest quality powdered iron materials (typically mix #26, 52). These are... not recommended for DCM operation.They can be okay in CCM, with a small ripple fraction (i.e., ripple current peak-to-peak being a small fraction of DC current). Basically what's good for maybe 10W in DCM, but can handle tons of DC current just fine, might be capable of 100W in CCM (I cover this in more detail here,https://www.seventransistorlabs.com/Articles/Core_Loss.html)."7." I never liked the phrasing "magnetic field collapse" -- it implies something that's simply not representative. It implies asymmetry, it implies... uncontrolled transient response, say. The fact is, the inductor simply does whatever you tell it to. In a switching converter, its terminal voltage is always perfectly well defined, clamped between Vin, Vout or some combination thereof, and therefore its rate of current change, and charge or discharge time, is all perfectly well defined.The least well defined part of all that, is when the impedance changes abruptly: switches turning on and off. And here still, we have well defined impedances, if we just take the time to look for them. Switches aren't ideal, they have resistance or capacitance; transformers aren't ideal, they have leakage, losses, capacitance and so on. In the waveform above, the voltage doesn't spike up, in 0ns, to \$\infty\$V, it's perfectly defined by the switch turn-off rate, node capacitance and transformer leakage. It is entirely our fault, as designers, if we choose to ignore these parameters, and don't take responsibility to control them, whether by design (e.g. transformer windup) or additions (snubbers and such).Well, in characteristic style, I've ended up writing quite a bit more into this margin than I initially led on, huh? Good luck, in any case.Tim

author: jonpaul
author_rank: Super Contributor
timestamp: February 20, 2021, 02:19:26 pm
content: Bonjour NorthernWingBRAVO for your aspirations re magnetic designs.I was a power supply and magnetics designer and consultant in the epoch 1970s..1990, worked on aviation, medical, cinema and lighting up to 12 kW and 55 kV.Even though I had magnetics courses in college, 90% of my design was based on experience and trial and error.The fundamentals are in the classic texts I have mentioned to others on the forum,RE Magnetics Simulations we used MicroCap in circuit sim and a magnetics design program called Intusoft Magnetics Designer.http://www.intusoft.com/mag.htmMicrometals site has some great app notes and design tools, but mainly for powdered iron materials.https://www.micrometals.com/Finally notice that new switch materials like GAN and SiC and modern topologies, class E, soft switching, etc have made much higher freqierncis possible so the industry migrated from 20 - 50 kHz>>>1 MHz >>10 MHz in recent years.Finally the old push pull is seldom used as the transformer is poorly utilized and needs a primary tap.Most today are single ended Fly back/forward/Buck or 1/2 or full bridge depending on power, etc.Hope this can point you in a good direction!Bon Chance,Jon

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: February 20, 2021, 04:21:43 pm
content: Everything I've used is very old. I've found the Col. McLyman book useful, though I got it on sale and probably wouldn't pay full boat today. My old copy has quite a few errors but is very practical. SMPS Design by Billings is good. Again, cheap in the old days, very expensive now. I also used a big binder of nomographs from Ferroxcube. I think it was free back in the day. They still have some downloads but nothing that leads you through the design process like the binder did. There really is a logical design process for these things. Excel is your friend! The one thing I learned over the years is there's a huge design difference between the common LV to LV switchers that are most common today, vs HV output designs that I usually needed. Transformer design for HV involves a lot of "secret sauce" in terms of the physical windings, to achieve what's always a compromise in the desired parameters.

author: sandalcandal
author_rank: Unknown
timestamp: February 20, 2021, 09:16:23 pm
content: Some notes on resources I've used for learning magnetics design here:https://www.eevblog.com/forum/projects/reducing-fringing-flux-problems-in-gapped-etd-cores/msg3280594/#msg3280594Jon Paul and other also posted some magnetics designs notes there too, though the main topic is dealing with fringing flux there was some useful general resources.

author: poorchava
author_rank: Super Contributor
timestamp: February 21, 2021, 07:49:22 pm
content: Google for "Magnetics Design" by TI. It explains everything from basics, although some stuff related to transformer design is quite non-practical.Mag Inc and Arnold also have some documents on designing power inductors.It also quite brutally points out that SI system should be used all they way, rather than CGS system, which is holding up strong in magnetics world for some inexplicable reason.In general the SI/CGS mess as well as MANY oversimplifications, ready-made formulas, different conventions etc make magnetics design have almost vertical learning curve for anything more serious than 'i need x microhenries from a core with Al of y'. You will either find already ready to use formulas, which are difficult to understand where they come from, or raw physics laws (eg. Base version of Faraday's law) which are completely useless for any practical application.Everything makes sense if you manipulate formulas long enough, but this is confusing as hell.

author: jonpaul
author_rank: Super Contributor
timestamp: February 22, 2021, 11:52:57 am
content: Bonjour a Tous:a few random old reference books.Steve Smith is an old friend....McLyman is oriented towards cut cores eg for avionics and space ..Basic college EE text: from City College of NYStatic Electromagnetic DevicesWilliam t Hunt and Robert Stein. 1970ISBN-10 : 125843654XISBN-13 : 978-1258436544Enjoy the reading!Bon Journee!Jon

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: February 22, 2021, 01:56:18 pm
content: Quote from: poorchava on February 21, 2021, 07:49:22 pm+1 on SI units. If you want to make mistakes, the fastest way is to mix unit systems.

author: NorthernWing
author_rank: Contributor
timestamp: February 23, 2021, 03:36:45 pm
content: First, thanks everyone for the suggestions - got a lot to go through here. I've added everyone's recommendations to the queue to look through.Quote from: T3sl4co1lWow, that's a lot to absorb. Out of school I worked for a defense contractor focusing on radar equipment and learned pretty quickly that everything is a transmission line if you try hard enough. I'm getting one key takeaway here... I was wondering if the best way to get a lot of this data is just to sit down and start measuring, and it sounds like yes, yes it is. I think I'll expedite getting some decent benchtop equipment... all I've got is a LeCroy 9354TM which is amazingly capable for being 30 years old, but I don't think just a scope and one good probe is gonna cut it where I'm going.Quote from: poorchavaGlad it's not just me then, because this is exactly what I'm feeling. I can follow app notes all day but once I start seeing a weird transient somewhere I didn't expect it and it isn't explained in their breakdown, I'm at a loss.Quote from: jonpaulThat's exactly how I feel - I've mostly just been trying things in LTSpice to see what does/doesn't work and how various parameters affect performance. Back in school, 90%+ of our EE course load was finding filter poles/impedances, step responses, and doing Fourier transforms/convolution by hand, or at least that's what I remember from it. A lot of people came out of it not even knowing how a MOSFET worked, let alone what something like the Qgatemeant and that you actually have to charge/discharge it. The word "snubber" was never used. Thankfully it sounds like after I'd left, they replaced our mostly useless VLSI course with some of this material that I'd consider missing.I could go on all day about the state of the EE/CompEn dep't at the University I went to, so I'll save that one for another thread.Quote from: jonpaulGood example of another problem I have right now. I had no clue that the turn-off behavior of a SiC diode was so much different/faster than something like an ultrafast Si diode. I'm sure there are a lot of other poor component choices I'm making just because I haven't ran into something newer/more suited in any searches yet.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 23, 2021, 03:59:26 pm
content: Hell, that's a better scope than I have! (TDS460 4 channel 350MHz 15kpts) Just add a function generator and you'll have the bare basics.Assortments of components, it goes without saying(?!). Power supplies of some nature. That'll get you the stuff to set up test jigs for this, doing resonance or frequency response and pulse testing.Quote from: NorthernWing on February 23, 2021, 03:36:45 pmHeh, it's not unusual for appnotes to not know where their transients come from, even when it's the very topic of the document. There's a, NexFET appnote by TI, concerning the switching loop transient of a buck converter, which utterly fails to address its headline claim. (Ah here it is, SLPA010, for those curious for a figure-out-what-the-author-is-missing test.)QuoteBest solution is to keep reading. Flip through distributor inventories, see what looks interesting, read up on things that are new [to you]. Get books to fill in the gaps or refresh your academic knowledge, push yourself to do some homework problems -- or make up your own. etc.Tim

author: uer166
author_rank: Super Contributor
timestamp: February 23, 2021, 11:59:44 pm
content: Quote from: T3sl4co1l on February 23, 2021, 03:59:26 pmHa: "the loop inductor, the loop resistor and the output capacitor of the sync FET form a series RLC loop and will resonate at a resonant frequency. This resonance will result in voltage overshoot and ringing at the switch node.". The loop inductor and output cap have nothing to do with voltage overshoot, ringing at Vds. Instead, it's the parasitic inductance in the layout between input cap/FETs?I mean they did address the layout issue, slowing down the FET switching speed with gate resistors, as well as reverse recovery current snap-off.

author: jonpaul
author_rank: Super Contributor
timestamp: February 24, 2021, 11:13:04 am
content: Hello again: Simulations have their use, we used MicroCap and Magnetics Designer for decades.Accuracy depends entirely on input parameters, simulation choices (eg circuit and sample rate) and the input of stray and parasitic capacitances and inductances.It often faster to simply to make a few assumptions and best guess at a preliminary design, build the transformer and evaluate in circuit.We often had it fine on the first try or had to make 1 - 2 iterations for a good production design.We never used a circuit sim to design magnetics, only for the power supply design.Magnetics designer is a great program to check windings and fit, choose bobbin size, etc.GAN and Sic parts are now mature and many manufacturers and technical papers can give you the required design guidance.The best courses are at Virginia Polytech, Dr Prof LEE power electronics group.SMPS and magnetics design is an intersection of physics, electronics, topology, and design consideration of available core materials, bobbin shapes and wire types.A combination of science, art and trial and error experience.The latest developments and best technical papers are from IEEE Power Electronics and Industrial Applications proceedings, as well as the APEC, PESC and Nurenberg conferences and proceedings.Hope this is of benefit,Kind Regards,Jon

author: harerod
author_rank: Frequent Contributor
timestamp: February 27, 2021, 07:58:01 pm
content: If I had to give a single recommendation:Erickson/Maksimovic: "Fundamentals of Power Electronics"provide an easily accessible foundation on this topic. I found the chapters on magnetics rather helpful for transformer design. The book is in active, albeit non-intended, use as I write these lines.Side note: When I start coaching electronics, I always make sure that the client thoroughly understands SI. No use building the roof before the walls.