Ćuk-buck Converter design - Page 1
Forum section: electronics / projects

author: EEVblog
author_rank: Unknown
timestamp: August 28, 2018, 11:31:17 am
content: Someone asked about this in Twitter but I have not really watched it yet.The guy (who it's named after) claims :QuoteLots of Tesla stuff on his channel which gives me the heebie-jeebies.

author: coppercone2
author_rank: Super Contributor
timestamp: August 28, 2018, 11:36:49 am
content: they should name it something else, it sounds like a weird french porno. I honestly would not want to reference this topology out loud in front of other people.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 28, 2018, 11:46:50 am
content: Quote from: coppercone2 on August 28, 2018, 11:36:49 amDoesn't bother me. Back in school, I had a prof who came from Serbia, or somewhere thereabouts, I forget what exactly. He made sure class understood correct pronunciation of the name, among other things like understanding history of certain other famous Serbian engineers.Tim

author: CJay
author_rank: Super Contributor
timestamp: August 28, 2018, 11:47:25 am
content: Quote from: coppercone2 on August 28, 2018, 11:36:49 amHah, only if you pronounce it incorrectly, we have a few Yugoslav/Serbian guys here and a C is often pronounced ch so it's meant to be pronounced somewhere between Tuck and Chuck

author: coppercone2
author_rank: Super Contributor
timestamp: August 28, 2018, 11:49:21 am
content: it's gonna be written as cuk on a white board some where and make people giggle, after there are some orders to snake some cables and something about logs and a control system its gonna reach critical mass

author: coppercone2
author_rank: Super Contributor
timestamp: August 28, 2018, 12:00:44 pm
content: Quote from: T3sl4co1l on August 28, 2018, 11:46:50 am

author: BBBbbb
author_rank: Unknown
timestamp: August 28, 2018, 12:19:56 pm
content: Quote from: CJay on August 28, 2018, 11:47:25 amNot sure a sound like that exactly exists in English, it is closest to the Italian 'ciao' (/tʃ/), usually ends up being translated to -ch. Also some other Slavic languages don't have it, just the ch version (Slovenian for example)Tesla reference - well a lot tech companies (and governments) from the exYu are riding the Tesla name heavily. Serbia being the most dominant, Croatia coming second (Tesla ethnic Serb born in Croatia).Tesla losing grip with reality near the end of his life really fuels the myth of him in the region in the general public and shitty clickbait media.

author: MiroS
author_rank: Regular Contributor
timestamp: August 28, 2018, 01:12:23 pm
content: Quote from: coppercone2 on August 28, 2018, 11:36:49 amBack to school my fried , take languages, geography and 'bonnes manières' classes. A few hundreds of milions of people have no problem with correct pronunciation , many professionals also posvitive/respectable conotation to this name .

author: Benta
author_rank: Super Contributor
timestamp: August 28, 2018, 04:52:02 pm
content: The very nice thing about the Cuk converter is, that it has both an input and an output inductor. This results in very low emissions. Downside is of course the need for two inductors.

author: chris_leyson
author_rank: Super Contributor
timestamp: August 28, 2018, 05:29:53 pm
content: One nice feature of the Cuk converter is that input and output ripple currents are non pulsating. In addition, the coupled inductor Cuk will give zero ripple on the input and output simultaneously but it's difficult the coupling spot on, it's even more difficult if want a transformer isolated coupled inductor Cuk as you need to wind the coupled windings on the outer core legs and wind the transformer windings on the inner leg. I once got it to work on the bench but not for very long, it's very sensitive to the amount coupling and really isn't practical. I'ven seen one transformer design where they drilled holes down through two of the outer legs to fit small tuning slugs to adjust the coupling it's that critical.Whereas the flyback is buck boost the Cuk is boost buck and a lot quieter. One downside of the coupled inductor Cuk is the poor transient load response. EDIT: Forgot to mention the Cuk class D amplifier, an interesting design but I've never tried building one.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 28, 2018, 06:01:57 pm
content: Mind this is talking about the (unqualified) Ćuk converter, not the Ćuk2 buck from the OP.The not-nice feature is the pulsating current is moved to the coupling node. Which is arguably worse because you can't spend much space on coupling capacitors there, and the full ripple current is very demanding on them.Another gotcha is the ground return current (from the switch and diode together, and as with the input and output currents, it is also continuous), which is the difference between input and output currents. That is, the magnitudes add, so if you have a 10A input and a -20A output, you need to handle 30A ground current.These are all just consequences of the converter not being anything fundamentally different, but a rearrangement of the underlying fundamental switching circuit. In a buck converter, the loop from diode to switch to input capacitor and back around, is equivalent to the Ćuk's loop from switch to coupling capacitor to diode and back around. The leakage inductance in the Ćuk transformer is equivalent to stray inductance to the buck's input supply.Getting the coupling correct, is no harder than getting the stray supply inductance correct. Anyone that needs to tune it is clearly doing it wrong (or doing something completely different -- perhaps you've conflated that memory with another experiment?).Tim

author: chris_leyson
author_rank: Super Contributor
timestamp: August 28, 2018, 06:39:19 pm
content: QuoteNope, try it sometime

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 28, 2018, 06:42:30 pm
content: Quote from: chris_leyson on August 28, 2018, 06:39:19 pmI've built several such converters without paying particular attention to coupling factor (I think they've all been >= 0.98, not particularly good). You'll have to be more specific.Tim

author: coppercone2
author_rank: Super Contributor
timestamp: August 28, 2018, 07:26:22 pm
content: Quote from: MiroS on August 28, 2018, 01:12:23 pm$50,000 of stuff right there, wanna pay friend?

author: David Hess
author_rank: Super Contributor
timestamp: August 28, 2018, 07:31:28 pm
content: With a simple buck converter, the step down ratio is limited by the minimum duty cycle and one way to get around this is to use a tapped inductor as shown to boost the output current and increase the duty cycle but nobody bothers to do this unless they have to like with a 48 volt input supply. How does he get 99% efficiency with 20 amps RMS through that diode?Does he have a patent on it? The Cuk converter was ignored for a long time because of patent protection. My old switching regulator books have a warning about using it so I gather that there was some unpleasantness.

author: chris_leyson
author_rank: Super Contributor
timestamp: August 28, 2018, 07:34:43 pm
content: Sorry Tim, wasn't specific enough. If you couple the input and output inductors on a Cuk then theoretically you can get zero ripple on the input and output simultaneously. I think the optimum K is about 0.7. I just happened to try it on an isolated Cuk and wound the two transformer windings on the center leg of an E-core and the input and output inductors were wound on the outer legs of the same core which were gapped to get the coupling down to about 0.7. Getting the gaps right proved to be extremly difficult as the slightest change in the gap length just made the ripple come back. It was beyond difficult to get it mechanically stable and even if you could nobody was going to pay for tooling for the bobbins. I abandoned zero ripple converters a long time ago, non pulsating currents on input and output are good enough.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 28, 2018, 07:44:12 pm
content: Right,zeroripple, that's a nulling operation. 10% might be reasonable for production parts. Zero, nah.Tim

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 28, 2018, 08:08:28 pm
content: From my understanding, he basically took a half bridge design and routed the primary current to cancel out the ripple in the secondary.Quote from: CJay on August 28, 2018, 11:44:35 amI rarely see the Cuk topology being used, the last time I saw it was on a quad Gigabit card to generate a negative bias voltage for the port switching circuit. Probably because a standard inverting converter would introduce too much noise.Quote from: David Hess on August 28, 2018, 07:31:28 pmSynchronous rectification is a wonderful thing. The diode in the schematic is to simplify the analysis.

author: JohnG
author_rank: Frequent Contributor
timestamp: September 01, 2018, 09:27:25 pm
content: Quote from: David Hess on August 28, 2018, 07:31:28 pmOf course there is a patent, although I'm not sure this is the right one:https://patents.google.com/patent/US7915874My other concerns:99% efficiency under what conditions, and at what power density?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 02, 2018, 12:28:42 am
content: The thing with most resonant converters, including this one, is the large reactive power. You might need to handle 2 or 3 times the DC output power as reactive power. That's extremely demanding on inductors, even capacitors, and in particular this being a half wave design, the diode peak current and output ripple are extremely high.A resonant converter pretty much demands using low loss ferrite, low mu powdered iron (like mix #2), or some other materials (Sendust and MPP are quite good, actually, though maybe not this good).It's a big advantage to be able to use cheap, crappy powdered iron chokes in a conventional buck with a low ripple fraction, needing below 10% of DC power in VARs. And so, for a #26 powdered iron with a typical Qaround 10, the losses are a similar fraction less than that, say 1% of DC output. Or if you opt for a ferrite core choke, the lower losses afford you a higher ripple fraction for the same efficiency.Tim

author: Calderan
author_rank: Newbie
timestamp: May 28, 2020, 08:07:44 pm
content: Hello Everybody.I just came across this post after ages of searching for some specific info about Cuk-Converters.For an uttermost brief introduction, some time ago I came across the task to charge batteries with power from solar panels, thus variable input voltage, and found the Cuk-converter (the “classical” one). Now I have scientific background but no electronic in-depth knowledge. So I started it as kind of a hobby / fun project. Anyway, I got a working setup including a funny concept for voltage control. But of course I also came a lot of detail-question that I just can´t address them all on my own.And this seems the only place in the whole internet, where people actually provide a factual discussion. So here I am.My first question is - as above - the magnetic coupling between the two inductors.If I use two air-core coils without magnetic interaction, the setup works.If I allow the two air-core-coils to interact (antiparallel orientation, as indicated in the respective Cuk-layouts) the efficiency is better.For control, an interaction in a parallel manner is less effective.Interestingly, if I put both coils on one ferrite-core (antiparallel) the efficiency is even worse than my above “control-experiment”.(And a parallel setup on the same ferrite-core, second control experiment, hardly works at all.)As mentioned 2 years earlier, for a ripple free current a value of 0.7 seems optimal. Now for charging batteries, ripples are of little concern, but if I can, I want high efficiency.Is for the Cuk-converter “ripple free” the same as “maximum efficiency”? Kind of a side note: does the duty cycle play a role?And is there a generic way of how to get at least close to a good solution, e.g. what kind of open core can be used? Despite all interest I am reluctant to salami-slice my ferrite core…

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 28, 2020, 08:55:45 pm
content: It's not... perfectly clear what you've done, if the stock circuit has been followed, how the actual coupling coefficient varies with described conditions, etc.; but it is the case that the overall efficiency is not a simple function of coupling, but varies up and down, and is affected in turn by the coupling capacitance and such. You generally want the parasitic resonant mode (leakage inductance * coupling capacitance) below switching frequency, or far above it, and efficiency can be unexpectedly low when it's inbetween.Tim

author: Weston
author_rank: Regular Contributor
timestamp: May 28, 2020, 09:32:27 pm
content: Has anyone actually spent the time to figure out the fundamental issues with the Cuk-Buck design? I a different variant a while ago and am very dubious but have yet to spend the time giving it a detailed look. The claims are pretty out there and I have yet to see any serious implementation or commercial interest in the design. I like to believe markets are at least somewhat efficient so there must be some unacknowledged issues with it.The conference paperhttps://ieeexplore.ieee.org/document/8066120(non IEEE hosted draft here:http://gecs.ieee.tn/wp-content/uploads/2017/03/GECS2017PaperEDITED.pdf) does not inspire much confidence and is pretty low quality.The most obvious issues are the large output ripple. Without doing a full analysis, I would also guess that the converter can only achieve the claimed efficiencies over a very narrow output conversion range, determined by the turns ratio on the inductor.

author: Dave
author_rank: Super Contributor
timestamp: May 28, 2020, 09:56:32 pm
content: Quote from: EEVblog on August 28, 2018, 11:31:17 amUnlike many nutcases that like to talk about Tesla, this guy is actually legit.

author: Calderan
author_rank: Newbie
timestamp: May 29, 2020, 09:30:11 pm
content: Hello All,(sorry for the long reply)Teslacoil, what I have done:Two separate air-core coils (1 mH each).For the first test, the coils were “far apart” and their axis’ orthogonal.Then I aligned the coils on one axis and approached the coils until the touched. Thus "zero" to "maximum" interaction, but as for air core coils, even “maximum” surely still has a lot of leaking inductance.To check the actual end with K= ca. 1, I used two 1 mH coils on a ferrite ring core.Today I checked "qualitatively" for the resonance effect you mentioned, by doubling the coupling capacitor. In case of the air coils this yields a slight drop in efficiency. So there’s definitively an effect.The setup with the ferrite core is not affected by this change (seems plausible, as a ferrite ring core should leak very little).But this means that the low efficiency of the ferrite ring core interaction is not caused by leakage, but actually by "K=1".However that’s not surprising, as also for “minimal ripples” the maximum is “in between”.But where?Problem is: even if I basically understand what you are talking about, there’s hardly a way for me to measure this the leaking inductance / coupling…Also, the only way I see to adjust the coupling (by try and error) would indeed be to drill/cut into my ferrite core (as mentioned for “minimum ripples”).But that I want to avoid unless I have at least a rough idea where I need to aim to.Cuk-Buck 2:I wish I would understand how the setup shown in the initial post actually works!But currently the traditional “Cuk” keeps me completely busy.Thing is: There are several designs that all are called “cuk-buck 2”.The links that Weston posted seem to refer to the “Original” design. For this there is a good discussion here:Basically the “transition” from “4 switches” to “2 switches and 2 diodes” is the pre-requirement to induce currents – but that actually costs efficiency.So maybe that’s already the entire “problem” with that setup.For the layout displayed in the first post here, I also have seen sketches without the resonance-inductor L(r). Also, there is what might be an "interim-version":https://www.electronicsweekly.com/blogs/engineer-in-wonderland/cuk-buck-2-dc-dc-converter-fine-art-pen-slobodan-2017-01/Anyway, I never had the time to think it through and understand how it works. Might be worth a closer look.And for something completely different:Come on, Tesla wasn’t a nut-case.He invented the all the AC-technology we use since a century and a lot more like the tesla coil and in turn HF-AC and transmitters. Problem is, he could not work out a reasonable theory for his observations. The whole “scalar wave stuff” surely was an interesting idea at his time, that fitted with his observations(!), but in the end there was nothing beyond electromagnetic waves and resonances.But anyway, we still use "his" AC – even if thanks to high voltage semiconductors, High Voltage DC transmission becomes a reasonable alternative. Seems nobody wants to invest / develop new technologies.I guess, in the very same video cited in the first post here, Slobodan Cuk actually complains about a similar issue, as they still use "sets of Buck-Converters". Now I can’t asses the design he proposes, but there’s another example: Loading car batteries with solar panels. The cheap devices available simply “heat away” any “surplus voltage”, resulting in high losses. No big deal for small panels - but if you “think big” (and efficient) and also consider that the voltage of the panel might actually drop below the battery voltage, then that "cheap approach" is hardly reasonable.And the Cuk-Converter, that is highly suitable with respect to both aspects, was invented 44(!) years ago, but seems nowhere to be found!Funny (sad!) enough, I did a lot of Google searches for “cuk” and also “cuk buck 2” - and since then, youtube, etc., shows a strange amounts of free-energy-stuff.I suspect that these converters were never fully investigated. There are numerous articles, but all cover only detail aspects. And granted, that tiny little layout has a quite complex behavior.So in summary, even Cuk-converters may seem kind of "obscure" - and that again seems to create the nut-case-link.So it would be nice to get all info together. And maybe get “something reasonable” going.

author: JohnG
author_rank: Frequent Contributor
timestamp: May 31, 2020, 06:39:09 pm
content: A couple of comments about the conventional Cuk converter:Pro: Two inductors is not a huge disadvantage in practice, because other converters with pulsating input or output converters generally need to filter them, and this is often via a second inductor. You can use a big cap bank, but this has it's own set of problems.Pro: You can reduce ripple via ripple current steering via control of inductor coupling.Con: Capacitor transfers most of the power. At low voltage, this requires a lot of capacitance, and they need to be low loss caps. This usually means a big and expensive cap bank.Con: RHP zero makes control loop design a chore.Con: Control of inductor coupling coefficient is not free, and even though it has been known for decades and built by many graduate students and even some professional engineers, there are very few commercially successful examples. In practice, the return on investment is usually poor, in large part because you still need to store and release energy from the inductor, so there is still flux ripple and core loss.On the Cuk-Buck(n) converters:They look like a combo switched cap/quasi-resonant converter. You get high RMS currents, high peak voltages, or complicated design with many switches and gate drivers, a lot of which are floating. Pick your poison.There is so little comprehensive data, and the designs I haveseenall have uninteresting power density. It's been years since the designs and ideas have come out. If it's so great, where's the data? By this I mean comprehensive efficiency or loss curves for a single good design over load and line, with a converter of known power density. You can find what I mean in a data sheet from any reputable power supply module manufacturer. But a few curves with almost no data on the design itself is not at all convincing to me.I mean, Cuk was a brilliant guy in Caltech days, but this later stuff has a weird, cultish vibe to it that really puts me off. If you don't believe me, go look him up on Linkedin. Perhaps I am missing the greatest breakthrough in power electronics since Fizeau invented the capacitive soft switching snubber in 1853, but I doubt it.Ok, enough pontificating...John

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 31, 2020, 08:39:01 pm
content: FYI, I find the argument about the coupling cap incomplete -- this requires a transform to see, but once this is done, one can see all the common converter topologies are simply rearrangements of the same general case.Which is to say: the coupling cap is equivalent to the buck input or boost output bypass capacitor. These topologies have supply and switch-loop stray inductance; the Ćuk has leakage inductance (which is simply more strays wrapped up inside a transformer) plus circuit strays.In other words, if you try to cheat on the Ćuk coupling cap, or fail to adequately dampen it or whatever, you'll have the same demise as using a too small, or underdamped, bypass capacitor in the topological equivalent locations.On a related note: series-parallel transformation is a mind-bending technique at first, but is perfectly algorithmic once understood. For a challenge: derive the current-fed half-bridge inverter topology.So useless, so weird!Tim

author: Weston
author_rank: Regular Contributor
timestamp: June 01, 2020, 06:15:13 am
content: If nothing else, the cuk converter serves as the basis for more challenging home work assignments in power electronics classesĆuk's recent endeavors are a bit puzzling. I wonder why he left Caltech, I assume he had tenure. That conference paper about one of his converter designs that I previously mentioned is bizarre enough where I am not sure if he is trolling or nothttps://ieeexplore.ieee.org/document/8066120(non IEEE hosted draft here:http://gecs.ieee.tn/wp-content/uploads/2017/03/GECS2017PaperEDITED.pdf). I assume it it was treated as a normal submission it would not have been published.I looked at the patent in the article that was linked by Calderan and it seems it has expired due to renewal fees not being paidhttps://patents.google.com/patent/US7915874. It looks like the patent on the other variant is active thoughhttps://patents.google.com/patent/US9231471B2

author: Calderan
author_rank: Newbie
timestamp: June 01, 2020, 07:53:17 pm
content: Cuk-Buck2:A discussed in the analysis-video, the 4-switch-setup has high efficiency for a 2:1 voltage ratio. And Prof. Cuk managed to overcome this hard-wired lock by replacing 2 of the switches by diodes. So far so good.But thereafter, here I agree with JohnG, it gets a bit strange, as there are no measurement-data for the new setup. There´s just a simulation, that also does not actually explain the function of the layout.That’s why I’m curious about these schematics.But as I said, currently my breadboard is filled with the classical setup, mainly the control circuit therefore.But quickly back to the magnetic interaction between the inductors. "Inspired from the setup of the CuK-Buck2", where L(r) is in series with the first coil of the hybrid transformer, I wonder:For the classical Cuk-converter setup, I should be able to put an air-core coil in series with the first of the ferrite-core coils, and a second air-core-coil in series with the second of the ferrite core coils. That way I can modify the magnetic coupling without actually drilling / cutting my ferrite core?But is that “linear”, like if I use 1 mH air-core coil(s) in series with a 1 mH ferrite core coil(s), will that (roughly) be K = 0.5?As it seems as nobody knows whether the K=0.7 for "minimum ripples" equals maximum efficiency, "giving it a try" seems the only option. But I’m quite limited with my equipment (and granted, also with my knowledge). Also: Does the duty cycle have an impact on the optimum K value for ripples? Or is there yet another parameter to investigate?But besides questions perhaps I can actually contribute something: The funny control mechanism.Allow me a little bit of background-considerations:If I “switch on” a cuk-converter while there’s still a current in the first inductor, that current goes to ground and is wasted. Thus a Cuk-converter must operate "discontinuously" (I borrow this term from the Buck-converter, where it describes whether or not the current in the inductor drops to 0).Now all control-ICs I encountered so far (again granted, not too many) use a fixed switching frequency and modify the duty cycle therein. So in order to use these ICs for a Cuk, I have to make sure that the whole duration of charging and discharging the first inductor fits in one period of the frequency. Especially when going from high input voltage to low output-voltage, this results in long discharge-phases, leaving only a very small range of Duty-cycle variation useable. Seems like these control ICs are made for Buck and also “can be used” for a Cuk. Anyway, this means that there must be some idle time reserve within each "control-IC-period", to accommodate for the variable lengths of the "cuk-period" (=Duration of charging and discharging of the first inductor), which of course costs efficiency.The funny thing is now: Actually the “Cuk-periode” is "controlled" by the Cuk-converter itself. All I need to do is to watch for the voltage drop after the first inductor / in front of the switch - and turn on the switch again.So basically I define the output voltage I want and adjust the charging time of the inductor based on whether the output voltage is too high/low. Then I switch off and wait for the “0-Trigger-Signal” from the circuit to start the next cycle. Control the power transfer is done essentially by varying the switching frequency.Does anyone has experience with this approach?(This approach might also be interesting for the Cuk-Buk-2. At least the early layouts seems to control the lengths of one switching phase, but the second part of the cycle seems to require “½ period of the oscillation” to complete...)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 01, 2020, 11:20:26 pm
content: Quote from: Calderan on June 01, 2020, 07:53:17 pmYes, you can add explicit leakage to decrease k. Assuming the new inductors are not themselves coupled, of course!I don't remember offhand if adding equal amounts results in k = 0.5. There's a sqrt(1-k^2) and I always forget which way it goes...Anyway, the formulas are here:https://en.wikipedia.org/wiki/Leakage_inductanceQuoteI've had fine success with very high k; last one I made, I used enameled wire in star quadrifilar arrangement, i.e., connecting opposite pairs in parallel. This gave something like, 20uH dual inductor with 5nH of leakage, basically fuck all, measurement error.A lot harder to get windups like that off-the-shelf, but I think for that reason (production cost), they prefer staying on the looser side, k < 0.98 say, which is enough that with some big ceramic caps on both ends of the choke, you can keep resonance below Fsw (which can be pretty high with modern composite power and ferrite core materials), and it's just as fine.I don't see that such a low k is necessary, 0.7 or less. It's harder to wind. You might as well go plain, uncoupled, beat the piss out of the coupling caps kinda thing, if you want to go that way?...QuoteErm, in what way is that current wasted? And, in what way is that any different from any other converter in CCM vs. DCM?QuoteYeah, constant-on-time variable-frequency controls are old and uninteresting. They don't offer high efficiency and the EMI is terrible.Namely: you're always switching at full current, and the EMI is generated at pretty much any frequency, from impulsive (very light or no load) to full frequency. Awful input and output ripple, impossible to filter; you'd actually need an LDO to clean that up, for anything that needs cleaner power.Being that they're old, you have found a key, early insight though -- it's the inductor current that matters, so we should control on-time and off-time to control that in turn. Voltage* can be regulated by controlling that current. (In this case, controlling its average based on pulse frequency, or perhaps varying the current too so we can get a bit more dynamic range out of it, before we have to vary the frequency too.)*Or whatever else we're regulating, output current (LED supply, welder?), or input voltage (UVLO?) or current (SSPC?) or power (MPPT?), etc. If you can get a feedback signal from it, you can wrap an error amp around it!So, in CCM (continuous conduction mode), inductor current is conserved. It only resets to zero when allowed to fully discharge, and that's DCM (discontinuous). If we switch on early, the current starts where it was, and charges up from there. That means we don't need as much on-time, and while the input ripple (for a buck topology) has 100% ripple (it's going full on and off), the output ripple can be lower (a modest percentage of the DC value), which saves greatly on inductor and capacitor ratings, and improves efficiency.The fact that current is conserved, makes trouble for us again: we need to take account of what the initial current is, in order to decide when to turn off the switch.Well, in a peak current mode controller, we don't worry about that at all, we just monitor the switch current instantaneously, and turn off when it reaches a setpoint. Done and done, right? Well, almost -- this actually leads tochaotic behavior, interestingly enough!* That's not a bad thing, by itself (a characteristic of chaos is that its evolution is bounded, i.e., it probably won't runaway and explode), but it's annoying enough that we should find a way to deal with it.*The technical explanation behind the chaos is: the system is equivalent to the Logistic Map -- a simple iterated function, which acts to fold its input back on itself, depending on an adjustable scaling parameter. As the parameter goes up, the iterated values first converge to a single, stable value; but at a certain point, suddenly it starts bouncing between two values -- bifurcation. Then quickly 4, 8, and pretty soon it's a jumbled mess. In the peak current mode control, the average DC current is the scaling parameter, and the pulse widths are jumping around chaotically.A typical real-world consequence of this behavior is the converter emits a hissing sound, and its ripple is somewhat higher. It's not generally a hazardous condition for the converter itself; the switch current is always carefully constrained, and it's only switching once per cycle -- power dissipation isn't increased much.Anyway, the usual mitigation is to include some PWM ramp in the comparator decision (slope compensation), which impairs the current limiting behavior of the control (now peak switch current depends on how long it's been on for), but allows stable operation down to modest ripple fractions (say 30-50%, i.e., Ip-p is about half of Idc).For lower ripple ratios, still another control method looks attractive. Average current mode control, is simply monitoring the average current (i.e., at a bandwidth somewhat below Fsw, so we don't mind very much at all the exact instantaneous current, or switching time), and controlling that as desired. This is safe because, with a very large inductor (very small ripple fraction), it takes many cycles for current to build up to nominal maximum anyway. Even under short-circuit conditions. We just control that as needed, then change the current setpoint as needed for our overall process control (output voltage or whatever).Here's a typical average-current-mode control, in a boost topology:Control:https://www.seventransistorlabs.com/Images/Flashlight2_Schematic.pngPower:https://www.seventransistorlabs.com/Images/Flashlight2Sch.pngPower source is a 18650 cell, so low-side current sensing is used. (If a common-ground source and load were required, the current sense might have to be on the inductor instead, probably requiring a differential sense amplifier, or an isolated sensor like a Hall-effect type.) As it happens, the control implements primarily a regulated output DC current, but also implements DC voltage regulation (to limit voltage in case of load failure), UVLO (undervoltage lockout: output throttles down when input voltage drops below 3.2V) and temperature limiting (output throttles down when load temperature rises above, oh I don't know, 60 or 80°C, something like that?).Note that it may be desirable to have these limits as latched faults instead (i.e., the supply stops abruptly when an extreme condition is detected), which may be hard latched (have to press a button or power cycle it?) or may be self-resetting. "Hiccup" type overload protection is an example of this, common in offline supplies.I also have an even more topical example of such a system:https://www.seventransistorlabs.com/Images/CukHack5.jpgThis is the same controller, but modified to interface with a Ćuk output stage; I don't have the exact schematic handy so I'll just show it off rather than discuss it in detail. (This is, in fact, the coupled inductor I mentioned above.) It's a pretty small module, handheld size, and delivers up to 20V at 20A.Heh, hmm, this was before I tested it at full load -- note the yellow 20A fuse and 12V battery. It's actually 30A input current at limit.Tim

author: Calderan
author_rank: Newbie
timestamp: June 10, 2020, 07:42:49 pm
content: Took me a while to reply. Teslacoil gave a lot of info to process - and I’m by far not through with it. A busy full time job just does not really help to speed things up.For technical reasons let me quickly summarize the Buck-converter:If the current through the inductance keeps flowing during the whole duty cycle, the converter is in continuous mode. If the current in the inductance comes to 0, the converter operates in discontinuous mode. This depends on some parameters as power-load, inductivity and operating frequency. There’s a nice summary about this here:http://www.ti.com/lit/ds/symlink/lm3578a.pdf(see fig 29, ff).Now on and off-phases need to be balanced, but as in the end all current charges the output capacitator, too much current will rise the output voltage and the control logic regulates down (direct feedback).For a Cuk-Converter (classical setup) I found no such info. So I did some own evaluations. (Maybe basics for an expert, but hey, I try to understand.)Current and energy in an inductance and power-transfer through the inductance (see attached graphs):I took a look at the standard formulas for time-dependent current and energy in an inductance (all parameters = 1). “E/t” has a (rather broad) maximum at 1.256 tau, not surprising as current is limited, charging time not.=> There is a power maximum. Charging for more than 1.256 tau decreases power transfer.But “left” of this maximum the transferred power is proportional to the charging time.Using this observation, I don’t sense any inductance current at all – I simply increase or decrease the charging time, based on output voltage. (With this setup I have not yet observed chaotic behavior, but sometimes quite regular oscillations of the operation frequency when using very large inductances (12or 16 mH). May well be a plain simple internal oscillation, I just could not investigate it yet.)As the charged energy needs to be transferred from the inductance to the capacitator, which takes an additional time proportional to this energy, the actual power output is lower than that. Now, ratio of “d” and “1-d” for a cuk-converter can be calculated with in- and output voltages, thus f(operation) >= d/(1.256*tau). Due to the broad maximum, the factor 1.256 is not that critical (use “1” instead) and then the formula looks quite similar to the “inverter”-formulas in above link, fig. 29. So it might actually be correct.However, an increase of duty cycle (resp. the on-time) as response to increasing load causes a voltage-drop at the output side (for a cuk, negative output voltage will go -> 0, so actually it’s an increase) if the operating frequency is too low and the charging time goes over the maximum, thus decreasing the power transfer.Anyway, if the duration of an operation cycle is kept below the threshold, then:One can control the on-time of the cuk-switch. With increasing power-demand, the on time will increase followed by an adequate off-time and, the rest of the cycle time is then idle. As soon as the off-time does not fit anymore inside the operation cycle the power in the inductance will not drop to 0. The lack in energy transfer will cause the output voltage to “go towards 0” and also a current remains in the inductance. But the controller will react to the output voltage “drop” with further increase of the on-time. Thus the inductance goes into continuous mode. The increasingly slow charging may actually stabilize the current below the limit defined by ohmic resistance. However, this only balances the first half of the Cuk-converter, but still not the output.This self-stabilizing current in the inductance actually allows for a second, theoretical mode to operate a cuk-converter: control the off time. Switch is always on, current is at the ohmic maximum and increasing power-demand is addressed by increasing off-times that transfers the energy from the inductance into the transfer capacitator and thus to the output side. Once again, as the ration between on and off is fixed, a too long off time will leave to little time to recharge the inductance, causing again a drop of the output voltage.Now running a high current through an inductance, resp its ohmic resistance to ground does waste energy. Also current increase over time is fastest when starting with I=0. Therefore the discontinuous mode seems the only reasonable approach to operate a cuk. But in order to minimize the idle time, the setting should be at the “border to continuous mode”.=> The on-time must be restricted in a way that the complete “cuk-cycle” fits into the time frame defined by the control logic.This makes the regulation of a cuk more demanding than e.g. a Buck. Key problem is, that a Cuk-converter actually uses three steps, charge energy in the inductance, transfer this energy into the transfer capacitator and transfer the energy to the output capacitator. Steps 1 and 3 occur during the on-phase, step 2 during the off phase. This seems to be an inherent disadvantage of the Cuk-Converter, as by design it has a slower energy transfer due to the mandatory off-phase of defined lengths.Also the boost and the inverter require 2 steps, thus share the inherent need for tighter control and thus share the same two stumbling blocks with respect to RHP zero.With respect to JohnG’s remark about power density, I wonder how all this translates to power density. Besides the controller and some “other parts”, a Buck requires one inductance and one capacitator. A Cuk-Converter needs two inductances and two capacitators (besides the “common rest”). And a Cuk needs longer to pass on the same amount of energy.Soft switching:If I get this right, turning on a current-free inductance is soft switching. Then switching on in discontinuous mode is “soft”, while switching on in continuous mode would be “hard”.Switching off is “hard” in both cases, DCM and CCM (but worse in CCM).Seems like a plus for DCM.However when it comes to low load, the switching frequency will increase and thus increase switching losses.Variable input voltageIf you have to deal with a varying input voltage, the whole thing gets even messier, as the “d-value” is not fixed anymore. Therefore I still think the frequency-controlled approach is a nice idea: You limit the on-time to “tau” and thus can use the full range in between to control the power transfer. The off-phase can take as long as it wants and the voltage drop at the end triggers the next cycle. (That’s maybe the actual trick: I watch the output voltage and control only the very first input parameter, on-time. Everything else I leave to the circuit and only go to the next cycle when I get the circuit’s “ready”-signal in form of the voltage drop.)This also avoids the high frequency oscillations (ringing?) that usually occur when the idle-time begins, see attachment "100us_5V" (red=control, yellow= voltage in front of the switch wkith lots of noise and an internal oscillation) compared to "50us_5V".(1. Yes, one could snub the noise away, 2. I’m aware that a voltage drop not necessarily means I=0 / soft swithcing. But if needed the timing should be "adjustable", 3. The high initial peak when switching off comes from some rather long lab-cables in my setup.)I also tested the air core / ferrite “hybrid” inductances.For comparison I used non-interacting 2 mH inductances (best result), two 2 mH coils on the same ferrite core (worst). Surprisingly the air-core / ferrite hybrid with an estimated K = 0.5 was better than both coils on the same ferrite, but still worse than the magnetically separated inductances.So either “max. efficiency” is absolutely not the same as “ripple free” – or there is a dependency on the duty cycle.I will need to take a second, closer look into this.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 10, 2020, 09:51:31 pm
content: Ah, I see -- your R/L time constants are all wrong. You never operate anywhere near the time constant, because that means massive dissipation in the inductor -- exactly as you say, energy storage peaks at some point, and at that point the efficiency of the converter will be something like 50% (or maybe it's 1/e or something, I don't know).So we use good quality inductors, and a high enough switching frequency, that the charging slope is a very small fragment of the exponential, so small that we can quite accurately (within some percent) model it as a linear ramp instead, and we can use the fundamental inductance equation to relate pulse times and voltages to current change:V = L ΔI / ΔtNamely, when the switch is on, V == V_in, and when the switch is off, V == V_out (which is negative in the Ćuk so no need to mind signs here).Note that for current to remain constant over a cycle, the fluxes must be equal:V_in * t_on + V_out * t_off = 0The controller only needs to make small adjustments to this balance condition, to compensate for losses, and to insert unbalanced flux so as to control the mean current to meet load demand. (Which is why compensation is different in CCM vs. DCM: the control loop gain is higher and the time constant is longer.)Behavior around the switch is the same as a flyback converter, or the inverse of an inverting buck-boost converter (i.e., a buck-boost with low side switch). (Which, because the circuit is rotationally symmetrical, is true of the diode as well.)Don't worry about the currents in the two inductors; treat them as one. Don't worry about the voltage on the coupling or bypass capacitors, assume they are constant. Understand the switching waveforms first!Later, we can separate the inductor currents, and include the capacitor voltages. We can then prove that they are a loosely coupled mode. That is, the currents in the two inductors can be separated into common and differential mode currents, and the common mode delivers switching energy while the differential mode is parasitic and can be made negligible.Tim

author: Calderan
author_rank: Newbie
timestamp: June 25, 2020, 06:09:51 pm
content: Once again it took a while...Still with reference to your second-to-last post, I finally managed to take a closer look at the layout you provided – and especially the elaborated control-inputs gave me some inspiration. Also the two coils on one ferrite core is interesting for its compactness. How much "noise" does this make with real load?Independent of the efficiency issue in comparison to air-core coils (see below), ferrite cores have here a little disadvantage, as they are quite "talkative", which is especially funny with "my" frequency-controlled approach.Magnetic dissipation: If I understand the info I found correctly, this is (sloppily spoken) the “magnetic equivalent” to the losses due to ohmic resistance, proportional to E, thus proportional to I^2.Now, the evaluation I did has the primary purpose to check for limits – and obviously there is one. (The first “do-not-cross”-border). I also stay well below this limit, after all my setup is on a bread-board, not on a toast-board.However: As the current is time-dependent, you can limit the maximum input current by timing, all without sensing. This of course does not replace an output-control (I like the idea of the hall sensor).Anyway (and now I hope again, that I did not miss anything), if losses (magnetic and/or electric) are proportional to E, then the actual current should be kept low, as losses increase by I^2. But if you need a certain power – and thus a certain input current – you hardly have any other option? One might use several converters in parallel, but with half the current (thus 25% of the Energy), one would need 4 converters in parallel for the same output in terms of “E”. So in summary the same loss, though distributed onto considerably more material.Sloppily: You lose a certain percentage of the energy you transfer. No matter what.Linearity and controller:Here you really gave me something to think / calculateTerms: t = on-time, tau = L/R (“that time constant”),d and (1-d) = duty cycle (in percent),TD = Duration of Dutycycle, TO = Duration of a switching cycle.Let’s say your controller adjusts the duty cycle between 5 and 50%, then the range covers a factor 10.If t << tau, then the current is proportional to t^2. Here simply the limiting e-function does not yet kick in, so it’s all in the range driven by the “^2”-part. If t/tau = ln 2 = 0.693 then the curve is roughly linear (~ t^1) and “later” it gets roughly time-INdependent (~ t^0).But let’s keep the maximum t = 1 tau because of the “first limit” from above. Then at the lower end of the duty cycle you will be around t/tau = 0.1, thus basically covering "everything". Only for short t, you definitively remain in the “^2”-domain. So really not linear.And it gets worse. Funnily I never noticed any hint that somebody takes any care of "TO" – even if everybody lists the energy-conservation-equation. Basically, the energy you store during “d” needs to be transferred to the output, which takes an exact “1-d”, both together taking a certain time, TD = t/d. As lined out before, when TO < TD then the inductance cannot discharge completely, current in the first inductance goes up, creating unnecessary losses (second “do-not-cross”-border). So TO must be > TD for maximum t.The other way round, this means there’s always some idle time in each TO, especially for short t. With increasing t, the idle time decreases, increasing the energy transfer by a factor of “t/TO”.So the energy-transfer is anywhere between t^2 * t/TO and 1* t/TO, thus between ~t^3 and ~t. Which actually is no issue, because as long as the curve is monotonous, all you need to do is increase or decrease t in order to increase or decrease E. Stick to the "first limit" and you get a decisive victory for the analog circuits.However, in order to avoid over-regulation and oscillations it might be interesting to get out of the t^3-range, as a very steep inclination of the curve might make regulation overly sensitive.With the frequency-control-approach, t/TO remains constant. This might actually be an advantage I did not realize before, as it removes one power of t from the equation, what may allow for smoother control.But let’s check that:As I use short t, E should be ~t^2, resp. P(out) ~ t, thus linear.I never really did any measurements in this respect (like long series with MANY points), but I have some data, see attachment.X-axis is P(out), y-axis is t. The light blue curve (2nd from bottom) are isolated air-core coils (K=0), the orange curve at the bottom are anti-parallel air-core coils (K<0.5???) and the yellow curve at the top are anti-parallel coils on the same ferrite core (K=1).I always was aware that “K” makes a difference, but until now I never actually quantified it…There’s nothing above good discussions!

author: nardev
author_rank: Unknown
timestamp: October 28, 2020, 02:28:58 am
content: Quote from: coppercone2 on August 28, 2018, 11:36:49 amI just watched his interview in Serbian, he actually explained the process of naming it. the techical name that he gave to the converted was something else, descriptive and long, so his professor at a time at Caltech University said, let's naem it simply "Ćuk Converter".

author: jonpaul
author_rank: Super Contributor
timestamp: October 28, 2020, 06:25:36 am
content: Hello I met Dr Solbodan Cuk and his collaborator Dr Middlebrook at Cal Tech in Pasadena, late 1970s.Very fine innovation, but the practical designs have some downsides, vs conventional topoligies.Also the need to a licencing fee during the patent term of 17 years, meant that Cuk-Buck topology is not widely used.Best regards,Jon

author: Le_Bassiste
author_rank: Frequent Contributor
timestamp: October 28, 2020, 07:02:14 am
content: FWIW,https://www.reddit.com/r/FPGA/comments/94ymor/hello_from_professor_slobodan_cuk/

author: JohnG
author_rank: Frequent Contributor
timestamp: October 28, 2020, 03:34:47 pm
content: Quote from: jonpaul on October 28, 2020, 06:25:36 amYes, interesting topology, but the RHP zero is a pain, ripple steering is also a pain in the real world. The middle capacitor takes quite a beating, and does not eliminate the need for input and output caps since there are not many applications where you want a high impedance input and output. All that weighed heavily against it's benefits. Once you added in licensing fees, it made no commercial sense. In addition to the fees, my understand is that he could be very difficult to work with (this based on folks I know who tried).Who knows, though? With digital controls, a lot of things that were once impractical have become a lot more practical recently.John

author: nardev
author_rank: Unknown
timestamp: October 29, 2020, 03:50:59 am
content: I'm wondering if he would be a good guest for an amp hour with Dave

author: koloko
author_rank: Newbie
timestamp: November 04, 2020, 11:24:08 am
content: Quote from: BBBbbb on August 28, 2018, 12:19:56 pmTrying to bait a few Croatians on the forum?

author: jonpaul
author_rank: Super Contributor
timestamp: November 04, 2020, 01:20:36 pm
content: Dr Cuk and his collaborator Dr Middlebrook ran the USC Pasadena Cal Tech Electronics dept for decades.Very bright guys but more academicians than practical designers.The classic topologies and some less well know ones do the job with simpler magnetics and capacitor requirements and no liscencing fees, flyback, buck, boost, foward and double foward.My old friend Nathan SOKAL (RIP) had much better luck with his amazing Call E RF converters, many IEEE papers and patents.I believe it was liscence fee free.Just the rambling of an old retired EE!Jon

author: 2N3055
author_rank: Super Contributor
timestamp: November 04, 2020, 01:22:17 pm
content: Quote from: koloko on November 04, 2020, 11:24:08 amAfter 2 years, and quite technical and smart stuff written in the post, that is only thing you have a comment on? Lol...And it is not even wrong..

author: nardev
author_rank: Unknown
timestamp: November 20, 2020, 05:02:13 am
content: Quote from: 2N3055 on November 04, 2020, 01:22:17 pmHajd sada se i ovdje poseri te jedni na druge.Kad moze Savfet beg Basagic biti Hrvat i Mehmed Mesa Selimovic biti SebinMoze i Tesla Bosanac