Merits/Pitfalls of LLC Resonant Converters & Other Conceptual Questions - Page 1
Forum section: electronics / general

author: TimNJ
author_rank: Super Contributor
timestamp: September 01, 2022, 05:04:53 pm
content: Hi everyone,I've worked in supporting roles for the development of various LLC resonant AC/DC power supplies, but never lead the design of one. As such, I've made many high level observations about their operation, through bench testing and measurement. But when trying to connect the theory to these observations, I've run into a few mental stumbling blocks. Maybe you can help me (and others).For the sake of discussion, I will call the lower resonant frequency 'f_res1' and the upper resonant frequency 'f_res2'.Question #1:If unity gain is achievable for all Q values (all loads) @ f_res2, and supposing that the conversion ratio of your design (input voltage, turns ratio, output voltage) allows you to operate exactly at M = 1...why does the switching frequency of LLC increase with light load? Or for that matter, why does the switching frequency vary at all, in practice?High level answer #1:If (2*n*Vout)/Vin is not actually equal to 1, the LLC must operate @ M ≠ 1 to compensate for this mismatch.Follow-up #1a:Is there a reason to operate to the left or right of f_res2 in a nominal condition (no line variation)? It seems reasonable to want to operate slightly above resonance (to the right of fres_2) to ensure that you stay squarely in the inductive region, even during heavy loading. So, if your resonant tank's actual fres_2 is 100KHz, and you want to operate above 100KHz, you could make (2*n*Vout)/Vin) > 1, which would require that the LLC works at M < 1 (and thus > 100KHz) to maintain the correct output voltage.----Question #2:So, if you are nominally operating to the right of f_res2, what's the point of using LLC anyhow? Doesn't this area generally resemble an series LC resonant converter?High level answer #2:An series LC resonant converter clearly has no ability for boost, which is an obvious disadvantage for line variation.Follow-up #2:I suppose, for an series LC resonant converter, if some "boost" functionality is wanted, the nominal M design point must be less than 1. But so what? Can't series LC maintain ZVS on the right side of its resonant frequency anyway? Is the full load to no load frequency spread greater? I suppose, even if ZVS is possible, the advantages of resonant conversion start to slip away, as the actual operating point moves farther and father away from resonance? Am I missing something about circulating currents, other other phenomenon that are not readily apparent in the frequency domain?...Well I guess I'll stop there for now.Some of my misconceptions I think would be cleared up if I was actually forced to do a design (and blow things up). But in the interim, these are some of the things which really make me scratch my head.If you have any other ideas about the merits/pitfalls of LLC, why it's gain so much popularity over previous resonant topologies, or really anything about LLC at all, I open your reply with open arms.Thanks!Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 01, 2022, 07:38:43 pm
content: I'm still a bit spotty myself, so joining to watch as well...So, take these comments as supposition, mostly:1: Yes; but when is it all perfectly balanced? If offline or battery powered, input varies over a nominal range; if PFC or other front end, input varies at least transiently (ripple + fluctuations). Output also varies, due to rectifier/output drops of course. And all the components vary, well maybe not the capacitor (any cap good enough will be very linear), but the inductors may vary slightly with amplitude; but more to the point, all the actual values will vary relative to their design intended values, to a similar end.Also, probably something about Vout(DC) not being exactly the ideal Vout(AC), whatever effect the harmonics have on that. (The harmonics don't go anywhere of course, being well above cutoff; but the mixing between fundamental and harmonics might affect this. I don't know enough about harmonic balance simulations to have a feel for this, hmm.)1a: staying about Fr2 is probably a good idea (it should never* go into capacitive phase), but I think it's uncommon that the inductance and gain ratios work out just right for this to be viable?*Up to nominal that is; overload/short will differ of course!2: Significantly above Fr2, at least, indeed it resembles a series L (we can even ignore C for that matter), with gain ratio given by turns and inductance ratios. At least with a transformer, we can just use a lower ratio to compensate; for an LLC buck*, we'd have little choice though.*Does anyone use this? Offhand, I see QR buck is a thing, but not LLC in the most directly analogous way. LLC should still work with half-wave output rectification, though it'll obviously be better with full-wave (so, a CT-grounded choke, but still short of a full-isolated transformer). Well, maybe not so obviously, since the Q is/can be so low that the load being absent during half a cycle has pretty significant implications. Eh, more things to ponder.And, in this region (LC or just L), gain is pretty flat, so the frequency change per required shift in gain is pretty considerable. I've seen frequency get pretty high during startup (>2x nominal!).Most of the controllers I've seen, seem to expect a rather generous current limit setting, or mask it off during startup; probably, startup just isn't a very happy process in general, and it's just something you grit your teeth and hope for the best. At least you usually have oversized transistors (to keep conduction loss down, since switching loss hopefully isn't a problem).Tim

author: Faringdon
author_rank: Super Contributor
timestamp: September 01, 2022, 08:18:12 pm
content: There is a EEVBLOG post on this i started some months back which showed series diode being used to make LLC safe even if it ever strays into cap region.I cant find the post now.It is not possible to be sure any given LLC cannot be strayed into capacitive region.....Thats why Infineon have a digital LLC which is supposed to detect onset of leading current before it actually happens so ruinous reverse recovery never actually happens.(!!!)......So either...1....nanny your LLC so it never strays into cap region, or if it does, and blows up, you dont care. (this is the way i believe most LLCs are designed, and the designer often just cuffs it)2...Use infineon method.3...Shovel a series diode in there.Also, when designing an LLC....make out the Vout vs frequency graph. The equation to get it is in fairchild app note AN-4151.When you make out that graph, things become obvious to youThe LLC folders in here say some more on it......SMPS course_little foldershttps://drive.google.com/file/d/1WvElhDNfHI0EBei4rouSNEdWWTO1oCtJ/view?usp=sharingYes you generally serve most loads at your upper res freq if designed right...but at very light load its just not possible......you can vary the Ls and Cs to get it to serve many loads with one similar frequency range....when you make out the graph as said, this becomes obvious to you.The folder i attach has the graph done for you "llc design template"AYK, LLC with half bridge...the LLC only sees half the actual vin.Also, when switching at the upper res freq, the LLC takes on the vout/vin = np/ns behaviour, since voltage across L and C become equal and opposite and cancel.

author: jonpaul
author_rank: Super Contributor
timestamp: September 01, 2022, 08:41:47 pm
content: Tim, the topic of resonant converters, soft switching and control technique has been well covered in textbooks, seminars and conferences, see IEEE power electronics and industrial applications proceedings, PESC, PELS, Nuremberg, PCIM and othersTwo old friends work can be mentionedDr Nathan Sokol, inventor of class E convertersDr Richard REDL, expert on these topics.After you absorb the theory you can understand the design choices, control techniques and topologies.bon courageJon

author: Faringdon
author_rank: Super Contributor
timestamp: September 01, 2022, 09:05:25 pm
content: Thanks, the "series diode" method that i spoke of, which gets LLC out of the mire, is not spoken of in any text or published document on LLC.....as such, please assist me in finding this post, its from only a few months back. It will help TimNJ out.I hope also that you find the "LLC design template" that i made.I find generally with SMPS, because its all been outsourced to the Far East, its not researched much now in the West.And what has been done, is kept an industrial secret, hence we love the forum.

author: TimNJ
author_rank: Super Contributor
timestamp: September 01, 2022, 11:00:59 pm
content: Quote from: T3sl4co1l on September 01, 2022, 07:38:43 pmThanks.Rectifier and output drops. Yes, that one is easy to understand. Vout is really Vout + Vd, where Vd depends on load current, so this variation would indeed cause a shift in the operating point on the M vs. f curve.I guess, in general, the handful of LLC converters I've worked on tend to work very close to fres_2 for let's say 50-100% load, and for lighter loads (let's say 25-50%) load, the frequency may shift up by 10-20% or so. (Below 25% load, let's assume it operates in burst mode.) If I recall correctly, some operated above f_res2 and some below f_res2. The decision making on where to operate each design seemed very much case-specific.Take for instance, in "above resonance" operation (to the right of f_res2), secondary rectifiers are hard commutated, to a varying degree depending on how far away from f_res2 you go. So, that's usually no good, since lack of ZCS on those will give you some overshoot which you will have to take care of by using higher voltage MOSFETs/diodes or a dissipative snubber. And, at light load, based on the shape of the curves above f_res2, it seems the frequency could go up quite a bit.On the other hand, go too far "below resonance" (to the left of f_res2) and the RMS rectifier currents will increase as they begin to take on a DCM current waveform. And, seems you have less ability to provide boost when operating in this region. So, if you have a hold-up time requirement, you may suffer if you have a 400V nominal design which is totally useless at -10% on Vbus.So, close to resonance still is logically the right choice. Which side is "better" perhaps depends on...a lot of things. (Waving my hands here.) But, all things considered, a little to the left of f_res2 seems overall like a good choice.Quote from: T3sl4co1l on September 01, 2022, 07:38:43 pmRegardingQuote from: Faringdon on September 01, 2022, 08:18:12 pmTo the point of capacitive region operation (and the fears of operating therein), I would say, with primitive control techniques, you need to be cautious, so much so that you might want to stay amply to the right of f_res2 for all loads, including overload. But, I am not really sure if the fear of capacitive mode operation isreallysuch a big deal, provided you are using a modern controller. Most (I believe) sense the phase of the resonant current, compare it to the instantaneous phase of the half-bridge, and can make a decision about whether operation is approaching the capacitive mode. Output is quasi-power limited. So, pretty much like you said, Faringdon.Point being, there's probably very little risk of destruction with modern control. Relying on this safety measure, you can more confidently move to the left of f_res2, achieve primary-side ZVS and secondary-side ZCS.Anyway, thanks for the discussion anyone. It is helping me think through some of this in a logical way.

author: TimNJ
author_rank: Super Contributor
timestamp: September 01, 2022, 11:28:10 pm
content: Quote from: jonpaul on September 01, 2022, 08:41:47 pmI come to the forum for theexact purposeof better understanding the theory. Of course, there are countless resources on the subject. Asking directed questions on forums is means to supplement and better understand what is already published.

author: Faringdon
author_rank: Super Contributor
timestamp: September 02, 2022, 08:45:26 am
content: QuoteAYK, in that situation, they are not hard commutated if you use a single sec coil with full bridge rect.This was discussed before and i can send the LTspice sim if you wish to see it.But yes, a well known semi-co has an app note (erroneously) saying that above res operation leads to hard commutation on the sec rects, but it isnt the case with single coil sec used with FWB sec rect.Above res operation does arise a situation when you have a split coidl secondary, and the 2 secondaries are inevitably not perfectly coupled with each other and the primary.QuoteThe ICE2HS01G doesnt do this, but i assume you are referring to micrcontroller based controllers?.....and the Infineon digital controllers.QuoteAYK, series LC resonant controller is a form of LLC...its just an LLC with high magnetising inductance....when you are using the template i spoke of, you can adjust the mag current and see how it affects the vout vs freq graph........and usually, i always find i need to reduce LMag to get a good performance at max load.........i often find i need to add an external Lmag.........but if that is done, then the txfmr needs to be sandwich wound, to reduce its leakage L. But yes, i always look to reduce iMag as far as possible.QuoteAYK, you sometimes find, due to the whole number nature of txformer windings, and the fact you cant get "5.3nF" caps etc, that you often end up away from the f_res2...you just settle to get fairly near it.Regarding line variation and having to serve it...yes, you always end up needing more iMag to serve it properly, and you see this when you play with the template.QuoteYes thats true, but AYK for a well designed LLC its near fres2 for most of the load range. At freq >> fres2, the pri current has not had chance to get "curled back down" so you get turn-off switching losses from this...having said that, the current is lower anyway when you are in lighter load so its not too bad.One good thing about operating at >fres2 is that your magnetising current peak is lower, and thus so too the Bpk.....so less core losses......well....slightly less, because core losses also increase with frequency.....but increase slightly more with increasing B than increasing f.If you do operate >>fres2, then you have to ensure dead time is big enough for the fet diode to get rurned on before FET_ON.....since the magnetising peak is lower and its the imag that gets used to slew the bridge node (FET capacitances)Ref cap mode operation, i think if any LLC is suddenly output_shorted, then it will end up in the ruinous capacitive mode, even if only for a short time. (Looking on the template and changing load to some extremely low value)The thing with capacitive mode, AYK, is that the severe reverse recovery that happens, may be enough to kill the fets, even if it only happens literally once....or it could weaken them.The thing with resonant converters, is that they are all "current sloshers"....they all slosh current around so the fet diode gets turned on before the fet.....and the thing with this, is that its wonderful so long as it doesnt all get out of sync...because when it does, the reverse recovery can then be extremely severe......because you can have a lot of current flow in that diode when it suddenly gets 400v shoved across it.......so, which kind of makes sense.......AYK, theres no such thing as a free lunch in SMPS, and the advantage of resonant op, is tempered by this terrible rev rec possibility.Think of reverse recovery in Boost PFCs, and how everybody is running to sic to get away from rev rec, or putting in all sorts of intricate cctry to get round it......and in a boost pfc, they slowly commutate the fet to reduce rev rec.Adding a series diode totally removes the possibility of rev rec in LLC......so then you can use a much lower rdson FET...since the FET VDS doesnt have to get high enough to properly recover the internal diode while fet is conducting...so the efficiency loss gets made up for, in that you can use a lower rdson fet....AYK, if you do use a series diode, then you need an overall parallel diode , aswell, which is Sic.Put a 1kW Full bridge and "standard" 1kW LLC on a repetitive short cct test rig...i garantee the full bridge will last longer.Gate drive:One of the great things about LLC, is that its very conducive to pulse txfmr gate drive...with none of the problems with sudden duty cycle change that you can get with other topolgys' gate drives. As you know, if pulse txfmr suddenly stops pulsing, then it can actually turn the fet on spuriously as the series cap discharges down. I have sims of this if anyone wants....thsi doesnt tend to happen with LLC...or rather, you can use the type of pulse txmr that doesnt have this problem, when you use LLC.Below fres2 operation:Remember with LLC, there is the dead time......so if your fet on time is 5us, then thats a 100khz operation......but you will be switching at <100khz, as the dead time has to be fitted in aswell. As you know, dead time is extremely important with LLC...and some people have micros to adjust dead time with loading.Severe reverse recovery:https://www.infineon.com/dgdl/Infineon-Evaluationboard_EVAL_3KW_2LLC_C7_20-ApplicationNotes-v01_01-EN.pdf?fileId=5546d462580663ef01582eb629b70118...the above tells that if you are not doing the digital signal processing effort described there, where they detect onset of reverse recovery before it happens, and then avoid switching on the fets, then you are liable to experience severe reverse recovery of LLC and dead LLC. They use their XMC4400 DSP to do the detection. AYK, they woudlnt bother with all that effort if it wasnt necessary. The only other way to avoid it is to use a series diode. Most LLC's , IMHO, just cuff it and hope they dont get those short circuit situations etc, that cause the dead LLC....or if it happens at startup, they hope the PSU only gets switched on once every year, etc etcMost likely, LLC's are mostly designed with this design flaw within them, but most of the time the use is such that they get away with it.........it costs money to make the LLC solid, and why bother spending that of you can handle a small failure rate, in those few that do get exposed to those x rated conditions.All "current slosher" power supplies are the same in this respect....PSFB, "Half bridge with 50% drive and leakage inductance only", etc etc.If you are providing for a server room say......what does it matter, there are always two PSU's in the rack...so the other one just kicks in till the duff is replaced.The danger is internally denying that the danger exists,