Do not waste energy in flyback converter snubber circuit, possible? - Page 1
Forum section: electronics / beginners

author: LinuxHata
author_rank: Frequent Contributor
timestamp: July 23, 2022, 05:58:23 am
content: Hello.Say we have flyback boost converter with transformer, say, using LM2588. As in datasheet, there are HER102 and P6KE30 in parallel with input winding, and they get warm noticeably, which means, there's some energy lost in them. So maybe there is some clever way of joining coils in a way, when that negative voltage in coil, instead of being wasted, is being added to output power?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 23, 2022, 07:37:04 am
content: Sometimes, a quasi-resonant snubber can be used to return that energy to the supply.They have many restrictions, though, such as: increased peak voltage and current seen by the switch (more switching area is required, but in return, you get higher efficiency?); they're hard to build with low enough strays to be useful, sometimes; and component parasitics have outsized impact on its operation (making them more useful at lower frequencies).As example, consider a flyback running at 200kHz; with 5us per cycle and around 2.5us on-time, you probably have less than 1us to handle the turn-off rise, or turn-off peak, or turn-on reset transients. Which means the QR snubber network needs good quality for operation at several MHz just to begin with.And that's quality in terms of not just Q factor, but also freedom from parasitics by a reasonable margin; say >= 10x higher stray SRFs.But you might have diode reverse recovery on the same order (100s ns), or if you use schottky, its capacitance might be on the order of the intended capacitor(s) in the network. Whereas to meet the 10x stray criteria, you'd want Cjo < C/10, kind of thing.If you're hand/custom winding the transformer, it may be easier to simply improve the winding (get better coupling between P/S, use more interleaved layers, or use twisted pairs or other transmission line design techniques). If not, it may be better to change to a more tolerant topology -- this is essentially why active-clamp flyback exists, but also forward (especially 2-switch) is very good at this. Or go with a half-bridge type like forward or LLC resonant. (Probably not worthwhile at this power level, but scales well at higher levels.)Tim

author: mag_therm
author_rank: Frequent Contributor
timestamp: July 23, 2022, 08:56:07 am
content: Looking at the LM2588 datasheet fig 27, (compared to Fig 24), there is some charge being returned to Cin2 depending on inductance to and in Cin2, and the actual voltage across Ds and Vz.(Vz+Vds) is higher than Vin which allows T1 core to run down to negative remanence for the next pulse. This reduces the core size of T1 and can increase the T1 efficiency.

author: Marco
author_rank: Super Contributor
timestamp: July 23, 2022, 03:14:03 pm
content: When I hear QR I think about the flyback circuit letting the secondary resonate and using that to switch the primary on the valley. QR switching is to my mind orthogonal to the snubber.The drop in regenerative snubber for flybacks is the Domb-Redl-Sokal snubber, you'd have to be careful with component choice and layout but it could probably be made to work with the LM2588.

author: Terry Bites
author_rank: Super Contributor
timestamp: July 23, 2022, 03:21:14 pm
content: This will interest you.http://ntur.lib.ntu.edu.tw/bitstream/246246/200704191001332/1/00654027.pdf

author: MrAl
author_rank: Super Contributor
timestamp: July 23, 2022, 10:53:53 pm
content: Quote from: LinuxHata on July 23, 2022, 05:58:23 amHi,One way to do this in principle is to try to divert some of the current back into the DC supply, not to the output directly.This can be done with a diode in some cases, but in other cases you may have to use a transformer to boost the voltage being clamped before you rectify it and inject it back into the DC power buss. If you wound the transformer yourself though you may be able to add another winding to work as the snubber winding, which would be used for snubbing as well as diverting some of the power back into the DC buss.Keep in mind that a snubber is usually there to protect the transistor(s) before the reverse diode can conduct. It's meant to be a very fast circuit so it has to be wired in close. That means in some cases even if you use a transformer you may need another smaller snubber to act as a super fast snubber as well as the higher current main snubber.Please tell us whatever you decide to do after you figure it out.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 23, 2022, 11:32:22 pm
content: Basically, leakage inductance acts in series between otherwise isolated primary and secondary inductances; when primary turns off, its magnetizing inductance discharges into the secondary through the leakage, thus the primary side overshoots. Or likewise during CCM turn-on.It would be convenient if that apparent series inductance were simply a component, that we could slap another winding on and dump it into the supply no problem. Unfortunately, leakage occurs precisely in the narrow gap between primary and secondary wires -- or indeed within them, to the extent permitted by skin effect. (Which is why litz cable has much higher inductance per length than solid: it is transparent to magnetic fields, which is exactly equivalent to saying it has evenly distributed current flow*. Keep this in mind when you need a high current, low inductance winding!)*Well, more precisely, it's that current flow isn't dictated by field distribution. It could be nonuniform of course, but this would be achieved by particular arrangement of the strands, or driving them with a current distribution (which will happen e.g. in short lengths, where lengths/positions/twists aren't matched evenly between strands).Wecanuse an auxiliary winding on the 1-switch forward converter, but that's for catching the primary magnetizing inductance, which carries much less current than flyback. There's still pri/sec leakage (carrying full load current at turn-off) that needs to be clamped (or switched slowly enough). To the extent pri/aux are tighter coupled than pri/sec, at least a little can be handled this way, but otherwise, pri still needs snubbing or whatever.Similarly, there are some QR/passive lossless snubbers that use tapped or multiple windings, which turn out largely impractical because the coupling factor simply can't be that high -- or the stray capacitance low enough, in the case of high winding ratios.Oh yeah, here's another reference:https://www.ee.bgu.ac.il/~pel/pdf-files/smnr13.pdfTim

author: David Hess
author_rank: Super Contributor
timestamp: July 24, 2022, 01:10:57 am
content: Tektronix did it in their blocking oscillator in the 465M and 455 oscilloscope. The inverter had to be made more efficient to operate with passive cooling.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 24, 2022, 01:45:56 am
content: That's a resonant class E design -- of a sort. It's important to note the sinusoidal output, meaning stray capacitance dominates, meaning, switching hard into the transformer will dissipate excess power -- a large transient as the capacitance is charged relatively suddenly through leakage, so a peak of Vin / sqrt(LL/Cp) (LL and Cp being primary referred quantities), with ringing and subsequent losses in whatever during that phase.Here, they opted to stretch out that ringing phase, by adding their own leakage; this stores evenmoreenergy, but allows it to be dealt with in a modest time frame. And in that time, they charge it up (the output "speeds up" slightly as the equivalent circuit is T550 pri in parallel with L554; this isn't really shown on the pin 5 waveform), then discharge it, dumping the excess back into the supply -- "lossless"!So it's also basically an LLC converter, but half wave, and with much higher reactive power (high Q) -- which, note the narrow conduction angle, so the power factor kinda stinks (that's fine, they can afford extra here, in exchange for cleaner output, and it's not like it's big; it's what, a couple 10W or so?).Having feedback from both transformers is a nice touch. Q556 as a synchronous rectifier(!) for the "leakage" flyback is cool. (Shame about the diode drop heh (CR557), but doing all that AND having high Veb would be a lot to ask for.)And as a blocking oscillator, the bias current, and cap value, will be dimensioned to avoid squegging, and to basically adjust conduction angle. It's probably not a wide range design (in terms of input voltage or output current ranges), but good enough, clearly, for their purposes!Tim

author: Marco
author_rank: Super Contributor
timestamp: July 24, 2022, 02:41:51 am
content: Quote from: T3sl4co1l on July 23, 2022, 11:32:22 pmVery hard to read and doesn't give much clue how to calculate the capacitor and inductor for the snubber. This is the grand daddy of passive regenerative snubber papers :https://www.semanticscholar.org/paper/Nondissipative-turn-off-snubber-alleviates-power-Domb-Redl/0730cac48acee2b9ba7eb8ab93107605d89cd58dUnfortunately not available on the web, but here's one paper using on using that snubber and how to get some reasonable values for a given design :https://www.researchgate.net/publication/317724256_Designing_an_optimum_non-dissipative_LC_snubber_for_step-up_flyback_converters_in_DCMA snubber with an auxilary switch, two switch flyback and coupling the winding in the snubber with the transformer can all doubtlessly do better ... but for the simplest regenerative drop in replacement for a dissipative snubber it's hard to beat an inductor, a capacitor and two (Schottky) diodes.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 24, 2022, 03:51:12 am
content: I mean, relations will be similar to the first one; you're doing a resonant circuit at any given point of commutation, so just take the impedance and frequency of the relevant components. I agree, that particular paper is rather opaque though...The LDCD snubber, has the limitation that it only works best for Vpk = 2 Vin, or something like that; so, can be used on half bridge (except you already have clamp diodes, so just get ZVS say with an LLC and you're golden), and boost/flyback/buck in limited cases (Vout/Vin or vice versa close to this ratio).I think I've seen one on a PFC boost, on an ATX PSU before. With the average case being ~120V to 400V, it's notsooobad of a choice; evidently good enough to win whatever percentage points they needed (I believe it was an 80 Plus unit). (I bet they wished they had SiC schottkys back then...)Tim