Anyone used this non-dissipative snubber design for SMPS DC-DC etc? - Page 1
Forum section: electronics / projects

author: theleakydiode
author_rank: Regular Contributor
timestamp: November 22, 2022, 11:12:52 pm
content: I've found lots of papers about them online but can't find practical examples or people discussing actual circuits using them, has anyone here ever used this scheme? And if so are they generally a good substitute for lossy RCD snubbers or more complex active clamps?I assume the diodes themselves will need snubbers too.

author: boB
author_rank: Frequent Contributor
timestamp: November 22, 2022, 11:26:02 pm
content: Rudy Severns may explain this one in his free snubber design book.https://rudys.typepad.com/files/snubber-e-book-complete.pdfI have used that particular snubber on a flyback and it worked well.It returns the snubbed energy back to the input source rather than dissipating it in the discharging of the capacitor.boB

author: theleakydiode
author_rank: Regular Contributor
timestamp: November 22, 2022, 11:32:08 pm
content: Quote from: boB on November 22, 2022, 11:26:02 pmThanks for the link. What were some real world limitations you found whilst using it and did those diodes need any additional RC snubbers? I've found them to be quite ringy.But it's odd how they're not more widely used.

author: Marco
author_rank: Super Contributor
timestamp: November 24, 2022, 01:24:05 am
content: Can't you use Schottky's?

author: coppercone2
author_rank: Super Contributor
timestamp: November 24, 2022, 01:27:57 am
content: I accidentally saw a circuit like that kind of work in a simulation before

author: jonpaul
author_rank: Super Contributor
timestamp: November 24, 2022, 07:06:33 am
content: Rydy Severens was an old friend, very specialized in SMPS topology.Low dissipating snubber, by return of power to bus, a very old idea.See Vicor line of SMPS, patented by Vincerelli decades ago, basis of very efficient and compact DC-DC modules.Jon

author: boB
author_rank: Frequent Contributor
timestamp: November 24, 2022, 07:10:09 pm
content: The circuit shows Schottky's I think.Have to choose an approximately correct inductor as well I believe.boB

author: theleakydiode
author_rank: Regular Contributor
timestamp: November 26, 2022, 08:45:05 pm
content: Quote from: Marco on November 24, 2022, 01:24:05 amDoubt it in my case, there's a few hundred volts coming from the primary coil at turn off.Quote from: jonpaul on November 24, 2022, 07:06:33 amCool! Got any schematics you could share? And has he ever chatted with you about these snubbers? I've had a look in that pdf but this style of snubber isn't really covered in depth.Quote from: coppercone2 on November 24, 2022, 01:27:57 amI've ran some basic LTspice simulations and built a flyback back converter using them, but I'm wondering if there's any real world pitfalls since they seem to be uncommon.Quote from: boB on November 24, 2022, 07:10:09 pmI'm using GUR460 diodes and an aircored inductor of a few uH.

author: mag_therm
author_rank: Frequent Contributor
timestamp: November 27, 2022, 02:14:11 am
content: I have done some simulation in qucs, and so far I give this snubber a fail.I don't remember this compound snubber, but it might have come from the old days " My Generation" the Who, 1965 when waveforms were more trapezoidal.Capacitor has charge(+) while switch_Q is On. At Off, capacitor takes a pulse of current and endswith charge(-). That is good if capacitor and diode can take the di/dt in a circuit with no defined R.At next On transition, switch_Q has to take the load current, plus a pulse re-charging the capacitor via the low side diode and (hopefully lossy) inductor.On EMI side, in total, there are 6 second derivative peaks of I_snubber, the highest one being in Off time when the dv/dt abruptly changes from (+) to (-)Perhaps I have used components with low side excessive di/dt in my simulation, but I think the expectation of high side diode and capacitor reliability is in doubt.In a high power converter, the high side diode will have to be rated for the falling di/dt into recovery.

author: theleakydiode
author_rank: Regular Contributor
timestamp: November 27, 2022, 06:14:56 pm
content: Quote from: mag_therm on November 27, 2022, 02:14:11 amSo it's very harsh on the snubbing components and requires expensive capacitors to take the current pulses?

author: boB
author_rank: Frequent Contributor
timestamp: November 27, 2022, 06:51:26 pm
content: I used this circuit in a flyback converter on years ago and it worked pretty well as I remember

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 27, 2022, 08:17:31 pm
content: Note that the current pulse is given by Vpp / Zo, where Zo = sqrt(L/C) in the loop. So, if it's "excessive", you've probably used too much C and not enough L.It is a downside, that to get complete reset in a minimum duration on-pulse, the peak current may need to be comparable to the nominal load current. The trouble with linear networks is, they're linear, everything's proportional. So, it's hard to have operation remain useful over a wide operating range.Tim

author: mag_therm
author_rank: Frequent Contributor
timestamp: November 28, 2022, 04:38:45 pm
content: The snubber of this thread title was described by:Domb, Redl and Sokal,"Nondissipative turn-off snubber alleviates switching power dissipation, second-breakdown stress and VCE overshoot: Analysis, design procedure and experimental verification"Published 1982Paywalled by IEEE, not a member, I might try to get a copy through Engineers AustraliaThe latest converter project here is mostly completed and running. it is a converter from 32V DC link to 14V 20Amp part of the 85 A.h. DC Ups for ham radio gear.The converter presently has what I call "floating" snubber.Here is qucs showing the "floating" snubbers. They do return part of the energy to the DC link, and they only dissipate about 2 Watt in this converter.https://app.box.com/s/2orjo7boyqa74c0siq88yy8ydzspus4i"Floating" Snubbers only conduct when the collector voltage is > 2 * 36VSo they do not reduce the off dv/dt which is set by the IGBTs.Here is the spectrum today of the converter with "floating" snubbers running at 7.9 Amp output.Measuring on the DC(+12V) output bus via a pad: Level is about -88dBm to the pad,from 2 to 20 MHzmarker at 18 MHz. (Note this spectrum is using snubbbers different from subject of this thread !)https://app.box.com/s/vtljdvw7wqwh1odq3h1zqvxo0m695j9g----------------------------Thanks Tim for your post #11,I will continue with that in qucs with R being of the transformer and the ESR of the DC Link capacitor.Here is qucs circuit for the "Domb" snubbers that are subject of this thread:https://app.box.com/s/vw2jfnoy2ejb40ch5be32pc9va1fc8nbI am presently having difficulty (long solving time, instability) with the transient solver due to snubber di/dt .Time steps are down to 5 ns but using the whole inverter circuit, it take 600 us for the start transients to die out sufficiently to zoom in on snubber.Also note that I have added Miller capacitors to the IGBTs to slow down the voltage rise time.That is for qucs solution and I might try it on next real converter too.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 28, 2022, 04:48:12 pm
content: If it's freezing up (so to speak) on the falling edge, maybe some loss/damping (R+C?) on the R+D is needed? I don't know QUCS quirks personally but that can give regular SPICE some trouble.Tim

author: mag_therm
author_rank: Frequent Contributor
timestamp: November 28, 2022, 05:12:30 pm
content: I just got the first complete run with the zoom at the end of 600 usec.The circuit is :https://app.box.com/s/vw2jfnoy2ejb40ch5be32pc9va1fc8nbNote that C_Miller is used to slow IGBT transitions.The graphs:https://app.box.com/s/6ix8or7x1cpmamp9qpj9jeqxxm3nwyzfLooks like smaller steps needed. Note(bottom graph) that the body diodes are now conducting

author: Neilm
author_rank: Super Contributor
timestamp: November 28, 2022, 07:15:08 pm
content: I have used this circuit in flyback transformers. It does need some careful work to ensure you don't overstress stuff and you do need to discharge on a cycle by cycle basis.It is more expensive and you may find you need a weird value or high pulse current for the inductor which makes it expensive.In the application I used it for, it replaced a 20 W resistor which improved the efficiency of the converter and saved the battery life considerably