Very low noise isolated DC/DC converter? - Page 1
Forum section: electronics / projects

author: exe
author_rank: Unknown
timestamp: October 26, 2024, 05:11:18 pm
content: I'd like to make a battery-powered linear power supply. In my current design (harrisson-like, see ), I need a floating point aux supply. What are my options? It will power opamps, reference, ADC and DAC. Something like 12V->12V or 12V->15V would be great. I'd prefer an integrated module that I can just "plug and pray".I'm not terribly concerned about efficiency, the power supply is not meant to be used for extended periods of time. But noise is another matter.References:- http://hparchive.com/Journals/HPJ-1962-07.pdf

author: Nominal Animal
author_rank: Super Contributor
timestamp: October 26, 2024, 06:14:58 pm
content: How about an isolated 12V to 15V/17V DC/DC converter followed by a linear low-drop low-noise regulator to 12V/15V?The same applies if you want a dual supply; many such converters you can follow up with a linear regulator.To get the noise even lower, you can use either a capacitive multiplier or a Pi (C-L-C) filter between the converter and the regulator.

author: PGPG
author_rank: Frequent Contributor
timestamp: October 26, 2024, 06:26:59 pm
content: Quote from: exe on October 26, 2024, 05:11:18 pmIf you make battery-powered linear power supply than it is already floating.

author: SiliconWizard
author_rank: Super Contributor
timestamp: October 26, 2024, 09:12:22 pm
content: Agreed, either there's something you didn't tell us, or this is a non-issue, as you shouldn't need to isolate anything if it's strictly battery-powered.But, again, there may be some missing requirement, like use cases where it could still be connected to a charger while operating, or another power source than a battery.Now, maybe the "isolated" part is a byproduct of the quest for finding a DC/DC converter solution?If you want a "ready-made" module kind of thing, then I'd suggest picking any DC/DC converter that fits your requirements in terms of in/out voltage and max power rating, and add proper filtering (as usually suggested in the DC/DC converters' datasheet, often something like pi filters at both the input & output), and yes, then followed by linear regulator as suggested by Nominal. Downside then is that often, those ready-made, encapsulated modules are not adjustable and you'd need to select an output higher than the final output voltage due to the linear reg dropout. So that may be the annoying part if you look for "standard" output voltages such as 12V or 15V, but some of these modules are adjustable.If noise is the primary concern and efficiency, not so much, my second suggestion would be to forget about DC/DC conversion altogether, and use a multicell Li-ion battery with the proper nominal output voltage, followed by a linear regulator down to your target voltage. For instance, for 12V output, you could choose a 4S or 5S Li-ion battery and then a linear reg down to 12V. That won't be super efficient, although with a 4S (nominal output 14.8V), you would have a "nominal" efficiency of 12/14.8 = 81% which you would hardly beat using a DC/DC converter followed by a LDO anyway. Just my 2 cents.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: October 26, 2024, 09:30:05 pm
content: What sort of power levels are you thinking about for the whole power supply?I am seriously considering building something similar myself, but starting from a 24Vdc power supply, for a starter, and later mayby higher voltage and current. but my first attept is modest, If I can get a neat 20V 500mA out of it I'm happy for the first try.Because auxillary mains transformers are also no option for me, I am probably going to use an isolated 24Vdc to 2x8V auxillary power supply.With a 1Watt module (costing about EUR 3) you have about 2x 60mA, which is enough for a uC, some opamps and a bit of bias current for the power section, although with a discrete darlington or Sziklai, you can use the main power supply to set the (minimum) operating point of the main power transistors. I prefer a symmetric power supply, because I don't like messing with "rail to rail" opamps, which never quite deliver what they promise.I know these small modules are not very clean. I am going to attempt to clean them up with RLC filters, probably a capacitor multiplier and proper design of the GND plane. And how clean does this power supply have to be? Opamps generally have a quite high PSRR, and the voltage reference needs very little power, which makes filtering easier if it's needed.My first attempt is not going to be a designed PCB, but built on matrix board. For the SMPS module + filtering I'm going to go dead bug / Manhattan style. My main goal is not to design a power supply, but to experiment with ADC and DAC techniques from a uC, and to gain more insight in analog circuit design. So I'm don't know yet what I'm going to build exactly yet, and it's likely to go though a bunch of iterations. All good reasons for going to matrix board.Another option for you: What about using separate batteries for the auxiliary power?And also: Realize that your auxiliary power supply does not have to be floating. If it's floating it can't deliver base current for the power section. Usually it's in series with the main power supply. The GND of the auxiliary power supply often is riding on top of the main power supply, If you want a +/- voltage for the aux supply, you can steal some power from the main power supply.Also, how is your Dutch? (Firefox has a built in (experimental?) translator these days, and in general, translating is becoming easier over time anyway. About 10 years ago "Blackdog" on the forum:https://www.circuitsonline.net/forum/view/110029has given his own swing on this harrison design. He had been working on details for about 3 years (form 2013 to 2016) on his design, and it started a few years earlier by improving the power supply he had (ELV-22532) There are a lot of interesting design details on that dutch forum, but there is also a lot of crud and distractions. It's inherent to how the 'net works I guess. If you want to skip all the Dutch, You can find his final version (42 of course) of the schematic on:https://www.bramcam.nl/NA/NA-01-PSU/NA-PSU-SCH-42.pngOr, if that link gets broken someday, a direct upload attached below.The older thread about the ELV power supply is:https://www.circuitsonline.net/forum/view/109400

author: exe
author_rank: Unknown
timestamp: October 26, 2024, 10:50:08 pm
content: Why I need isolation in a battery powered power supply: the harrisson design needs to power rails: one "main", and another one "floating" on positive output.Quote from: Nominal Animal on October 26, 2024, 06:14:58 pmI don't have much experience with SMPS, so not sure if that's will be sufficient. That's why I wanted a ready-module that would optimized by professionals.Quote from: Doctorandus_P on October 26, 2024, 09:30:05 pmVery modest, 12-15V, 200mA output.Quote from: Doctorandus_P on October 26, 2024, 09:30:05 pmYess, that's the one I use for inspiration).PS Mijn Nederlands is niet goed)

author: David Hess
author_rank: Super Contributor
timestamp: October 26, 2024, 10:56:14 pm
content: Small pulse transformers with 12:12 and 12:15 ratios are available which can be used with sine wave drive to make very low noise isolated outputs.

author: bson
author_rank: Unknown
timestamp: October 26, 2024, 11:42:47 pm
content: How much power is it switching, buck or boost, and what is your noise budget?Edit: oh, and what is the concern: radiated or conducted noise? Those can both be significantly mitigated with minimal effort by using various standard techniques - like inductor orientation. Do those, then see where that puts you.

author: exe
author_rank: Unknown
timestamp: October 27, 2024, 04:07:19 pm
content: Quote from: bson on October 26, 2024, 11:42:47 pmThat's the thing, I don't know(. Let's say, I'm looking for less than 1mV p-p noise output. May be higher is ok if there is no "high-speed" content in it (which is quite likely the case).As of radiated vs conducted, I'd love to not have any of them.I was hoping to find something like LTM8080:https://www.analog.com/media/en/technical-documentation/data-sheets/ltm8080.pdf.There are plenty of integrated modules from TI (https://www.ti.com/power-management/dcdc-power-modules/isolated-power-modules/products.html). But TI doesn't provide noise specs, not just spectrum plot missing, any numbers related to noise are missing in DS for a few modules I checked :/

author: Postal2
author_rank: Frequent Contributor
timestamp: October 27, 2024, 04:22:30 pm
content: It's elementary. Use converters with any noise, just turn them off for the duration of measurements, powered by charged capacitors.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 27, 2024, 04:47:20 pm
content: The Harrison topology does not need a high voltage to power the controller part. Chances are some +-5 V would be enough and not much power needed either. So one could use a chip like LT3999 to drive a small transformer. If low noise is more important than efficiency an audio amplifier like LM4991 could be an option too. For a compact ciruit consider a ready made DCDC converter (e.g. AW) or SN6501 hard switching driver.The main point of the Harrison topology is having the output stage as a controlable current source. One can get this also without an extra floating supply. For battery supply or starting with a single voltage this may be the easier option.

author: schmitt trigger
author_rank: Super Contributor
timestamp: October 27, 2024, 05:07:22 pm
content: If you’re not concerned about efficiency, you can deliberately slow down the turn on/off of the main power Mosfets.Another trick is a resonant push pull, with a cap across the primary. You then trim the switching frequency to obtain maximum amplitude. (Resonance).

author: exe
author_rank: Unknown
timestamp: November 08, 2024, 10:59:01 am
content: Small update, I did a bit more research on the topic. Particularly I read "the art of electronics, x-chapters", there is a discussion on the topic. I'm a bit disappointed that it takes so much effort to make a clean SMPS. Like, everything matters, every tiny-tiny detail, like layout, slew rate, component selection, etc. Since designing a quiet SMPS is a big topic on its own, I decided not to push it this way and try to build my power supply without the need for an isolated power rail.Having said that, for the sake of science, I'm building an isolated power supply on the breadboard just to verify that it's difficuilt). So, I'm building something like on the image attached. Unfortunately, I don't have ideal parts to build this, I'll just use the parts I have in my "trash bin".One thing that already caught me off-guard is how much slew rate I need from opamp. I wanted to use LM358 because of its slow edges, but, turned out, its slew rate is not enough to drive even 5KHz 15-30V p-p trapesoidal signal ("power bandwidth").

author: Andy Chee
author_rank: Super Contributor
timestamp: November 08, 2024, 12:01:22 pm
content: Why not use a 555 timer for your oscillator? Surely you have one in your junk parts pin!You can also make an oscillator from a schmitt trigger CMOS inverter 74HC14.

author: exe
author_rank: Unknown
timestamp: November 08, 2024, 12:24:33 pm
content: I was just trying to get a slow edge. Otherwise 555 is good).So, I get my trusty MC33072 opamp produce a trapezoidal signal. It was nice and clean, and I could get some 3us rise time.But then I had a brillian idea that I can simplify circuit and use a power opamp to generate signal directly. So, I picked UTC2003 which I got from LCSC for like 20 eurocents and I got... nothing. Scratching my head, I opened the schematic, and it doesn't look like an opamp to me (where's long-tail pair?). So, I replaced it with LM1875 from HGSEMI which costs whooping 60cents... Pricey upgrade I would say).And I worked. Kinda. I have horrible oscillations which I can't get rid of. Decoupling doesn't help. The opamp is presumably requires minimum gain of 10, but I thought in oscillator mode this is irrelevant. Anyway, it badly oscillates and I don't know why and how to fix this (. Randomly placing capacitors didn't change anything.May be the issue is that I only place 100nF ceramic caps, but this is a beefy IC, and it needs some bulk electrolytic caps for proper functioning... I'll try to add those too.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 08, 2024, 01:07:48 pm
content: Quote from: Andy Chee on November 08, 2024, 12:01:22 pmThe NE555 is quite noisy, with a relativey large current spike at the supplies, even though it is BJT based.In a different thread about powering the VFD filament someone suggested a LM4871 audio amplifier that could be suitable and also provide some power up to maybe 2 W.

author: iMo
author_rank: Super Contributor
timestamp: November 08, 2024, 01:13:21 pm
content: The cleanest DC/DC you may get with harmonic signals only, imho. A colleague of mine (talking mid 80ties) made, as a part of his thesis, a DC/DC converter for a 1/f noise measurement system (afaik a higher voltage one). The idea was the filtering will be much easier, no high energy spectral mess generating by any switcher..Also its frequency components would be better defined considering the spectral analysis measurement.. He spent most time manufacturing the transformeras I can remember, he was running it at 10+ kHz (we measured the noise up to 10kHz), driven by a low distortion "power" amplifier (for a well defined sine signal).

author: exe
author_rank: Unknown
timestamp: November 08, 2024, 01:31:30 pm
content: Quote from: Kleinstein on November 08, 2024, 01:07:48 pmYeah, original NE555 is noisy. I have much better results with a CMOS version of the chip.LM4871 is a nice suggestion, adding to my shopping cart. I like that it is unity-gain stable and that it seems it provides symmetrical output drive.Quote from: iMo on November 08, 2024, 01:13:21 pmI agree. I was just thinking that by controlling slew rate I could simplify circuit a little bit. According to X-Chapters, they found that in their isolated power supply, there is not much difference between a clean sine wave and a triangle wave. They also suggested that if triangle waveform works fine, then trapezoidal signal should work fine too (with obvious advantage of higher RMS voltage, afaik). That's why I'm experimenting with trapezoidal signal.

author: temperance
author_rank: Super Contributor
timestamp: November 08, 2024, 02:32:30 pm
content: You can try an open loop LLC converter.Edit:-You can increase the separation between prim and sec. winding without any problem. The leakage inductance can be used for the LC part.-Efficiency in this case is not a concern. so you can use small resistors in series with the sec. rectifiers.Disadvantage is that for low power you will need to reduce the magnetizing inductance such that the converter always stays in ZVS. A core with an air gap will be required.

author: Postal2
author_rank: Frequent Contributor
timestamp: November 08, 2024, 02:59:16 pm
content: If the main goal is to have no noise, then you can cascade thermoelectric elements and heat them.

author: Manul
author_rank: Super Contributor
timestamp: November 08, 2024, 03:08:17 pm
content: For low power one-off lab solution, LED array module driven by good linear CC and optically coupled to a solar cell is both isolated and clean. At a bit bigger distance the capacitance is also very low. It's also funny to show to people, cause it's a bit ridiculous.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 08, 2024, 03:36:19 pm
content: For very low power there are even ready made PV opto-couplers. There can deliver some 10-50 µA at some 7 V. That is enough to power some OP-amps if really needed, though the main intended use is to drive a MOSFET gate.The efficiency is not great (e.g. ~ 3% for a TLP3906), but not as bad as one may think as IR LEDs are quite efficient and PV cells have there best efficiency at around this wavelength.

author: Manul
author_rank: Super Contributor
timestamp: November 08, 2024, 03:51:33 pm
content: Quote from: Kleinstein on November 08, 2024, 03:36:19 pmYeah, I once used VO1263 in a design (both channels in parallel), works well if only small current is needed.

author: Andy Chee
author_rank: Super Contributor
timestamp: November 08, 2024, 06:44:35 pm
content: Quote from: exe on November 08, 2024, 01:31:30 pmWhoa hold on.....I thought you were just using parts available in your junk bin?If you're just going to purchase low noise parts from online retailers like Digikey, then that opens up an entire set of possibilities!

author: exe
author_rank: Unknown
timestamp: November 08, 2024, 07:40:16 pm
content: > that opens up an entire set of possibilities!You're right, if do so, I'll never finish anything)I considered photovoltaic cells, but that's too much, imo). I need a big array to have any meaningful current. At such efficiency peltier elements or, maybe, piezo-effect something doesn't sound too bad)Anyway, I did the experiment. I just assembled the whole thing without much thinking and tuning. This is what I get: p-p noise around 4mV (~5MHz BW, probe is 1x, oscilloscope set to 20MHz BW limit). Baseline noise seems to be around 800uV p-p when everything around is powered off and probe shortened (but with a long ground lead).Setup: Trafo is some random audio trafo (EI14-600:600). Thenschottkydiode bridge (DBL206G), caps 100n+low-esr electolytics. Final smoothing is AMS1117-3.3. Load is 330 ohm resistor (10mA load).Current problems:- trafo (EI14-600:600) has resistance of ~120 ohm each winding.- diode bridge (DBL206G) is not schottky- opamp wildly oscillates with base freq about 1MHz, but there are some high-freq components because it's a saw-tooth oscillation.- frequency seems to be 35KHz instead of 5KHz as I wanted. I messed up while messing up the opamp.- voltage headroom is minimal. Just on trafo it seems voltage drop is whooping 20V p-p!I consider this a mild success. I might do a bit more experiments, not sure. I think, if driven slowly and gentle, sub-1mV noise is achievable without much trouble. But trafo size will be significant.

author: Postal2
author_rank: Frequent Contributor
timestamp: November 08, 2024, 08:09:37 pm
content: Quote from: exe on November 08, 2024, 07:40:16 pmIn general, rectifier diodes are the weakest point of the design. I may be confused, but it seems that Schottky diodes produce more noise than regular diodes. But regular diodes can also make noise, you need to select them, perhaps germanium ones will do. Also, the weak point of your design is the desire to use the key mode, albeit smooth. You will have to use a sine wave (and the generator can be directly on your transformer, on 2 transistors).By the way, good oscilloscope, I have such.

author: peter-h
author_rank: Super Contributor
timestamp: November 08, 2024, 10:42:04 pm
content: I've designed loads of these, often for low power eg 1mA input at 5V.The way to get lowest noise is to generate a sinewave (not anything square) and feed that into the transformer. The "noise" will be just the one spectral line.Then you rectify it and the diodes will have a narrow conduction angle, which creates harmonics, but you can trade that by having large ripple and filtering it afterwards.In modern products the EMC issue tends to be addressed with MHz-area DC-DC converters which pushes their emissions into the MHz region which, if you keep it all compact, doesn't register too badly on the test lab gear

author: iMo
author_rank: Super Contributor
timestamp: November 09, 2024, 08:11:40 am
content: If your diodes in the bridge generate noise -> add a small capacitors across the diodes. That is a common practice even with modern 50/60Hz rectifiers since ever (1..10nF). With 5+kHz the capacitors would be like, say, 100pF. That noise is not easy to see on a o'scope as it usually annoys people from radioelectronics in ~XX MHz regions of spectra..

author: exe
author_rank: Unknown
timestamp: November 09, 2024, 08:49:04 am
content: Quote from: peter-h on November 08, 2024, 10:42:04 pmCould you please share which frequency do you use and which inductors?

author: peter-h
author_rank: Super Contributor
timestamp: November 09, 2024, 10:05:50 am
content: I have tended to use 50kHz to 300kHz. That works well with cheap ferrite magnetics. Everybody and their dog makes ferrites.Toroids are the most efficient although the least convenient for production, and not cheap to wind. One solution is to get a subcontractor to mount them on a little PCB with pins, like thisActually that one (hand wound in china with tefzel wire-wrapping wire) is an amorphous iron core, from Allied Signal, expensive, but super low loss, used in a flyback PSU which has a wide input range and the core is thus hammered with nasty harmonics, and ferrite would heat up a lot.Then you can buy these little oneswhich are better for production. Many years ago I used this style for manchester encoded dataThere are thousands of these, made in china etc.

author: radar_macgyver
author_rank: Frequent Contributor
timestamp: November 09, 2024, 05:46:57 pm
content: LT made a number of slew-rate limited switchers, the idea being to reduce harmonics and therefore reduce amount of post-filtering needed. Some, like the LT3439 are pure transformer drivers (no regulation) and can be followed by an LDO to produce a regulated output. Others, like the LT1683 have regulation built in, but the feedback required for this breaks the isolation barrier. Recommended transformers are listed in the datasheets, and are often found in PoE applications.

author: Postal2
author_rank: Frequent Contributor
timestamp: November 09, 2024, 06:23:03 pm
content: Quote from: iMo on November 09, 2024, 08:11:40 amYes, for a rectifier bridge rectifying alternating voltage of ~12 volts 50/60 Hz from a conventional plate transformer, it is advisable to connect a 100 nF capacitor in parallel to each diode. However, for high frequencies (> 60 kHz), this does not work. For high frequencies, ferrite cylinders with a hole through which the diode lead passes are used. By choosing the right ferrite, interference can be reduced to zero.

author: peter-h
author_rank: Super Contributor
timestamp: November 09, 2024, 07:55:36 pm
content: The above is rubbish.

author: Postal2
author_rank: Frequent Contributor
timestamp: November 09, 2024, 08:29:26 pm
content: Quote from: peter-h on November 09, 2024, 07:55:36 pmI can't tell you much about using ferrite tubes. More than 30 years ago, I was struggling with interference on TV screens with a bad signal. First, I tried to connect each diode in the power supply through a short coil of 8 turns with a diameter of 8 mm with a wire of 0.5 mm. The leads of the diodes were shortened, and almost immediately passed into the coil. This gave a very good effect, but I wanted to completely eliminate interference. I assumed that the scattering field of the coils affected it, and to reduce it, ferrite was needed. I went to the local radio market and bought a lot of different ferrite. This completely eliminated the interference. The problem is that I do not know the parameters of this ferrite. There are ferrite-looking tubes that conduct current like metal. These are not suitable for a power supply, but they completely remove interference from thyristor converters. I also disassembled various TV coils that had a tube-shaped core, in some cases they were better suited, but again I don't know the parameters. I've been installing these tubes for 30 years if I need to remove interference. Sometimes it's impossible to install them constructively. That's my experience.

author: nctnico
author_rank: Super Contributor
timestamp: November 09, 2024, 08:37:32 pm
content: Quote from: Postal2 on November 08, 2024, 08:09:37 pmActually, this depends on the type of diode. Regular diodes have a nasty reverse recovery spike. Schottky diodes have much less of this effect IF they don't have a guard ring which forms a regular diode in parallel (which gives the reverse recovery spike). I forgot the exact term for Schottky diodes without a guard ring, but Google knows that for sure.However, nowadays I'd go for a DC-DC converter with a synchronous rectifier. Much less losses and no 'switching noise' from a diode to deal with.

author: Postal2
author_rank: Frequent Contributor
timestamp: November 09, 2024, 08:53:47 pm
content: Quote from: nctnico on November 09, 2024, 08:37:32 pmYes, when I first saw this solution, I was amazed at how cold they were.I have a 12 volt 5 amp adapter with a synchronous rectifier, it is about 20 years old. I can take a photo of its board, the mosfet is initiated by the coil, which is a green ring.

author: Zoli
author_rank: Frequent Contributor
timestamp: November 11, 2024, 01:13:55 am
content: Quote from: Postal2 on November 09, 2024, 06:23:03 pmPlease read thru pages 7 to 11:http://www.hagtech.com/pdf/snubber.pdf

author: Nominal Animal
author_rank: Super Contributor
timestamp: November 11, 2024, 01:53:44 am
content: Quote from: Zoli on November 11, 2024, 01:13:55 amHuge thanks, Zoli! As a hobbyist knowing the math but with very little useful practical experience, those (athttps://www.haglabs.com/pages/frontpage) are pure gold for me. It is finding these in the first place that is so hard –– and will become even harder in the future, with LLM hallucinations filling every conceivable niche starting from search engines.I particularly like Jim Hagerman's article style, because he combines the practical explanations with precise mathematical formulae. Much like Horowitz and Hill in TAoE, in my opinion.

author: Andy Chee
author_rank: Super Contributor
timestamp: November 11, 2024, 02:15:50 am
content: If you're really keen on snubbers, it might be worth a look at Rudy Severns' book "Snubber Circuits For Power Electronics" (which can be found floating around the interweb as a PDF ebook).https://www.cde.com/resources/technical-papers/design.pdfhttps://web.archive.org/web/20160305161056/http://snubberdesign.com/

author: Postal2
author_rank: Frequent Contributor
timestamp: November 11, 2024, 02:21:33 am
content: Quote from: Zoli on November 11, 2024, 01:13:55 amGood research, thank you. I advise you to make it simpler. We will still use a diode bridge, unlike the researchers, so you will have to resolder 4 capacitors at once. You just need to put an old AM receiver close, turn it on and start the experiment.To analyze interference from 50/60 Hz diode bridges, as well as to search for electrical wiring, I use this receiver (diagram and link below).https://rw6ase.narod.ru/index1/konstr/rk_radiopriem/junost105.html

author: Nominal Animal
author_rank: Super Contributor
timestamp: November 11, 2024, 03:02:46 am
content: Quote from: Andy Chee on November 11, 2024, 02:15:50 amNot on snubbers in particular. It's just that Jim Hagerman's article just slotted in perfectly to what I did not understand in practical DC-DC (isolated and non-isolated) converter topologies/schematics. Stuff like"Since the snubber resistor must be in parallel with both the inductance and capacitance, it can be placed across either the diode or the coil", combined with the math and explaining how and why it should work. Not only does it explain what I can occasionally see, but it also explains what I can do myself to combat/avoid ringing, and what I need to know to design that in.All my DC-DC converter use cases thus far have been based on designs from datasheet suggested application schematics or TI Webench. As you know, they give a good starting point, but are likely not complete for any specific use case. For example, my projects are usually powered from USB 5V, which has a capacitance limit of 10µF (unless you limit inrush et cetera), and I need most current at 3.3V, 3.8V (4G/5G/LTE modems), 2.5V/1.8V/1.2V/1.1V and so on, occasionally post-regulated for some audio circuits. Sometimes I also need some ±12V or ±15V for opamp rails at tiny currents, so either voltage multipliers or low-power boost controllers, occasionally low noise. While the power budget itself tends to not be a problem, the waste heat often is; even 1W of heat in a tiny USB device can make things difficult.Just consider this thread, and what it would have meant for us all if someone skilled in DC-DC design (including the math) had created a similar type article on low-noise DC-DC converter implementation, showing three or four techniques of de-ringing and filtering the output, a couple of options to ensure no EMI induced on the inputs, and the practical effects of each. (I suspect the optimum technique for any particular case would especially depend on the frequency range used in the converter.)

author: Zoli
author_rank: Frequent Contributor
timestamp: November 11, 2024, 07:00:56 am
content: Quote from: Postal2 on November 11, 2024, 02:21:33 amThat radio will emit back a lot of radiations thru the antenna; I understand the simplicity, but it defeats the purpose; I would avoid it. Simplicity: other way to deal with the bridge ringing is to add a snubber over the secondary(parallel with the AC entry to the bridge), and ferrite beads/common mode choke in series with the DC output of the bridge(see picture).

author: peter-h
author_rank: Super Contributor
timestamp: November 11, 2024, 08:24:41 am
content: QuoteIndeed - never saw these before. Funny that he is usingpassiveRIAA equalisation. At univ, 1977, I built a phono preamp using this method. Spent days writing FORTRAN code to simulate it. Used a MAT101 on the input stage; back then the lowest noise option. The result, feeding a 400W power amp and with max volume, your ear had to be inside the speaker cone to hear the noise

author: Postal2
author_rank: Frequent Contributor
timestamp: November 11, 2024, 01:43:29 pm
content: Quote from: Zoli on November 11, 2024, 07:00:56 amThe thing is that the "ringing" is the second problem. The first is this:Quote from: peter-h on November 08, 2024, 10:42:04 pm

author: mtwieg
author_rank: Frequent Contributor
timestamp: November 11, 2024, 01:58:32 pm
content: Quote from: exe on October 27, 2024, 04:07:19 pmThis is your problem. There are several independent phenomenon one could describe as "noise" in SMPS, and there are countless potential methods to mitigate them. If you ask for something like "no noise" then you're going to get suggestions pulling you in completely different directions.Actually starting with an example of a "noisy" SMPS and pointing out specifically why it's too "noisy" will allow us to give more useful feedback. Looks like you're already experimenting with something, which is great. But you didn't provide a schematic, so it's hard to give specific feedback.

author: David Hess
author_rank: Super Contributor
timestamp: November 11, 2024, 10:37:21 pm
content: Quote from: Zoli on November 11, 2024, 07:00:56 amNot all line rectifiers have snap-back; I have seen it crop up in production between different manufacturers and different lots of parts.My preferred solution is to use fast recovery rectifiers which never seem to have the snap-back problem, and sometimes I replace old rectifiers with fast recovery rectifiers if that is convenient.

author: r6502
author_rank: Regular Contributor
timestamp: November 12, 2024, 05:59:31 am
content: Hi all,there is a chip from former Linear Technology, calledLTZ3439, that offers slew rate controlled switching. This ist highly recommended for low noise SMPS applications.You may also watch a video from Marco Reps where he showed how to get a SMPS with very low common mode noise using this chip for a supply of a LTZ1000 reference:Guido

author: peter-h
author_rank: Super Contributor
timestamp: November 14, 2024, 09:49:33 pm
content: I would not use anything from LT or whatever they are now called. The covid lessons have already been forgotten?

author: r6502
author_rank: Regular Contributor
timestamp: November 17, 2024, 07:23:17 am
content: @peter-h:What's wrong with chips fron LT or now Analog?If you do not like LT / Analg, you'll find something similar in the portfolio of TI. these kind of SMPS controllers just limit the output slew rat for ths switching MOSFETS and you get mutch lower ripple noise at the output.Guido

author: Kleinstein
author_rank: Super Contributor
timestamp: November 17, 2024, 08:53:37 am
content: AFAIK Ti has no direct alternative to the slew rate limited transformer drivers like LT3439.One may get a similar effect with power OP-amps or the LM4871 audio amplifier, though with a little less power and a fixed slew rate and as full bridge drive (may be a small advantage).At some point one may have to settle for a specific part/manufacturer. This would also happen with an ADC, µC , FPGA.For a hobby project there is no real need to look for a 2nd source for every part.Today a redesign of the PCB is no longer that bad. Thing are more tricky with a µC, as a change there may need porting the code.

author: tszaboo
author_rank: Super Contributor
timestamp: November 17, 2024, 10:49:34 am
content: Quote from: mtwieg on November 11, 2024, 01:58:32 pmI agree. There are standard ways to build SMPS, and one should try that before deciding if thye need to use the non-standard ways. You can use standard 1W Traco (or other) DC-DC converters in precision circuits, there are ways to clean up the output, post regulation, common mode filters. Sometimes you can even rely on then PSRR of the ICs. And you shouldn't decide on the topology of a circuit, before making the requirements. "As little as possible" is not a requirement.

author: Vovk_Z
author_rank: Super Contributor
timestamp: November 17, 2024, 11:08:18 am
content: I guess you can have sub-1 mV noise with a regular integrated isolated DC-DC too. And, I guess you can have sub-0.1 V noise (which is close to low-noise) by filtering 'sub-1 mV noise'. E.g. if you need 12 Vdc you may use 15-24 V rated DC-DC and then put some differential mode inductor and a linear regulator etc. This can be fine for many applications. For example, I used this appoach with an ultra-low distortion 1kHz oscillator.