Designing to minimize power supply noise - Page 1
Forum section: electronics / projects

author: ezalys
author_rank: Frequent Contributor
timestamp: October 09, 2020, 01:07:32 am
content: Let's consider two different power supplies for a DC-10 KHz 4 channel single ended 18-bit DAC. 18-bits corresponds to 108.37 dB of SNR:In one I use an external power supply such ashttps://www.digikey.com/en/products/detail/inventus-power/MWA020005B-10A/2764481. Inside I use a high quality LDO. 1% pk-pk ripple from the datasheet is -20 dBV voltage, or -40 dB power relative to the supply, and then on top of that I can use a high end LDO such as the TI TPS7A4901 which features 70 dB PSRR, for a total of 110 dB... ideally. I place the brick outside the enclosure and add a ferrite to the DC line to fight common mode noise. Maybe I add some ferrites and pi filters inside the enclosure before the LDO to get the last bit of performance out. Given that the references of the DAC and the DAC itself have their own PSRR, I would imagine I'm in the clear.In another design, I use a much larger chassis and an Acopian A5MT510 which outputs 5 volts and has 0.25 mV ripple. That's -40 dBV or -80 (?!) dB power relative to the supply. I suppose I could add an LDO there and potentially under ideal circumstances beat 110 dB but 110 dB seems quite low, and I have to imagine I'd be limited by other things at this level.My question is this... is there really any sense in buying a high end linear power supply outside of simplicity and reliability when you can get a decent switcher and pair it with an LDO and a little filtering and do essentially well enough? If this is true now, when in the history of electrical engineering did it become true and what was the reason?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 09, 2020, 01:48:11 am
content: I mean, test equipment has used switching supplies, of some sort or another, for a very long time. (You're welcome to argue whether a particular CRT flyback supply is class C or D...) Just a matter of filtering it well.Back in the day, switching supplies just weren't economical against linear supplies, whatever the noise requirement. Added filtering or post-regulation just made it worse. When PoL switching regulators became cheap, and ran at frequencies reasonable to filter (100s kHz), they went in everything, from consumer equipment (e.g., the 1-transistor blocking oscillator PSU in the Apple II, and similar in VCRs, etc.), to test equipment.I can imagine some other reasons which might be stronger. The test equipment market tends to have longer life cycles, less priority for low power dissipation / high efficiency, or low cost, and more priority towards accuracy and reliability. That'll tend to keep old (often linear) designs going long past their prime. Meanwhile, reliability tends to disfavor new, poorly tested designs. They certainly used new, bleeding-edge designs -- HP and Tek are famous for their custom chips. But only in the critical signal path, where no other solution is possible. And those are sometimes notorious for chronic issues (like the Tek 2465 horizontal driver, I think it is?). Why risk using new, unknown chips in something as pedestrian as a power supply?Management aside -- yeah, just filter it enough.You do occasionally get pernicious offenders, like a sync buck IC with just enough dead-time to momentarily forward-bias its substrate diode, which dutifully snap-recovered, spraying low-GHz harmonics throughout the power planes. (Alas I forget the exact part number for that one.)You can do some work with layout and filtering to mitigate those sorts of noises. Most chips don't make much noise above 100 or 200MHz, still reasonably tractable on the PCB. At worst, a metal-can shield will keep most all of that inside.Then again, with more GaN-based and fully-integrated regulators coming out these days, operating frequencies in the 10s and 100s of MHz, and harmonics in the GHz, are becoming more common. Use responsibly!Tim

author: ezalys
author_rank: Frequent Contributor
timestamp: October 09, 2020, 03:09:26 am
content: For a low noise application I have no interest in using such high speed, high power switches. My go-to is an isolated push-pull converter run open loop with slew rate limiting. SN6501 for instance.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 09, 2020, 07:43:37 am
content: There is still some value to a good old mains frequency transformer: they offer good common mode suppression at low frequencies. The injected common mode signal from the typical small DCDC converters is very hard to filter unless you have a low impedance path back (e.g. Y capacitor) to a good ground. With test equipment one may want good isolation and this low capacitance to ground.I see no problem with the SN6501, but the tricky part is getting right transformer go with with it. The usual small transformers do not offer good separation.Common mode injection is more tricky when starting from a high voltage. So the SMPS supplies from mains have quite a few extra problems with this. So for a sensitive part I would consider 2 steps of separation, possibly also a classical mains transformer followed by a higher frequency DCDC.Ripple specs of -20 dBV are ridiculous poor: this would be 100 mV ripple. The "V" is the reference power level - so relative to 1 V (RMS) amplitude. It does not mean this would be 40 dB for the power.When at very low ripple values / high damping, one also has to take care of he layout, especially with the higher frequency parts. There are sometimes paths that bypass the filters and it is not so difficult to get a ground loop wrong.

author: ezalys
author_rank: Frequent Contributor
timestamp: October 09, 2020, 02:58:53 pm
content: Sorry I meant 1 percent voltage ripple!

author: ezalys
author_rank: Frequent Contributor
timestamp: October 09, 2020, 09:54:27 pm
content: Also, in what way are transformers likehttps://www.digikey.com/en/products/detail/w%C3%BCrth-elektronik/750315371/5799562a poor choice? When you say the separation is bad, could you be a bit more quantitative and point me to where I might tell what the relevant figures are or otherwise figure out how it might be unsuitable for my application?

author: KT88
author_rank: Frequent Contributor
timestamp: October 09, 2020, 11:38:44 pm
content: The main challenge with switch mode DC/DC converters is common mode noise. Cheap off-line PSUs are often times terrible in that regard. I have seen some that spit out half of the mains voltage (>100V here in Germany) at some tens of microamps...Medical grade PSUs are typically very usable as they havE very tight limits for that.Ripple can be filtered pretty easily. A good low-noise LDO plus some caps are usually sufficient. But make sure the LDO is also very fast. The best ones I know start with LT... Expect them to be north of one or even two bucks.CheersAndreas

author: ezalys
author_rank: Frequent Contributor
timestamp: October 10, 2020, 03:08:12 pm
content: Now I see what you mean about the Y-cap. Let’s say I’m Audio Precision or Keithley or someone and I’ve decided to use an SMPS in my design but I also want to avoid injecting common mode noise AND creating an AC shunt to ground. What do I do?

author: KT88
author_rank: Frequent Contributor
timestamp: October 10, 2020, 03:35:07 pm
content: Your enemy is the capacitance between primary and secondary.A few strategies are easy to understand but hard to implement in the real world.First of all you want minimal capacitance.Shielding is next.Symmetry has almost super powersThe first two relate mostly to the design of the transformer.The third topic is mostly dependent on the topology of the SMPS.Also low harmonic content helps by using soft switching or zero voltage switching.CheersAndreas

author: julian1
author_rank: Frequent Contributor
timestamp: October 10, 2020, 10:08:36 pm
content: QuoteSo, for transformer coupled smps eg. push-pull (versus inductively coupled), wind primary and secondary on opposite sides of a toroid. Like a common mode choke, except the windings are in the same direction? For non-gapped e-core use adjacent windings on the bobbin, rather than winding the secondary over the primary?Also use a capacitor multiplier (in conjunction with LC) for the better PSRR at higher frequencies compared with an LDO?.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 10, 2020, 10:15:57 pm
content: Quote from: julian1 on October 10, 2020, 10:08:36 pmVersus what, they're both inductive?QuoteYes, split-bobbin or bank-wound transformers are used in various places. The excess leakage inductance makes these more practical with resonant designs, rather than square-wave (forward/flyback) or open loop (unregulated chopper) designs.You can indeed use a CMC this way, with the downside that you can only get a few watts at useful voltage (open loop nonresonant), due to the limitations of saturation flux and leakage inductance.Tim

author: julian1
author_rank: Frequent Contributor
timestamp: October 10, 2020, 10:45:55 pm
content: So the design trade-off appears to be, that one can reduce capacitive coupling, but generally at the expense of creating more parasitic inductance. Presumably that can be *partly* addressed with primary-side snubbers or active clamps.But I am wondering, if a toroid using a trifilar winding - with a disconnected middle winding - used just to create physical separation between primary and secondary might be a useful place to start experimenting. eg. versus the extreme point of trying to wind it like a CMC.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 11, 2020, 12:30:44 am
content: Just a continuum of cases -- you can increase the separation distance using thicker insulation for example (twisted/multifilar magnet wire, vs. twisted hookup wire or zip-cord), or spacing the turns, of course the more space you leave, the smaller the winding factor is, and the bigger the transformer must be overall.The underlying theory is that you're making a transmission line, wrapping it around a core, and driving it odd-mode so that the transmission line impedance is in series with the load (rather than in parallel, as in a CMC, like wrapping coax around a bead for a current choke balun). The equivalent inductance of the TL becomes the leakage inductance, and the equivalent capacitance becomes parasitic capacitance. You can improve isolation by raising Zo (Co falls), but leakage rises proportionally (Zo = sqrt(Lo/Co)). You can use a shorter TL (L and C drop) but then you can't handle as much flux.Tim

author: eplpwr
author_rank: Regular Contributor
timestamp: October 11, 2020, 11:55:00 am
content: I find this discussion very interesting. I have sometimes need of a power source "like a battery" that is essentially free of any coupling primary-secondary. I also like to DIY and tinker ... but not inductors in any form or size. I've come to realize I'm not skilled enough in either mechanical work nor math to make any progress with inductors.Thus I wonder, is there any commercially avaliable inductors (or even complete DC/DC "bricks") that have performance good enough to function as a "battery"? Examples of use: powering voltage references while measuring voltage difference (GNDs connected), powering nV amps, powering fA amps.Any hints to commercial solutions would be most welcome.

author: ezalys
author_rank: Frequent Contributor
timestamp: October 11, 2020, 03:44:40 pm
content: https://www.analog.com/media/en/reference-design-documentation/design-notes/dn316f.pdfSo in this document they shunt the two grounds together with a 2.2 nF cap. At 100 kHz, this provides about 1 kohm of imaginary impedance. Why is potentially making an AC ground loop ok here? How do you pick the impedance of your ground path?

author: KT88
author_rank: Frequent Contributor
timestamp: October 11, 2020, 04:12:01 pm
content: The driving source of the CM noise is on the primary side. The cap provides an AC current return path. Important to know is that the higher frequency harmonics are the biggest challenge here. The impedance between primary and secondary is also lower for these harmonics. The 2.2nF cap creates an attenuator with the tens of pFs of parasitic capacitance for the complete spectrum.CheersAndreas

author: ezalys
author_rank: Frequent Contributor
timestamp: October 11, 2020, 06:14:56 pm
content: https://imgur.com/a/BZdtUyxI guess what I’m trying to figure out is how to connect P, Q, R, and the chassis. The +/- leads will be connected to outer-conductor-isolated BNC cables. Unfortunately the thing I’m connecting this to has BNC connectors on it.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 11, 2020, 09:51:39 pm
content: Reducing the capacitive coupling tends to also reduce magnetic coupling. This tends to reduce the efficiency as more energy ends up in the snubber / clamping didoes.It helps to start from a relatively low voltage so that there are fewer windings. So less capacitance and less voltage at the windings. So it can be worth going from mains to some 3 or 5 V and only than do the low CM injection transformer.For very low coupling there are acoustic transformers: no magnetics, but acoustic coupling with some PZT or similar transducers. The coupling "core" in this can be a rod of quartz glass.

author: David Hess
author_rank: Super Contributor
timestamp: October 11, 2020, 11:51:57 pm
content: Quote from: KT88 on October 09, 2020, 11:38:44 pmAnd the common mode noise once generated gets everywhere. It does not matter when an instrument is tested in isolation but sure does once it is connected to anything else.Quote from: T3sl4co1l on October 10, 2020, 10:15:57 pmMaybe he means using the transformer for energy transfer instead of energy storage like with a flyback converter?QuoteI have seen "high isolation" transformers of various designs but not for switching regulators. I have not seen an electrostatic shield used so I assume that fails at high frequencies and maybe it does not matter; what would you connect it to anyway without recreating the original problem? At power line frequencies, electrostatic shields are common.Quote from: julian1 on October 10, 2020, 10:45:55 pmIf you need the high efficiency of a switching power supply, then there is nothing which prevents using a line frequency transformer on the input side to provide galvanic isolation. This comes with the obvious weight penalty though.Careful design to limit switching dv/dt and di/dt, at a small expense of efficiency, helps considerably but that likely means designing your own power supply, which would be another reason to use a line frequency input transformer for isolation.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 12, 2020, 01:00:29 am
content: Quote from: David Hess on October 11, 2020, 11:51:57 pmThey're out there:https://recom-power.com/en/products/dc-dc-converters/rec-s-RxxP06.html?0I recall reading appnotes from them about a toroidal split winding (like CMCs), or maybe I'm imagining that. They have some nice offerings in medical and power families.Shields I think are also not too uncommon, I just haven't taken apart much medical kit to see it, for example.Shields aren't a problem for switching, it just increases the distance between signal conductors. Consider wires over a ground plane: you get a microstrip TL, of modest impedance, maybe 50 ohms for enameled wire over foil and tape. That's equivalent to a single layer winding over, or under, a shield. Simply enough, the impedance is double, or 100 ohms. So, LL is higher. (Though not that much higher, as the two single layers windings will give about the same impedance themselves, give or take insulation thickness.)And you can always use windings in parallel, wider winding area, or multiple interleaved sections with their own shields, to reduce the winding impedance.Leakage is higher still if you use double shields, and also Cp is about as high as it's going to get (the shields are spaced only by insulating tape), but that can still be good enough where shielding is critical.Shields are returned to RF ground, usually DC link. Simple enough. Maybe not the cleanest point in some designs (DC link picks up the noise from main-switch-to-heatsink capacitance, when applicable), but a lot better than without, anyway.I think the main downside is the complexity of winding, and, maybe foil isn't so easy to automate? Idunno. If it can be done with layers of wire, that seems to be the cheaper way. So, balanced circuits are better: 2-switch and full-bridge over half-bridge, etc.Tim

author: David Hess
author_rank: Super Contributor
timestamp: October 12, 2020, 01:54:11 am
content: Quote from: T3sl4co1l on October 12, 2020, 01:00:29 amI suspect that limits the effectiveness; it would have been just as easy to decouple the primary and secondary at RF, which is the modern practice that leads to the problem, so an electrostatic shield is not a solution, or at least not a panacea.QuoteI have a very old high isolation power module somewhere which oddly enough has a clear enclosure. The transformer is like nothing I have seen elsewhere; I cannot even describe it. I suspect Old Ones technology as it looks like something Escher would design. Maybe a clear enclosure was used so people could see it and be driven insane.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 12, 2020, 04:02:15 am
content: Quote from: David Hess on October 12, 2020, 01:54:11 amWhat do you mean, the "Y" cap? A switching supply can't have both ends of the windings decoupled of course...The "Y" cap is there to shunt the current from the primary and secondary capacitance, which depends on their common mode voltages. In an unshielded one-switch flyback converter for instance, the primary common mode is Vsw/2, rather substantial. With shield connected to DC_N, the common mode is, well, if not zero, something a lot less than Vsw/2.Or in the two-switch flyback, the primary CM is ideally zero, but the propagation delays to each switch won't match perfectly. The residual CM has a peak amplitude +/-Vsw/2 and width equal to the difference in delays. The component at Fsw can be small, but the harmonics are rich. Easily CM filtered, but would still benefit from shielding.QuoteIt's not made by Sony is it? They liked to use old techniques like saturable reactors: four-peg transformers were a typical sight in Trinitron chassis. But I'm not sure what use that would be there (besides the usual)...Tim

author: BrianHG
author_rank: Super Contributor
timestamp: October 12, 2020, 05:17:03 am
content: Filtering a switcher's output is child's play.Routing that output takes a little more skill to prevent transfer through of switching noise.Isolating that bridge to AC mains requires specialty switcher transformer and core.It sounds like you are making something like an instrumentation or audio dac. In the audio DAC world, these are the types of transformers some of the pro use: (Just scroll down, you'll see all the special transformers. Some with both an internal and external shield where you get GND connections for you mains GND to further separate mains and secondary.)https://nl.aliexpress.com/af/Type-R-audio-transformer.html?d=y&origin=n&SearchText=Type+R+audio+transformer&catId=0&initiative_id=SB_20201011211252

author: KT88
author_rank: Frequent Contributor
timestamp: October 12, 2020, 09:57:25 am
content: As mentioned earlier ripple or differential mode noise is not a challenge like you confirmed.But any isolated power transmission comes with some level of common mode noise - except photovoltaic power transmission over considerably long optical fibers. But both efficiency and cost rule them out in the vast majority of applications.Another one would be charging and replacing batteries which the TS asked about a replacement for...Among the other isolated power transfer methods there are a lot of techniques to improve the common mode noise transfer.And no, an an audio transformer does not come without CM noise...CheersAndreas

author: ezalys
author_rank: Frequent Contributor
timestamp: October 12, 2020, 02:27:00 pm
content: Are people able to look at the diagram I posted? I think ultimately I still have basic questions about avoiding the creation of ground loops in a lab environment. Ultimately this is going to be used in an experiment where I have to single-point-ground everything to a cryostat containing the DUT in the center of the room, and will be connecting to the cryostat with SMA and BNC cables where the shield of the cables is shorted to the cryostat. We have a “clean earth ground” in the room, though I’m not even sure what that buys. It just seems to me that if we connect a BNC cable between instrument and cryostat, and then we earth the cryostat and earth the instrument, now there’s a ground loop! Will the common mode chokes I added on the BNC connectors break this loop? Should I just earth everything through the cryostat?

author: mag_therm
author_rank: Frequent Contributor
timestamp: October 12, 2020, 03:50:14 pm
content: I would suggest to connect P close to the incoming power supply and the other two Q & R, initially connect them together to a ground bolt near your output bnc's.Are you using the twinaxial BNC connectors as shown marked "out" on you diagram?I recently finished a pwm dc-dc converter project were noise mitigation was a priority.The dc-dc converter acts as a boost regulator, sitting on the unregulated 5Y3 + 125V anode voltage rectifier of a Lafayette HE30 HF receiver.The +V supply runs all over the receiver wiring including to the plate and screen of the first RF stage.I have no way here to measure the noise on output of the dc converter except it has on/off switch which shows no change in the reception noise.The dc-dc converter is mounted in a separate steel chassis with plated steel floor, allowing short bolted grounds to the circuit.The only problem I have yet to fix is that when a strong signal or RF gain control sends the tubes far into cut-off,the inverter phases right back to the bottom of the sawtooth and bounces there causing loss of regulation and AF noise.