Noise model of an SMPS - Page 1
Forum section: electronics / projects

author: ezalys
author_rank: Frequent Contributor
timestamp: August 08, 2021, 04:20:42 am
content: I'm trying to find a rough, general circuit model for the noise and pass characteristic of a switch mode supply, ignoring the voltage change. As far as I know, they look like a voltage source adding harmonic content, and a high pass filter of some kind that cuts in around the loop frequency. I reckon there's also a step down in the impedance seen by the load of the supply. I really just want to answer basic questions like: If I use a decent medical SMPS brick and follow it by a fancy slew rate limited push-pull (like a LT1533) do I wind up with noise that looks more or less like the medical brick alone, or is it improved by the LT1533? Is this noise easy to filter? How do I know what will be attacked by the passive filters, the LDOs, and the switchers and by how much? I think common and differential mode mode models would be very useful, too, as common mode noise is a pretty big deal in my application.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 08, 2021, 05:15:28 am
content: You're in need of a few more questions first -- what kind of noise? Conducted, radiated, CM, DM? For EMI purposes, or merely functional?In other words, your questions are basic, yes, but I'm afraid simply too basic -- the only honest answer they could get is: "it depends".To be specific -- how would that LT1533 circuit be designed (schematic and layout)? How is the transformer wound? Does it have internal shields? Does the output share an AC ground reference with any outside connections (mains earth, comm ports, chassis)? Does the output care about CM at all (e.g. the load is all inside its own metal box and has no other connections)?There is an unlikely response of "probably / yes", because -- assuming the push-pull stage is transformer isolated -- then that acts in series with the power brick's own isolation, and yeah LT1533 should be fairly quiet, or at least easily filtered. So it could be improved. Who knows. There are cases where it's worse, too; and in any case, it's impossible to say by how much, without those missing data. And if you're doing it common-ground (as if a buck-boost stage or whatever, no isolation), you're probably not doing anything more to DM noise than the equivalent number of LC components would've done, and by definition you're not doing anything to CM.But even more so, you should be doubly, triply leery of any such value judgement language -- what does it mean to be "good" here? Is it "good enough"? Is it "too good", or "overengineered" (whatever that means)? Nah, get this messy English out of the way, we need numbers! Well defined ports! Impedances, decibels and hertz!Also, to be clear, when you say "pass characteristic", you mean the AC response from one port to the other, e.g. ripple input to ripple output, uncorrelated to the converter's own input and output ripple (disregarding internal noise)?If so, generally speaking a regulated SMPS has a bandpass characteristic, where PSRR and regulation (that's dVout / dVin, and dVout / dIout, respectively) are very good at DC (not perfect, but limited by error amp DC gain), rising at middle frequencies (where error amp gain is falling as frequency rises), peaking somewhere below the crossover frequency, and falling again above there (dominated by passive filtering at the input and output).But there's also a nonlinear component due to the mixing effect of the modulator (particularly evident in full-wave converters, where supply voltage and PWM are amplitude modulation around Fsw), so we should be careful about which things we mean.To serve the first case, we might choose a cycle-averaged SMPS model, which categorically excludes switching ripple, harmonics and conversion products, focusing on just the baseband response. In such a case, we can consider the small-signal two-port response, just as easily as we would an audio amp. But such a model is also only valid up to the cutoff frequency, or to Fsw, at best, so it can't answer all our questions. (It would be an effective method to calculate immunity to mains ripple, but not to the ripple from other converters.)Tim

author: ezalys
author_rank: Frequent Contributor
timestamp: August 08, 2021, 05:59:09 pm
content: QuoteLet's restrict to conducted noise for purely functional purposes. This is again about the 6 channel 20-bit DAC project for bias voltages that are sinked inside a cryostat that you've so graciously answered so many questions about.QuoteFair enough -- I ask somewhat general questions that are qualitative in hopes to find places where these problems are treated in generality. Ott's Noise Reduction Techniques has been amazing, for instance, but I also understand that these are questions that only have answers through experiment, experience, or in the collective consciousness of the greybeards.QuoteI would like to consider a power supply like the followinghttps://cdn.shopify.com/s/files/1/1631/5609/files/Blog_LowNoiseSplitRail_01.PNG?v=1496934506, except with an LT1533, substituted so I can slow the slew rate down. I would likely use a medical grade SMPS brick instead of the USB port in the interest of reducing common mode noise. I would choose a similar Wurth SMT transformer, though alternatives with Faraday shields that can run at 500 kHz-2 MHz in similar push-pull configurations would be super useful. The grounding well, I don't know. I imagine I'll have to experiment. I will probably add grounding posts to the PCB and play with grounding the input stage to the chassis, the output stage, the grounding of the chassis to the input brick, all these things. I really don't know. All I know is in the end, this whole circuit will be inside a big metal box, and I'll have SMA connectors connecting the DAC outputs to devices in a big metal shielded cryostat, and in the interest of breaking ground loops, I would like the option to float the common ground of the converters such that it can be shorted to the cryostat.QuoteSo yeah, in this design, the push-pull is transformer isolated, and I suppose the Brick is too. I also guess that grounding these in different ways (brick to earth, 1533 to instrument gnd vs brick to instrument gnd and 1533 to instrument gnd) will yield different noise characteristics, also depending upon the load and the load's connectivity to earth. I think what I really want is to know what can be done about the conducted switching harmonics of the brick.QuoteSo I guess I can try to be more concrete. The instrument I'm designing only needs to provide DC biases, so a pass band of 0-20 Hz. I'm trying desperately to not spoil the noise characteristics of the AD5781, 7.5 nV/rtHz, whose buffered (OPA2210) +/- 10V output will bias a 50 fF load. Again, 6 converters, all sharing an instrument ground, which is galvanically isolated from the earth ground, such that the instrument ground can be bonded to the cryostat in a star-grounding configuration. The goal is to do all this in a 1U metal rack-mount chassis and power it from a high quality SMPS brick I can buy on digikey. I have the budget to do this with an Acopian linear supply and use a few more rack units, and that's what I likely will end up doing in the interest of saving time. However, I've been fascinated by this problem of trying to make a 7.5 nV/rtHz +/- 10V voltage supply powered by a commodity brick and feel like I've learned a lot asking questions about how to go about it.QuoteYes that's exactly what I mean. I was looking for a common mode and differential mode pass characteristic.QuoteThis works for DM noise, but what about CM? I guess that would be mostly the shunting parasitic capacitance of the primary and secondary, divided by the Y-cap? What else?QuoteYeah... that's good for 60 Hz. I'm currently fixated on how to "clean up" the brick, which I reckon probably has CM and DM content in 100 kHz-1 MHz fundamental and harmonics out to hundreds of MHz.

author: ezalys
author_rank: Frequent Contributor
timestamp: August 08, 2021, 06:02:46 pm
content: I think the full circuit would look like the following (stolen from a previous thread)https://imgur.com/a/BZdtUyx. I just don't know how P Q and R should be connected to the cryostat or earth or chassis.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 08, 2021, 11:32:29 pm
content: What were you using for the signal outputs again, just a whack of BNCs? I suppose I wonder if a D-sub might be better, so they can all be under one shield; and I guess capacitive coupling between them makes very little difference, thanks to the low bandwidth, and AC-coupled load (RC bias tee, wasn't it?).But anyway, yeah, those all come off the cryostat, which we might treat as AC ground for sake of analysis.There might be some difference between cables, due to induced loops; these will be more or less unavoidable, and if significant enough (requires knowing ambient field intensity / inhomogeneity in the lab area, relative to the signal level), may dictate some combination of ferrite beads and bypass, or a differential buffer (output signal = cable ground + input signal; cable ground is used as a reference level, and allowed to move a modest amount with respect to circuit ground, at least at low frequencies; it should probably be bypassed for HF, so ferrite beads can do some good).Likewise between bias units, likely a larger difference, due to mismatched cable paths and/or lengths. So it may be unwise to bond grounds between units, and indeed, you've got isolation on USB and power to cover that.This may be adequate consideration already: the signals are shielded, the bandwidth is low, and the transmitters are referenced to the cable as well as can be. That's good for more than a few decades already.If you don't want to worry about SMPS noise (at least from outside the circuit; the onboard converter(s) are easy enough to LC filter and/or LDO), you might still consider using a low capacitance (toroidal or split bobbin? medical grade) mains transformer, and good old fashioned linear unregulated supply (in turn with PoL switching regulator, again because it's easy to filter).The question of, if SMPS brick then what kind and how much filtering, or mains frequency transformer; does indeed depend on the expected noise level of the mains input, and the CM transfer function of the unit in question.So a good point might simply be to avoid that problem entirely, say with a line voltage conditioner. This might simply be a modestly effective filter, grounded somewhere around the star ground (so, probably on the cryostat), or a whole-ass ferroresonant transformer (generally cleans up the mains quite a bit in the process, it has a bandpass characteristic), or even inverter I suppose.We could move the star ground inside the rack along with the bias units, bringing galvanic ground along with the BNC cables to avoid loops; indeed perhaps as an additional shield, if possible (get a bigass Sealtite conduit to run everything in?).Which would also be a fine place to ground the, whatever the USB is hosted on, PC I suppose?A certain amount of filtering is afforded even by just an isolation transformer, which can be placed in the rack, and its output neutral tied to chassis ground, thus shorting out the common mode inside the rack itself; mains filtering then amounts to simple normal-mode filtering.You can always apply CM filtering after an SMPS brick as well; the ground gets referenced to the load instead, leaving a high CM impedance facing the brick. You can't have any lower noise than (filter attenuation) * (noise between mains and local ground) this way, of course. And the same applies to USB paths, which should be well isolated (low capacitance). (Believe I mentioned last thread, there may be value in a C or R+C across the USB isolation path; this is equivalent to an R||L at the host end of the cable, which if it is known, as in this case say -- a ferrite bead there should do about as well.)Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 09, 2021, 01:21:26 am
content: Oh yeah, and this thread may prove interesting reading:https://www.eevblog.com/forum/projects/need-guidance-on-everyones-favorite-subject-grounding!/Tim