LT1533 + LT3045 Low noise DCDC pcb, suggestions needed - Page 1
Forum section: electronics / projects

author: Echo88
author_rank: Frequent Contributor
timestamp: March 06, 2023, 12:10:09 pm
content: As a voltnut one needs an isolated supply with as low differential/common mode noise as possible.So heres my idea of that concept using a suitable dcdc-controller LT1533 + LT3045 as the best LDO currently available for that task concerning PSRR and noise.I followed the layout suggestions of the LT3045-Evalboard closely (i think):-overlapping +-Vin strip of a few cm length to cancel AC-coupling to Vout-output cap of the DCDC is not sitting next to the LDO, therefore no DCDC ripple magnetic coupling to Vout of the LDO-LDO output cap layouted in 4-wire fashion-added shielding (according to page 20 of AN-159)But im still worried about the correct layout thats necessary to get the full PSRR-spec of the LT3045 and would like to know what to improve.I dont know if the separated ground layers are correct on my pcb:-on the bottom layer the overlapping V- strip connects via a short trace (in my case a jumper) to the rest of the many LDO-ground layers, so thats why i separated the ground layers between the DCDC rectification/filter-stage and the LDO-stage -> is that correct and would connecting the ground layers cancel the PSRR-improvement of the overlapping +-Vin-layer-strip?Whats still missing so far/needs to be improved as far as i can see:-missing ferrite bead against high frequency spikes, maybe as an EMIFIL + shield as used by TiN or as suggested on page 20 AN159-deleting ground layer beneath common mode choke/filterinductance?-decrease the size of the rectification/filterstage to allow more room for the DCDC-controller-more gound stitching vias on the layers-improve common mode noise measurement BNC jack with shorter traces?-the ferritebeads (Toshiba Amobeads) in front of the transformer/directly behind it were added as suggested by AoE3 X-Chapters to suppress high frequency spikes, populate as neededHappy to hear your suggestions.Relevant documentation:https://www.analog.com/media/en/technical-documentation/data-sheets/lt3045.pdfhttps://www.analog.com/media/en/technical-documentation/user-guides/DC2491AF.PDFLT3045 Evalboardhttps://www.analog.com/media/en/technical-documentation/application-notes/an70.pdfLT1533 Appnotehttps://www.analog.com/media/en/technical-documentation/app-notes/an-159.pdfLT3042 Appnotehttps://xdevs.com/article/lxref/LTM8049 + LT3045 power supply by TiN

author: Kleinstein
author_rank: Super Contributor
timestamp: March 06, 2023, 04:29:03 pm
content: Ferrite beads between the drivers an the transformers make very little sense. One usually want's low parasitic inductance here, as this part of the magentic energy feeds ringing and ends up in snubbers.For the promary side it is more to consider snubbers, though not absolutely needed with slow "switching".I don't think the direct capacitve loading of the secondary side makes sense.For additional filtering one could consider a common mode filter direcly at the supply input. One would than have shielding still from before the filter.The supply and outtput filter are mainly with polarized caps, which is unsual. Normally there should be some MLCCs too. Depending on the frequency one may skip the eletrolytic ones, but hardly the MLCC fitler capacitors.The foot-print for the transformer is tiny, which suggest a rather high frequency or low power. With low power the filter inductance at the output side is a bid on the low side.A big part for a low noise is the choice of the transformer, to reduce the common mode coupled signal. This part is hard to filter and thus best avoided at the "source".Low capacitance transformers tend to be a little larger to allow for some distance to keep the capacitance low.For use with relatively low voltage a full bridge rectifier causes more loss than a center taped secondary and only 2 diodes.If the PWM dirver is used with regulation one would need the inductors for L1,L3,L5, L10? and not common mode chokes (at least if low leakage).With anyway an LDO on the secondary one may get away with a fixed transformer ratio and thus without the feedback.If in doubt there could be another fitler stage between SMPS and LDO part, just in case or maybe just ferrite beeds.

author: AnalogTodd
author_rank: Regular Contributor
timestamp: March 06, 2023, 06:18:21 pm
content: I would agree with Kleinstein on his comments. Ferrite beads between drivers and transformers as well as direct capacitive loading of the transformer secondary sides are probably not what you want.With all that said, there are a couple things that are helping you here:1. The LT1533 allows you adjustable slew rate to help control what is happening in terms of switching edges. Slowing the edges is a hit to efficiency, but it helps in controlling the high frequency that is so hard to clean out of a supply line.2. Filtering can help prevent signal from getting to the LT3045 to begin with. Parasitic effects of components must be considered however.3. The LT3045 gives excellent supply rejection as long as attention is paid to layout and an understanding of the coupling possibilities is understood.The LT1533 gives better noise performance with the slower slew rate because you bring the switching edge down to a frequency that is easier to filter/reject. Switching edges of 1nsec are over 1GHz frequency content, while slowing that to 1usec now puts things in the MHz range. Higher frequency means higher energy content and easier transmission and coupling of the signal.When it comes to adding ferrite beads and capacitors as a way to limit noise/signal that might reach the LT3045, one should understand what capacitors and inductors look like across frequency. Each component has parasitic factors that change the resultant impedance across frequency: capacitors have series resistance and inductance, inductors have series resistance and parallel capacitance. What this mean is that at some high frequency, capacitors will look like inductors and inductors will look like capacitors.The way an LC filter works is it is an impedance divider; at low frequencies, the inductor is low impedance and the capacitor is high impedance, signal passes through. At the resonant frequency, these impedances are equal and the signal through it is 50%. Beyond the resonant frequency, the capacitor is lower impedance than the inductor and low amounts of signal get through; imagine it as a resistor divider where the values change across frequency. The issue at high enough frequencies is making sure the impedances are correct, and parasitic effects reverse things. Standard inductors look capacitive at high frequencies and the capacitors look inductive; the impedance divider is going the wrong way! A ferrite bead works well because it doesn't have the parasitic capacitance and it is high impedance at the frequency of concern (switching edge frequency). You want to couple that with a capacitor that has a low impedance at that frequency, which may be 100's of pF up to the uF range depending on switch edge transition speeds. MLCC's are nice because of their low series resistance.As for the layout, this is what turns out to be the most critical aspect of using the LT3045. You need to think about where your AC currents will flow. The reason for the strips of copper to and from the LT3045 is to force the AC current to return directly under the feed. This minimizes the loop area and the field created by the loop. At the same time, the output device, load, output capacitor, SET pin capacitor, etc. all create minor loops except that these are orthogonal to the input loop. This severely reduces the coupling coefficient between the input and output.Because of your slow switching edges and distance, you may get away without shielding. Again, higher frequency edges are harder to control because of parasitic effects, with a slow enough switch transition you get away from where those effects come into play.

author: Martinn
author_rank: Frequent Contributor
timestamp: March 06, 2023, 08:10:26 pm
content: Quote from: Echo88 on March 06, 2023, 12:10:09 pmNice, they are back on stock... 93 pcs at Mouser. Last time I checked they were unobtainium.Recently came acrosshttps://www.ti.com/product/UCC25800-Q1"ultra low EMI resonant transformer driver" - maybe an alternative?I like high impedance (kOhms) ferrite beads in output LC filters.When using LC filters at the output, make sure you don't accidently hit their resonant frequency with your switching frequency, this will give disastrous results (guess why I know this).See this AN:https://www.analog.com/en/analog-dialogue/articles/ferrite-beads-demystified.html

author: Kleinstein
author_rank: Super Contributor
timestamp: March 06, 2023, 09:36:09 pm
content: The LT1533 is more made to keep the EMI emissions for the chips low. Many of the examples in the DS are for non isolated cases, where the coupling capacitance is not that important. A transformer with poor coupling may still work, but may well need more snubber network / lower slew rate and less efficiency.The typical applications for the UCC25800 are not really very low noise ones. It is more like applications that need low capacitance of the transformer to isolation against common mode noise from another source (e.g. high frequency inverter). So the US25800 may still be emitting some EMI.A resonant converter may work well even with a not that great coupled transformer and this can help to build the transformer for low capacitance.The design is quite a bit more complicated with getting the resonance capacitors and inductance right. This is OK for a high volume product, but not that great for a more low volume thing.The suggested transformer from the DS example has only 0.7 pF of coupling capacitance, but no shields.So I am afraid the UCC25800 is not really an alternative, though one could still give it a try. It may still work out OK.

author: Echo88
author_rank: Frequent Contributor
timestamp: March 08, 2023, 06:15:20 pm
content: The pcb was designed back in 2021, the point of the secondary caps C7/C2 escapes me right now (probably CRC-snubber-idea) and quite a few things like the rectifier diodes/ferrite beads were designed to be populated as needed.Anyway, since the pcb really didnt have defined filters and was generally not quite thought through i redesigned it as attached.As suggested i did a filter-analysis, seems to me it should work quite nicely (at least with standard 50R-source and load, real life might differ considerably) at the chosen 8kHz LT1533 switching frequency without filter resonance problems.The design also throws away the idea of using standard smd transformers for this task.It concentrates completely on using the pickering transformer, to prove its claimed common mode isolation superiority, as thats what really differentiates a DCDC with CM-noise but low DM-noise from a battery.Any thoughts or found errors on the new design?Edit: The UCC25800 and its transformer also sitting on my shelf, havent yet come around to design a board yet thoughEdit2: I hope the parasitic values for the components are correct, couldnt find any ESL for the electrolytics and the ESL/ESR for the MLCC were guessed. Happy to hear if thats completely wrong or if the pcb will screw the filtercurve anyway severely.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 08, 2023, 08:36:20 pm
content: 100 nF are readily avaiable - no real need to go smaller in the filter, so why only 10 nF. If possible ringing is an issue, one may want space for resistors in parallel to the inductors.The inductors like L3 and L4 should have a bit more space in between. At least some types ( e.g. simple bobin type core) can couple to it's neighbor.I think 8 kHz is rather low for the Lt1533 and it would need quite some turns. It would prefer more than some 15 kHz to get away from audible frequencies, even though the magnetorstriction ideally would be 2x the AC frequency.In most cases one would not need a CM choke on both the primay and secondary side. One could still have both positions to test.The Regulator feedback can normall use a more normal, slower opto-coupler. not need for fast one - especially not with a low clock frequency.

author: Echo88
author_rank: Frequent Contributor
timestamp: March 08, 2023, 09:15:32 pm
content: Good suggestions, thanks.The 10nF were chosen due to the idea of less ESL and therefore effective filtering at higher frequencies.But maybe its irrelevant due to my layout at really high frequencies, so using 100nF will of course also be okay.Havent thought about the inductance coupling between L3/L4. I will create more distance between them.I tried to mostly use the component values based on the Fluke 7000 from branadics article, which includes the 8kHz, transformer specs and the used fast optocoupler. Dont really know why they used this low frequency of 8kHz (even lower 4kHz in the older F7000 versions) and therefore chose to make the Sync-input available as well as the pots for the slewrate-setting.Personally i would just let it run without feedback at 50:50 duty cycle for testing first.Im unsure about the effectiveness of the CM-chokes in this exact case as well, populating/shorting them will depend on CM-measurements as well as the exact fitting value for the cap between the transformer shields.

author: David Hess
author_rank: Super Contributor
timestamp: March 09, 2023, 02:52:38 am
content: For an isolated but truly low noise supply, besides the LT1533 I would also consider linear sine wave drive to a transformer, and then to remove common mode coupling, use two transformers in series with the coupling winding grounded.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 09, 2023, 08:36:29 am
content: Quote from: David Hess on March 09, 2023, 02:52:38 amTo really get low coupling it needs 2 shields, one on the primay and one on the secondary side. The idea with 2 transformers and grounding the coupling is still good - just have 3 and to have 2 series windings to ground. In this case also sine wave drive makes absolute sense, though with the often somewhat nonlinear magnetics the current may still be somewhat distorted. The downside with sine drive is extra heat loss.

author: Echo88
author_rank: Frequent Contributor
timestamp: March 10, 2023, 11:44:43 am
content: The usage of two transformers in series is indeed worth testing, its also described as a variation in the pickering patent, grounding coupling windings isnt mentioned though. ->https://xdevs.com/doc/branadic/PSU_P1/Ref18.pdfPage 7 and 8The sine drive sounds logical for smooth low noise performance, but in reality thats apparently not needed:-for differential mode noise slew rate limited square drive is good enough as proven with the LT1533, were the noise becomes so low its very difficult to measureSine drive might have a slight advantage here though, i just consider the LT1533 + LT3045 good enough for this test so far-for common mode noise tests done by Horowitz and Hill indicate that sine drive doesnt prove better than trapezoidal/triangle/slew rate limited square wave drivehttps://x.artofelectronics.net/wp-content/uploads/2019/11/9xp14_low-noise_isolated-pwr.pdf"However, the drive waveform need not be sinusoidal, as long as it does not have discontinuities – it’s OK to use a triangle wave (or a slew-rate-limited square wave), as in Figure 9x.59."

author: branadic
author_rank: Super Contributor
timestamp: March 11, 2023, 07:39:22 am
content: Hopefully, you have spot the note?Quote-branadic-

author: Echo88
author_rank: Frequent Contributor
timestamp: March 11, 2023, 03:47:50 pm
content: Hmm, as far as i can see my schematic follows this note and your schematic apart from the exact cap value which will be subject to CM-noise measurements or did i miss something?Im thinking about setting a jumper between secondary screen and secondary ground, so it can be tested how the CM-noise changes with/without connected screen to ground.

author: gyoung
author_rank: Newbie
timestamp: February 11, 2024, 08:16:11 pm
content: Quote from: David Hess on March 09, 2023, 02:52:38 amDo you mind providing a sketch of what you mean by the series transformers and grounded winding? Thanks!I've attached an image of what I think you mean...

author: David Hess
author_rank: Super Contributor
timestamp: February 12, 2024, 08:10:58 am
content: Quote from: gyoung on February 11, 2024, 08:16:11 pmThat is it, although it looks odd when you draw it. This is a alternative to having a transformer with electrostatic shielding. I have seen it done in some test instruments like multimeters which have a galvanically isolated set of inputs.