Power supply for voltage references - Page 1
Forum section: electronics / general

author: branadic
author_rank: Super Contributor
timestamp: December 16, 2020, 02:18:59 pm
content: Hi all,triggered by the W/F7000 I thought it would be interesting to talk about power supplies for voltage references. Battery power is indeed a proper solution and Andreas showed a design that provides the voltage out of a set of 12x NiMH for the LTZ reference via an LT1763, while applied with BMON (battery monitor) a charge circuitry is present.But what about mains power, with extrem low coupling capacity?John Pickering obviously developed a DC-DC converter solution that seems to work pretty well. So I was looking for info about it on the web and came across the following papers, describing the solution. Carefully reading all three of them about the xformer together with the pictures of W7000 available on xdevs provides an overall picture how it's done:Design of a 10 mA DC current reference standardDesign and evaluation of a 10-mA DC current reference standardA new compact isolated power supply for electrical Metrology at low signal levelsSo I wonder if anyone else created a similar solution, providing high isolation between primary and secondary, so that it is possible to power the reference from mains. What xformer was used?Edit:Guidance on eliminating interference from sensitive electrical circuitsEdit:Design and Evaluation of a 10-mA DC Current Reference Standardstates a frequency of 8 kHz for the DC-DC converter, which agrees with the frequency I can measure on my F7000 based on LT1533-branadic-

author: Kleinstein
author_rank: Super Contributor
timestamp: December 16, 2020, 03:05:19 pm
content: For extremely low capacitance / high voltage, there are acoustic transformers (piezo - isolator - piezo). However they are relatively exotic and usually for low power (like 1 W).The usual small higher frequency transformers work better with a low voltage. So this would likely be in 2 stages: 1st a conventional mains transformer to some 5 V DC and than a special, low capacitance ferrite or similar transformer. Dave sowed a nice example from the Clever-scope.If efficiency is not that important, on can have the primary and secondary windings on opposite sides of a relatively large ring core.

author: exe
author_rank: Unknown
timestamp: December 16, 2020, 03:20:20 pm
content: Am I correct that you want a tranny with low capacitance between windings to avoid getting noise from mains?R-core transformers?I resist buying one, although I may pull the trigger. Alternatively, a toroid with a shield between primary and secondary might also be an option. May be even a conventional EI transformer with a gap between windings would suit you?Anyway, even if we eliminate capacitance between windings, magentic coupling is still quite strong up to some tens of kHz or even beyond. I know this because people use small EI-trannies as injection transformers.

author: doktor pyta
author_rank: Frequent Contributor
timestamp: December 16, 2020, 04:23:06 pm
content: Yup.Resonant current fed push-pull operating at about 20kHz.As far as I remember parameters were:Supply:12 V, output 16V @0.3A. Efficiency 50-60%.Capacitance C(in-out)=4pF.Pros: very easy to build with commercial components, I've never observed problems with start-up.

author: BravoV
author_rank: Super Contributor
timestamp: December 16, 2020, 04:34:38 pm
content: Quote from: Kleinstein on December 16, 2020, 03:05:19 pmThis one ?

author: tszaboo
author_rank: Super Contributor
timestamp: December 16, 2020, 04:44:27 pm
content: I have two ideas for you:- multi stage transformer (230 to 48V, 48 to 12)- Jim Williams magic:https://www.analog.com/media/en/technical-documentation/application-notes/an29f.pdfThere is a circuit, where he drives a transformer with a 1-2 KHz sine wave that comes from a current boosted opamp. Crap efficiency, but small and not noisy, so who cares.

author: notfaded1
author_rank: Frequent Contributor
timestamp: December 16, 2020, 05:29:41 pm
content: Does anyone know why they refer in the IEEE paper to "The value of 10 mA, based on engineering considerations, seemed suitable for all of the following applications, allowing a single design of dc current reference, referred to as the PBC, to cater for each." Anyone have any idea why they called them PBC? Just curious.Another couple questions... maybe dumb but it says "The five prototype units were each connected to a 100Ω resistor and measured daily with an MIL 8000 against Fluke 732B standards traceable to METAS/Bern. The long term drift can be seen in Fig. 2. When the measurement resistors were replaced by 1 kΩ , the inter-comparison could be made directly with the Fluke 732B 10-V standards through the DataProof scanner and uncertainty reduced. It is estimated that a group of four units can easily provide the 10 mA with an uncertainty of five parts in 10^7 constantly maintained.What's a MIL 8000 and a DataProof scanner? I'm guessing the DataProof scanner comes from here:http://www.dataproof.com/Bill

author: Vgkid
author_rank: Super Contributor
timestamp: December 16, 2020, 07:23:35 pm
content: Measurements international binary voltage dividerhttps://mintl.com/products/model-8000b-10-volt/

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: December 16, 2020, 07:54:11 pm
content: The construction details of the GR shielded bridge transformers are interesting-https://www.ietlabs.com/pdf/GR_Experimenters/1935/GenRad_Experimenter_Oct_1935.pdfYou can also shield a toroid with copper tape.Me, I'd probably use a couple solar cells, lit up with some big incandescent lamp! Can't get much better isolation than that.

author: dietert1
author_rank: Super Contributor
timestamp: December 16, 2020, 09:40:34 pm
content: After experiments with high frequency converters (art of electronics) i am now using something very simple and old fashioned. It's based on a small two chamber mains transformer with low coupling (20 pF) plus two extra 2K2 resistors on the input side (RF suppression). Like all two chamber transformers the secondary needs a proper snubber. Then there is a LM723 regulator with a 30 mA current limit. Outputs are adjusted to 15.000 V with fixed resistors. Temperature drift remains below +/- 3 mV in our lab. Image shows a 750 Ohm resistor i used for test (20 mA at 15 V). This circuit will inject less than 1 uArms current at 50 Hz into the ground line, so with reasonable wiring errors will be a fraction of a microvolt.I found that our LTFLU references shifted by about 0.15 ppm when i replaced the previous LM7815 based power supplies (e.g. from 14.79 to 15.000 V). That means even a well designed reference will change when you change the supply voltage by such an amount.Regards, DieterPS: I made six of those and have parts for more. Open two chamber transformers where you can ground the core are superior, but the minimum size i know of is 12 VA (much bigger) and the difference in performance is small.

author: babysitter
author_rank: Frequent Contributor
timestamp: December 16, 2020, 10:05:57 pm
content: I digged trough a old Siemens patient monitor with exchangeable modules, they used a IR communication and two pot-core ferrites for power transmission, mounted like "E 3" . Think inductive charger. They did it for low leakage and cleanable surface, but should give low capacitance too. Maybe a royer circuit would be a good start for the primary part. Load the secondary to prevent high output voltage.

author: SilverSolder
author_rank: Super Contributor
timestamp: December 16, 2020, 10:57:39 pm
content: Quote from: Conrad Hoffman on December 16, 2020, 07:54:11 pmI have a couple of double shielded tranformers made by Singer/Gertsch and they are also 1:4.What is the reason for these double-shielded transformers often being made with a 1:4 turns ratio?

author: branadic
author_rank: Super Contributor
timestamp: December 16, 2020, 11:22:58 pm
content: QuoteI must add that W/F7000, F7001, F7004 and F7010 are powered by a Hitron Model HER-57-312 wall adapter with 12.88V and 1.2A. This powers the DC-DC converter, so it's already kind of a two stage concept.Attached is a measurement of the averaged output on F7000T, running the modules from their internal batteries in the first half and from the wall adapter in the second half. As far as I can see there is absolutely no difference. One can repeat that of coarse on the individual units too, just in case there was some cancelling effect going on.-branadic-

author: niner_007
author_rank: Frequent Contributor
timestamp: December 17, 2020, 05:27:38 am
content: I was thinking of starting a thread on it the other dayMy thoughts on this are to use rechargeable batteries, stored in the same heavily shielded metal enclosure, alongside the reference, like in a Fluke 732. For the batteries, something practical with lots of energy and long life, like LiFePo or LiIon. I do not know what kind of noise these have, if lead acid has lower noise, that can be an option too. Keep two independent battery banks, one that is being charged, and one that is used to power the reference. As one bank is completely discharged, you switch over to the other bank. That way you remove any noise from ever entering the system from the outside, with runs with the assumption that the batteries are relatively low noise.For regulation or step down to a practical range, I was thinking of a simple npn - zener regulator, using a low noise zener diode (1N5518, 1N5519, 1N5520, 1N5521, 1N5522, 1N5523). Another option that could be great to try is using the LTC6655 low noise reference. It could also be used to reduce the noise of the LT3042, which on its own has something like 10uV of noise 0.1 to 10Hz, and 2uV of noise 10Hz to 100kHz.

author: TiN
author_rank: Super Contributor
timestamp: December 17, 2020, 06:53:22 am
content: QuoteOuch, that means good bye shipping for calibration without paperwork/etc.

author: dietert1
author_rank: Super Contributor
timestamp: December 17, 2020, 08:21:08 am
content: I liked the Cleverscope transformer, thanks. Using a twisted pair bifilar transformer with symmetrical drive in resonant mode (like in the art of electronics proposal) may also remain below 1 uArms of current injection. It's just a matter of designing a 3D printed part to hold off the secondary. That's something to try. I still have the circuit somewhere on the bench.The AD paper AN29 mentions 30 uV somewhere, that's a lot. And high frequency is more difficult to absorb into wiring due to cable inductance.How to measure the injected signal? Use two scope channels, one to trigger on the converter frequency (proximity, ground lead remains open), the other one to measure injected signal over a 1 KOhm resistor. One side of the resistor gets connected to converter output ground and probe tip, the other side gets connected to the ground lead of that probe. Then use averaging to raise injected signal above noise. With a modern DSO this will get you down to some 100 or 500 uV sensitivity, which then corresponds to about 0.1 to 0.5 uA injected current. Need to avoid clipping though: The scope power supply also injects its own asynchronous noise. Clipping will screw up averaging, so you may have to adjust scope sensitivity before.Regards, Dieter

author: notfaded1
author_rank: Frequent Contributor
timestamp: December 17, 2020, 12:41:51 pm
content: @niner_007 Another nice thing about using two sets of rechargeable batteries is you can change out the one side with new batteries while the other side is running. You'd never have to come off of battery power that way ever and always have good fresh batteries.Bill

author: branadic
author_rank: Super Contributor
timestamp: December 17, 2020, 01:00:09 pm
content: Repeated the measurement battery vs. wall adapter this time on cell1 only, I leave it up to you what you can spot.-branadic-

author: notfaded1
author_rank: Frequent Contributor
timestamp: December 17, 2020, 01:16:20 pm
content: It's definitely doesn't look as stable as before. Downward slope.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: December 17, 2020, 06:06:10 pm
content: Quote from: SilverSolder on December 16, 2020, 10:57:39 pmNot a clue, but no doubt there was some sensible reason, possibly a desirable impedance conversion.

author: niner_007
author_rank: Frequent Contributor
timestamp: December 18, 2020, 12:21:38 am
content: Quote from: TiN on December 17, 2020, 06:53:22 amCan't have everything in lifeYou can build it modular, if you don't want batteries, it can be mains power. You can enable any chemistries also, and any capacities. Volumetric energy storage is what I mean, the batteries can be as low as 0.5Ah, it all depends on how long you want the system to run without external power.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: December 18, 2020, 12:33:37 am
content: Hi,The LT1533 is well suited to these applications. It has controlled slew rate for both current and voltage. This allows you to trade efficiency for low noise.Link:https://www.analog.com/en/products/lt1533.htmlIf you don't need isolation, you can look at the very similar LT1534.Regards,Jay_Diddy_B

author: dietert1
author_rank: Super Contributor
timestamp: December 18, 2020, 01:41:49 pm
content: The LT1533 specification of the injected signal seems to be 100 uVpp, including high frequency transients as visible on the scope trace, so once more a lot. That is because they are using switches, not linear drive as in the art of electronics proposal. Of course it also depends on the transformer geometry.I remember from my experiments with the art of electronics circuit, that depending on the transformer the injected signal at the secondary ground could be between 300 uArms and 1 uArms. It got lower with lower frequency e.g. 20 KHz instead of 50 KHz and when using a slotted copper foil separating primary and secondary coils. I also used plastic insulator to hold both coils away from the core. This was with a resonant circuit and with primary side amplitude regulation and after a 100 uH common mode choke on the secondary side. That circuit gets tricky if you want to reach 1uA rms or less.Regards, Dieter

author: fcb
author_rank: Super Contributor
timestamp: December 18, 2020, 02:43:35 pm
content: Interesting subject. We've used a hugely oversized E3 transformer running at 100KHz in one of products to lower capacitance between two circuits.I liked the Cleverscope arrangement, but my favourite is the Keithley DMM7510 teardown Dave did that uses coax windings. Newtons 4th (voltech) have an interesting method using single turns on ferrite rings with copper tape guards.To get round capacitively coupled noise on something i'm finishing at the moment I've taken to shutting off the PSU, taking a measurement (80us at 30Hz), and then restarting the supply (100KHz).

author: fcb
author_rank: Super Contributor
timestamp: December 18, 2020, 03:05:24 pm
content: This I've seen done with a single winding (like the joined secondaries) and with a copper tape screen on the primary.I'd probably do it with a 3D printed bracket to guide the windings off the core (reduce capacitance further). This is also used for data transmission in some power-analysers.

author: dietert1
author_rank: Super Contributor
timestamp: December 18, 2020, 04:18:28 pm
content: Yes, a similar setup was also used for data transmission in the isolation interface of the HP 3456A DVM. Don't know whether they use that interface during measurement or autozero. I think if the core is kept off by plastic insulator or spacer this will have more capacitive coupling than with (partial) air spacing as in the Cleverscope image. But that's something to decide by measurements.Up to now i decided to make something old school that is cheap and hard to beat. Anyway, i can't imagine running that mains transformer close to a voltage reference. Neither can i imagine running a high frequency converter near a voltage reference. Integrating and optimizing such a setup will be a major engineering effort.Regards, Dieter

author: doktor pyta
author_rank: Frequent Contributor
timestamp: December 18, 2020, 08:36:32 pm
content: AFAIK one of the Fluke/ Hart Scientific thermometers used two akkus or super caps. One was charged and the other was connected to the load at the time.This was patented solution.

author: Echo88
author_rank: Frequent Contributor
timestamp: December 19, 2020, 12:07:58 pm
content: Links to the circuit in the F1595a mentioned by doktor pyta:https://download.flukecal.com/pub/literature/Recent%20Advances%20in%20Resistance%20Thermometry%20Readouts.pdfhttps://patentimages.storage.googleapis.com/94/8a/8b/fdbc915129e6b9/US8120327.pdf

author: fcb
author_rank: Super Contributor
timestamp: December 19, 2020, 03:01:55 pm
content: Quote from: doktor pyta on December 18, 2020, 08:36:32 pmI quite like that approach. Could probably work if you can get a high-enough isolation/low enough capacitance between across a relay.I thought about using a motor, long plastic shaft and generator (in reality two stepper motors), if driven by low frequency could be low noise. Could even mount motors onto a PCB and use a belt. Risk of creating a van-der-graff generator? Although you could run the belt past a numer of generators if you needed to power a few channels.

author: The Soulman
author_rank: Super Contributor
timestamp: December 19, 2020, 04:38:25 pm
content: Quote from: fcb on December 19, 2020, 03:01:55 pmIf you want to go mechanical, why not use two (brushless) electric motors with a belt or a plastic axle between them, one driving the other?The switched battery/supercap idea is kinda clever tho, something like a beefed up ltc1043.

author: fcb
author_rank: Super Contributor
timestamp: December 19, 2020, 05:32:47 pm
content: Quote from: The Soulman on December 19, 2020, 04:38:25 pmI literally said that and you've quoted it.

author: The Soulman
author_rank: Super Contributor
timestamp: December 19, 2020, 05:44:39 pm
content: Quote from: fcb on December 19, 2020, 05:32:47 pmSorry about that, first time I've read that to quickly and thought you were thinking of a motor driven commutator touse instead of a relay.Reading again, I don't know what I was thinking.

author: guenthert
author_rank: Frequent Contributor
timestamp: December 19, 2020, 06:55:58 pm
content: I don't quite see the need for extreme measures for the power supply of a DC voltage reference as typically they just feed long scale DMMs measuring DC, which take their sweet time integrating the signal (Fluke 8508A takes a whopping 25s for the conversion at best resolution -- twice as fast as the old Solartron 7081) or being compared to other sources using a null meter (Fluke 845A specifies a patience-testing settling time of 5s at the most sensitive range). Substituting a nano voltmeter like Keysight's 34420A for the null meter speeds up the measurement considerably, still for best results it'll integrate over multiple power line cycles. That of course assumes that there won't be some rectifying of the signal in the meter. For null meters AC rejection is specified and can be tested.I'd think other instruments, e.g. spectrum analyzers, have much more stringent requirements for the noise-lessness of their power supplies. How are they doing it?

author: SilverSolder
author_rank: Super Contributor
timestamp: December 19, 2020, 08:06:29 pm
content: Quote from: guenthert on December 19, 2020, 06:55:58 pmOne thing I've seen done is the use of a shielded transformer for the mains supply, to avoid common mode noise capacitively leaking across from primary to secondary, I guess.

author: exe
author_rank: Unknown
timestamp: December 19, 2020, 08:17:30 pm
content: Quote from: guenthert on December 19, 2020, 06:55:58 pmLooking at tear-downs, there are many-many individual power rails in modern instruments, and most if not all of them are smps. I think in R&S instruments I've seen completely integrated modules that don't require any external components. Probably, this way parasitic inductances are small, thus reducing the noise.I've also seen decoupling capacitor networks, where multiple caps are in parallel. But you can't just wack in several caps and hope they they will have a combined effect. Usually it only makes things worse. So it takes some engineering to do it right.

author: Dr. Frank
author_rank: Super Contributor
timestamp: December 19, 2020, 09:45:08 pm
content: Quote from: guenthert on December 19, 2020, 06:55:58 pmHi guenthert,I thought just like you, until I found out in practice, that ordinary linear power supplies, w/o shield and isolation, obviously create ground loops which may shift the reference voltage by several tenths of ppm. These may also manifest in disturbance from mains into the circuit, which may create additional shifts.The worst effect was observed during the difference measurement between two mains supplied LTZ circuits, using a DMM like 3458A or 34465A, I don't remember. That measurement was complete toast, only huge spikes.I did not repeat that measurement any more by using my KEI 182A nV meter, but maybe I'll do that now that I have a Fluke 7000, against a mains or battery supplied LTZ.Frank

author: branadic
author_rank: Super Contributor
timestamp: December 19, 2020, 11:03:37 pm
content: At least a yesterdays measurement with K3458A in 100mV range worked, measuring one of the F7000 (cell2) powered from mains against a F731B, also powered from mains. The readings were stable and consistant. So it is possible, at least on commercial products.-branadic-

author: niner_007
author_rank: Frequent Contributor
timestamp: December 20, 2020, 04:05:24 am
content: Quote from: fcb on December 19, 2020, 03:01:55 pmThere are a lot of relay options out there, series connected relays might also be an option

author: Kleinstein
author_rank: Super Contributor
timestamp: December 20, 2020, 08:30:56 am
content: Ideally the reference should not be very sensitive to mains hum an similar. In most cases the circuit where such a reference is used / needed is relatively low impedance and thus should not be effected very much. It is more like that most meters would also inject some signal. In some cases good isolation at the reference could compensate for not so ideal isolation at the meter. The main critical cases I see would be using a KVD or Hammon type divider, as these have relatively high impedance.So chances are one could get away with a relatively simple mains supply (like classical mains transformer with separated windings and linear regulator to avoid possible surprises from EMI). For the few really critical measurements one could use battery power - this is kind of the standard to compare too.

author: Dr. Frank
author_rank: Super Contributor
timestamp: December 20, 2020, 08:42:38 am
content: A few years ago, I made measurements with 3 different setups, always using a 3458A, NPLC 50 in difference or absolute configuration. I used 3 references, Ref_1 and Ref_2 in one case on same supply, LTZ #5 in separate case and with separate supply:1. difference between two separate references, with 2 separate mains supplies, about 9mV apart2. difference between two references in one case, on same mains supply, about 47mV apart3. absolute measurement on one of the referencesFor all measurements I then calculated the relative deviation in ppm.First case made really ugly spikes, up to 100 times bigger than the absolute measurement.Even the 'quiet' areas between 7h-8h were much noisier compared to the absolute measurement.LTZ #5 meanwhile runs on batteries, so I might repeat this test.REF_1 and REF_2 were my 16 years old prototypes, standard LT circuit, LTZ #5 is Andreas design, practically immune to external disturbances.Absolute measurements on latter references, as well as on the FLUKE 7000 usually show noise of max 0.2ppmpponly, in 'quiet' areas even half of thatFrank

author: dietert1
author_rank: Super Contributor
timestamp: December 20, 2020, 04:43:57 pm
content: Here is todays 24h log of two LTFLU references, difference measurement with HP 3456A. Vertical scale is +/- 0.5 ppm again. Those two references are running continuously on mains, using the old school power supplies as shown above. That setup includes a second HP 3456A to measure temperatures, two unmodified Arroyo temperature controllers (with SMPS) and a HP 59401A isolated GPIB DAC for temperature control. This represents development status of May 2020.Regards, Dieter

author: MK
author_rank: Regular Contributor
timestamp: December 24, 2020, 02:16:44 pm
content: Quote from: SilverSolder on December 16, 2020, 10:57:39 pmPerhaps that guarantees that the output volts count as "SELV" even with 230V going in?

author: MiDi
author_rank: Frequent Contributor
timestamp: December 24, 2020, 10:52:48 pm
content: Quote from: MK on December 24, 2020, 02:16:44 pmSELV has to be <= 50 VAC or 120 VDC in air and w/o protective earth connection.

author: branadic
author_rank: Super Contributor
timestamp: December 25, 2020, 07:41:11 pm
content: Xmas time, reverse engineering time.LCR meter powered up to measure some inductance, multimeters to measure coil resistance, caliper to get some dimensional measurements and handheld multimeter to measure frequency on the F7000 with LT1533.Based on that I will try to replicate this mystical Pickering xformer circuitry for some DIY voltage references I have. A spice circuit was already build to prove the reverse engineering was correct.-branadic-

author: dietert1
author_rank: Super Contributor
timestamp: December 25, 2020, 08:40:04 pm
content: What is the coupling capacitance from primary to secondary of the transformer?Regards, Dieter

author: branadic
author_rank: Super Contributor
timestamp: December 27, 2020, 10:05:56 am
content: Forgot to measure it and when I thought of it, the unit was already put back together. But I can measure it once my replica is done. Boards already designed, need to order them in the next step.-branadic-

author: Andreas
author_rank: Super Contributor
timestamp: December 27, 2020, 11:39:27 am
content: Hm,in the paper I read from 2 "shield screens" (primary + secondary) consisting out ouf a polymer.Where are the shields connected to? Case or Guard?With best regardsAndreas

author: Echo88
author_rank: Frequent Contributor
timestamp: December 27, 2020, 06:45:15 pm
content: Measured the coupling capacitance of two DIY-Pickering Transformer without shield:13pF and 21pFOther normal COTS transformers suitable for the LT1533 have the following coupling capacitance for comparison:CTX02-13834 (suggested in the LT1533 datasheet): 9pFWE 314619 (substitute for some other CTX02): 25pFWE 30804 (substitute for some other CTX02): 86pFMeasured with HP4192A at 1MHz.In the end the ultimate test is to see wether someone can spot the difference when the powersupply is switched over from battery to DCDC, while measuring the 10V reference in opposition-mode.

author: dietert1
author_rank: Super Contributor
timestamp: December 27, 2020, 07:07:38 pm
content: I'd guess the one shown above by fcb will be like 1 pF.There is nothing mystical about that pickering transformer. The two core idea is the same, except the position of the two cores is different, since it is meant for power transfer. So you don't want one but more than one turn for the coupling coil.From the images i can see that the DIY Pickering transformers have more distance between the cores than the original one. So the original one will have more coupling capacitance, maybe 30 or 40 pF. And the shields don't really help, since the guard can't absorb high frequency spikes very well.Regards, Dieter

author: branadic
author_rank: Super Contributor
timestamp: December 27, 2020, 08:55:02 pm
content: Andreas, primary shield is tied to primary GND and earth / case, secondary shield is tied to secondary GND.-branadic-

author: Kleinstein
author_rank: Super Contributor
timestamp: December 27, 2020, 09:20:08 pm
content: I have build an ugly DC/DC converter based on a relatively large core with distance to the windings. Still no shield, but the coupling capacitance looks small, at some 3-7 pF (hard to measure) range.

author: Andreas
author_rank: Super Contributor
timestamp: December 27, 2020, 09:45:09 pm
content: Quote from: branadic on December 27, 2020, 08:55:02 pmEarth / housing is that what I would have expected.But obviously the connection is not short (< some mm) and routed via GND.The transformer position on a PCB is not a optimum. It should be tied directly to case.Quote from: branadic on December 27, 2020, 08:55:02 pmI would have expected to a guard (inner housing)Secondary GND does not make any sense for me. (will not reduce coupling capacity).with best regardsAndreas

author: fcb
author_rank: Super Contributor
timestamp: December 28, 2020, 01:22:57 pm
content: How are you measuring the capacitance? LCR meter?One way I've done it is to measure the leakage current increase at 3200V 50Hz. Seems to correlate quite well with an LCR meter when measuring 10-20pF. And we seem to be able to measure reliably down to 1pF with the 3200V/50Hz - which the LCR meter seems to struggle with (the only 4 terminal LCR meter we have was homebrew).Maxim (MAX14436) make some nice couplers that are around 1pF per channel BTW.

author: Echo88
author_rank: Frequent Contributor
timestamp: December 28, 2020, 05:13:30 pm
content: https://eu.mouser.com/datasheet/2/427/vom617a-1767067.pdf0.3pF and no EMI from the Digital IsolatorsThere might be even better Optocoupler, i just found it after a bit of searching.

author: fcb
author_rank: Super Contributor
timestamp: December 28, 2020, 05:35:57 pm
content: Quote from: Echo88 on December 28, 2020, 05:13:30 pmVOM617, great device, use on numerous projects. But it can't do 200Mbps

author: Echo88
author_rank: Frequent Contributor
timestamp: December 28, 2020, 06:10:55 pm
content: Sorry, was thinking about power supplies and feedback-coupling and not about your data transmission project.

author: branadic
author_rank: Super Contributor
timestamp: December 28, 2020, 08:00:49 pm
content: QuoteIf you carefully look at the first picture you see a nut between primary and secondary side of the transformer, to which the case is connected to.QuoteThere is neither a guard nor an inner housing in this units.Board design finished and order placed.-branadic-

author: branadic
author_rank: Super Contributor
timestamp: January 01, 2021, 07:29:49 pm
content: Received some ring cores before new year and started to wind some copper on them. Since we still have lockdown and even the shops where you can buy some enameled wire are closed, I had to use what I had in the drawer, thus I used some teflon wire.The result I found is, that 2x 16 (17) turns on the primary side (~4.2mH per coil) and 2x 32 turns on the secondary side (~14mH) do match the inductance I measured on F7000 with LT1533, thus a ratio of 1:2. The transformer is wound in a bifilar fassion. Not sure if twisting the wires would make any difference though.The boards I designed already arrived in Germany and will hopefully show up soon. It will take some additional time to draw the resistive shells in CAD and have them printed. Not yet sure if my idea works, but stay tuned.-branadic-

author: Kleinstein
author_rank: Super Contributor
timestamp: January 01, 2021, 08:39:24 pm
content: I think it should be OK with these cores (low magnetostriction), but with soft ferrite cores one should keep in mind the effect of mechanical stress on the core. It can reduce the Al value a lot - those ferrite cores can make sensitive force sensors.For the coupling coil it can help to have quite some distance. Probably no need for that many coupling "turns / loops": 3 - 5 may be sufficient. Multiple separate loops may be better than more turns in series. There is still the one "turn" in parallel to the core that could upset things.

author: robert.rozee
author_rank: Frequent Contributor
timestamp: January 02, 2021, 03:47:04 pm
content: doktor pyta (reply #3) and Kleinstein (reply #50):i'm picking you're both using a driver circuit similar to this:https://en.wikipedia.org/wiki/Royer_oscillator#/media/File:Royer_oscillator.gifare you able to post a schematic to what you are using with some component values? i'm aiming a fair bit lower myself, just after something to go from a 6v battery up to 20v with relatively little noise to run a LM399. i'm led to believe that my present solution - an LM2577 boost converter - is frowned upon in present company!cheers,rob :-)

author: doktor pyta
author_rank: Frequent Contributor
timestamp: January 02, 2021, 10:33:48 pm
content: @robert.rozeekind of, but I'm using 2xNMOS.In other project I've successfully implemented circuit described in article below (but with center tapped primary coil).This is a very good article worth reading.https://www.mdpi.com/1996-1073/11/10/2653

author: Kleinstein
author_rank: Super Contributor
timestamp: January 03, 2021, 08:57:17 am
content: My converter uses an external drive signal for the transistors, so not self resonant, but a fixed drive push pull transformer. Different from common implementations there is a relatively long dead time and quite some RC damping.

author: Gyro
author_rank: Super Contributor
timestamp: January 03, 2021, 11:56:44 am
content: I hesitate to post this as it's acompletelyoff the wall idea. I've accumulated a couple of old Oral B style electric toothbrushes over the years which have small magnetic charging bases - the ones with the little plastic spigot that goes into the base of the toothbrush.I haven't done any tests yet, but inside the toothbrush, there is just a simple coil on quite a large diameter standard former. No ferrites. It occurs to me that this has the basis of a really compact mains powered, high isolation, low capacitance PSU. The large gap and full mains insulation of the charger base would make it really easy to apply grounded screening foil (obviously no shorted turn). There's enough energy transfer to charge an AA sized NiMh in around 8 hours, so it ought to be sufficient for a low current reference with a coil rewind.Obviously this is a one-off type solution, and maybe the switching waveform is too noisy, but it might just trigger some ideas.

author: macaba
author_rank: Regular Contributor
timestamp: January 03, 2021, 01:35:31 pm
content: Quote from: Gyro on January 03, 2021, 11:56:44 amAs it happens, I had the same toothbrush in my disposal pile!So some quick approximate measurements with DMM6500 in Digitize Current mode measuring from one side of the battery inside the toothbrush to PE on the back of the DMM6500.DMM6500 noise floor: 0.005uAppWithout charge base nearby/on: 0.4uAppWithout charge base nearby/on, inside thick steel box section grounded to PE through 1M resistor: 0.2uAppWhilst on charge base: 1.5uAppAs is, no good; the modifications you suggested would be a good start but not something I'm interested in doing at the moment.Edit: I'm unsure if there might be some interaction with the DMM6500's own leakage (0.14uApp)

author: exe
author_rank: Unknown
timestamp: January 03, 2021, 01:44:58 pm
content: Quote from: Gyro on January 03, 2021, 11:56:44 amhttps://youtu.be/JJgKfTW53uo?t=1320. This, of course, rerpresents only one model from one brand. Other may do it better.

author: Gyro
author_rank: Super Contributor
timestamp: January 03, 2021, 05:49:54 pm
content: Quote from: macaba on January 03, 2021, 01:35:31 pmWow, some impressively quick measurements, thanks!As you say, it sounds like no good as it stands, but scope for experimentation. There's certainly enough physical gap, without the toothbrush body to implement some sort of primary screen, and a separate guard screen on the secondary.Quote from: exe on January 03, 2021, 01:44:58 pmThanks exe, I hadn't realised that Dave had done a teardown, well of the toothbrush side, I think all of the charging bases are potted internally for waterproofing. The one that I've pulled apart is a single winding with a bridge rectifier rather than a lower loss 2 winding two diode full wave arrangement.Dave showed some interesting waveforms, interesting about the 800Hz charge pulsing - I had assumed that they were 'dumb' HF oscillators but maybe there's some (unreachable) intelligence in there.I will report back any findings.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 03, 2021, 08:33:15 pm
content: The charging intelligence is at the receiver side, not at the driver side. The receiver side would be rebuild and should avoid too much of a variable load.A main problem may be electric fields from the drive side, as the coils are likely high voltage. So a primary shield can do a lot.With a not so well closed magnetic field, there can also be some trouble from the stray magnetic field, not just the electric field. So it may need some distance to the circuit.

author: dietert1
author_rank: Super Contributor
timestamp: January 03, 2021, 09:50:01 pm
content: Quote from: macaba on January 03, 2021, 01:35:31 pmThat is a very low number and it made me look through the DMM6500 teardown on xdevs.com. There is a double mains transformer, both cores with two chambers each and with shield. There could be an additional high frequency converter hidden under an aluminum cover.In my understanding one critical test case are the kilovolt pulses arising from mains switches of inductive loads. That cannot be solved by a low coupling capacitance alone, because coupling below some pF is unrealistic. So there should be a shield connected to the guard net. A 12VA commercial shielded mains transformer i measured recently had coupling from primary to core+shield of about 260 pF (!) and a similar coupling between secondary and shield. With that capacitance even a 15 Vac secondary at 50 Hz injects several uA into Gnd. Definitely not what you want, although the coupling around the shield is only 2.7 pF.Regards, Dieter

author: TexasRanger
author_rank: Contributor
timestamp: January 03, 2021, 09:51:14 pm
content: Why not use a little hydro generator - Oil - cooling pump combination?https://de.aliexpress.com/item/33009775552.html?spm=a2g0o.productlist.0.0.7eda53a6MhrDSy&algo_pvid=d646b410-b034-47a4-8a78-a6cd88366715&algo_expid=d646b410-b034-47a4-8a78-a6cd88366715-0&btsid=0b0a0ac216097101883665950e81bd&ws_ab_test=searchweb0_0,searchweb201602_,searchweb201603_Would for sure be a funny project and would achieve femto range parasitic capacitance and MV proof insulation.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 04, 2021, 08:58:21 am
content: The point of he shield is to reduce the voltage to an absolute minimum, like less than the voltage per turn - ideally a small fraction of this if the point were the shield is connected is chosen right. With a small mains transformer it is something like 1/10 V per turn. If one has 15 V at the shield there is something wrong.260 pF coupling sounds relatively high, but with a low voltage this may still be acceptable. It only needs a good ground to connect too.The capacity from primary to shield is usually not important. The point is the capacitance between the 2 shields.In the DMM6500 only the transformer for the analog part needs to be shielded. The digital/output part may be even grounded - so no real need for a special transformer for that part. I doubt there would be another switched mode converter - at least in the analog part.

author: dietert1
author_rank: Super Contributor
timestamp: January 04, 2021, 12:43:00 pm
content: Maybe i need to explain better.You want to connect the shield of the shielded transformer to your guard network to keep away noise coming in from primary (mains). Then you build some circuit (rectifier whatsoever) that makes a connection between the secondary and Gnd/Low of the device you want to supply. Now you will have AC current along the capacitance between shield and secondary injected between Guard and Gnd. This current flows through the Gnd/Low network to the one point were you wanted to connect Guard and Gnd/Low originally. On its way it generates error voltages in the Gnd/Low network. When i calculate that current at 15 Vac 50 Hz 260 pF i get 3.5 uApp. Compare this to the number 0.14 uApp given above for the DMM6500.The water generator proposal is interesting. If those devices work well with oil, one could combine two of the same, one as pump the other as generator, except it will make some noise and last "only" about 3000 hours. That's what they claim. Maybe it can last longer with the low power required for a voltage reference.Regards, Dieter

author: robert.rozee
author_rank: Frequent Contributor
timestamp: January 04, 2021, 02:40:53 pm
content: Quote from: doktor pyta on January 02, 2021, 10:33:48 pmthe article is interesting, but a tad more theoretical than i was hoping for.however, this posting:https://www.eevblog.com/forum/projects/inductive-charging-how/msg139940/#msg139940points to an actual implementation that someone has built here:https://www.mikrocontroller.net/articles/Royer_Converterhttps://translate.google.com/translate?hl=en&sl=de&u=https://www.mikrocontroller.net/articles/Royer_Converter&prev=search&pto=aue(translated)for my own requirements (no isolation, 6v battery) i'm thinking i probably don't even need the secondary winding - if the primary presents a clean enough sinewave, this can just feed a pair of cascade voltage multipliers to get a clean DC output for the reference (20v at probably less than 30mA).cheers,rob :-)

author: Echo88
author_rank: Frequent Contributor
timestamp: January 04, 2021, 04:33:44 pm
content: Remember to choose the right oil and test the tightness of the oil drain plugs on your voltage references.Just like in those Fluke 720A.btw:Attached is a short simulation of the supercap-switching-circuit like used in the Fluke 1595A discussed a few threads ago, to get general ballpark numbers.If relays like the AGQ200 (we want very low capacitive coupling) would be used with its lifetime of minimum 100000 cycles at nominal electric load, then we could expect a lifetime of the relay in this simulation of at least 3 years. Stick them into sockets and replace every 3 years as a maintenance-cycle...certainly easier than replacing the oil-pumpshttps://www.buerklin.com/medias/sys_master/root/h5c/hea/9313902428190/8873768026142.pdf

author: exe
author_rank: Unknown
timestamp: January 04, 2021, 08:23:24 pm
content: The next thread would be what type of oil is the best for volt-nutting.

author: Gyro
author_rank: Super Contributor
timestamp: January 04, 2021, 08:41:56 pm
content: I think that one's already been done.https://www.eevblog.com/forum/metrology/oil-for-a-leaking-tinsley-standard-resistor/

author: antintedo
author_rank: Contributor
timestamp: January 04, 2021, 10:14:23 pm
content: Quote from: Echo88 on January 04, 2021, 04:33:44 pmPickering 103 series might be more suitable - they have guard shield and the capacitance is specified in the datasheet, it's on the order of 0.1pF. They have much higher lifetime as well. Some small signal relays have high frequency isolation specs. I've seen this in G6K series, capacitance derived from the graph was <0.2pF.

author: Echo88
author_rank: Frequent Contributor
timestamp: January 05, 2021, 03:30:28 pm
content: Unfortunately the Pickerings arent easily available from Mouser/Digikey. But the general idea to use Guarded Relays is nice. Can you explain how you derived the coupling capacitance via hf-isolation spec, since im not a hf-guy?

author: antintedo
author_rank: Contributor
timestamp: January 05, 2021, 05:21:52 pm
content: Quote from: Echo88 on January 05, 2021, 03:30:28 pmMe neither, but my understanding is the isolation test can be modeled as a voltage divider with a capacitor in one leg and 50 ohm termination in the other. Capacitor impedance will be 1/(2pi*f*c), so substituting numbers, for G6K -86dB at 1MHz the capacitance turns out to be 0.16pF.

author: dietert1
author_rank: Super Contributor
timestamp: January 05, 2021, 07:23:36 pm
content: A small reed contact from my drawer wasn't more than 0.1 pF (a bit difficult to measure). Another, larger one had less contact distance and it gave about 1 pF. Then with a reed relay there will be additional coupling along the coil, so to keep coupling low a metal foil gets inserted between contact and coil. The Pickering relay has that shield and researching the web i saw others like that. I also measured an Aromat RF relay in a VXI E1473A multiplexer, they are below 0.1 pF as well.The same was true for a Teledyne 732-12, once i connected Guard to the metal can. That one is DPDT, so it replaces four of the Pickerings. But it's small and the wires close and difficult to guard from each other.General purpose DIL type relays in plastic enclosure had about 0.4 pF. There are special RF relays like Axicom HF3 with 60 dB isolation at 2 GHz. -60 dB in a 50 Ohm system indicates an isolation impedance of 50 KOhm at 2 GHz. That is a capacitor of 0.0016 pF. Strange. Have seen some RF relays where they create shorts on the inactive ports to improve isolation.If this discussion is about building a multiplexer for two rechargeable batteries, then there will be eight contacts, with four of them open at a time. Then you put the batteries into two separate guarded compartments and you may want to switch those guards between primary and secondary, so maybe five open contacts at a time. Plus there will be stray capacitance in the wiring near the relays. If you get down to about 1 or 2 pF coupling in the end that would be a nice result, similar to the DMM6500 number given above.Regards, Dieter

author: antintedo
author_rank: Contributor
timestamp: January 05, 2021, 09:15:14 pm
content: Maybe this image and an old version of the datasheet will shed some light on the construction of HF3 relay:http://mt-system.ru/sites/default/files/docs/documents/File/TYCO_PDF/HF3.pdfIt might be that there is no direct line of sight between common and switchable contacts. There are some recessed spaces in the base of the relay where signal contacts are supposed to be. When the armature is not connected, it might be shorted to guard - big yellow contact on the left of the image. I could not find any relevant patents, so the best course of action would be to buy one and disassemble it.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 06, 2021, 11:29:46 am
content: The relay looks very much like using 2 contacts in series and maybe even connecting he movable part to ground when open. This can explain very good RF isolation. However the good RF isolation does not mean there is low capacitance of the additional ground (would be an extra shield potential) is not there.Even when not using the extra shield / ground it can still be a good relais.

author: branadic
author_rank: Super Contributor
timestamp: January 12, 2021, 06:35:33 pm
content: Some progress to share:- boards designed, ordered and received- cores for primary and secondary side wound- CAD design of the shield and arrangement drawn- first samples of the shield fabricated and checked for fitting into each other- finalized the CAD drawings (dimensions slightly adjusted to tighter tolerances)Now the final parts are being fabricated. Components on order, so hopefully a first version can be assembled and tested soon.-branadic-

author: EmmanuelFaure
author_rank: Regular Contributor
timestamp: January 12, 2021, 11:27:56 pm
content: Nice job branadic! Is the plastic shield made of some conductive plastic for EMI shielding?

author: branadic
author_rank: Super Contributor
timestamp: January 14, 2021, 10:36:33 am
content: Exactly, the shield is made from a conductive material.Meanwhile I got all parts done to complete at least the transformer.With finally installed transformer cables to the board, but rest of the PCB unpopulated, I can measure some 17 - 18pF between primary and secondary ground. However, on the original 7000 board some 1nF/1kV cap (blue) can be seen. So not yet sure what this number means by itself.-branadic-

author: dietert1
author_rank: Super Contributor
timestamp: January 14, 2021, 06:07:59 pm
content: In my opinion that 1 nF capacitor is a mistake unless it is necessary to protect the optocoupler from malfunction. Are you using an optocoupler with built in guard?Like Andreas proposed above, with two shields of the transformer you have several options how to wire them. When making a guarded solution you can connect the coupling wire to guard - it doesn't have voltage on it. Then the optocoupler should move next to the transformer and maybe get a guard trace as well.Regards, Dieter

author: fcb
author_rank: Super Contributor
timestamp: January 15, 2021, 01:03:55 pm
content: 17-18pF seems quite high, is the complexity worth it? I can see that the isolation voltage would be reasonable and noise shielding possible.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 15, 2021, 02:33:39 pm
content: 17-18 pF looks quite high. This should be capacitance between 2 shields - though I somewhat don't see the connection in the pictures.So the injected current can still be small, as there should be essentially no AC voltage across the capacitor.The shown transformer could be relatively high power - from the sizes I would consider 20-100 W possible.If needed one could probably reduce the capacitance with a little more space between the coupling windings and the inner part. For lower power thinner and less (e..g. 4-5 loops at 0.6 mm²) wires should be Ok too.

author: branadic
author_rank: Super Contributor
timestamp: January 16, 2021, 10:04:26 am
content: Just to clarify, the transformer shown above is a direct replica of the Pickering in F7000 with LT1533, so same dimensions, same amount of copper etc.With respect to the report linked in the first post, this circuit and transformer was designed for 7W.The capacitance was measured between primary GND/shield and secondary GND/shield.-branadic-

author: ignilux
author_rank: Unknown
timestamp: January 17, 2021, 03:44:56 pm
content: Hi, all-I just want to clarify my understanding of the concern over transformer capacitance. The principal issue seems to be the coupling of noise between the outside world (primary side) and the circuit (secondary side), correct? Magnetic coupling is largely mitigated with a toroidal core, but that does nothing for electrical coupling through the parasitic capacitance. Hence, trying to minimize that quantity. Am I all good so far?Here's where it gets a bit fuzzier for me. On first glance, I would assume that most of the electrically coupled noise is common mode, since the noise would have coupled equally to both the hot and neutral conductor in unshielded mains wiring (Romex, in the US). If that's the case, why couldn't the noise be reduced with the use of a common mode choke on the primary and/or secondary side of the transformer? There must be more to it than that, otherwise all of the knowledgeable people here would have suggested it by now.Anyway, I'm in the process of building a power supply for my AD587JQ and was thinking of starting a thread on the subject, so I'm glad to have found this discussion. Branadic, what type of toroid core are you using in your DIY Pickering? Sorry if I missed that.Cheers!

author: Kleinstein
author_rank: Super Contributor
timestamp: January 17, 2021, 04:33:17 pm
content: The main problem with a simple transformer is common mode injection due to point with different voltage in the coils. The shield can reduce the voltage to less than the votlage of one turn - below that one has to balance and chose the grounding point right. So even with a shield, low capacitoance is good.A common mode choke is of limited use, when at a very good isolation level. It is only am additional seires impedance and this may not be much compared to some 20 pF of capacitance at the transformer.It mainly helps to the the higher frequency part. For the lower frequency part a low capacitance transformer is more effective.I would still consider a CM choke worth while, but the transformer should still be low capacitance, especially if the shield is not good.There may be also the case that the rest of the circuit has some CM voltage to ground - so good isolation is definitely helping and even just capacitance to ground can be negative.

author: Andreas
author_rank: Super Contributor
timestamp: January 17, 2021, 04:36:36 pm
content: Quote from: ignilux on January 17, 2021, 03:44:56 pmHello,the frequency ranges where a shielded transformer (< 1 MHz) and a common mode choke (> 1MHz) work effectively is different.So usually you have to use both.Quote from: ignilux on January 17, 2021, 03:44:56 pmMhm,for a AD587 reference (< 5mA) I would always use batteries. 12 AAA cells last 7-10 days before you need to recharge.See also my AD587LW design:https://www.eevblog.com/forum/metrology/ad587lw-10v-precision-travel-standard/msg1449494/#msg1449494with best regardsAndreas

author: dietert1
author_rank: Super Contributor
timestamp: January 17, 2021, 06:23:07 pm
content: Yes, a common mode choke helps, too. And it isn't easy to make one with low parasitic capacitance, either. Sometimes people combine common mode chokes with coils and sheath current filter (ferrite on cable). A primary side common mode choke works best when combined with a guard. Then you get an inductive divider between the choke inductance and the effective inductance of the guard net.As always some people try to avoid own studies by "best practices" approach, others try to get own hands-on experience. When i made a short on the probe tip (with its own ground clip), put the probe onto the table without any other connection, adjusted the trigger for mV and turned on/off the microscope illumination (transformer inside!), i captured how that mains switch sparked. For precision measurements you don't want to have spikes of several mV or tens of mV in your circuit, so it will have to be shielded/guarded, like "best practice" tells you for any other audio or RF setup. Some people even connect an amplifier/headphone to critical signals and listen to them..Regards, Dieter

author: ignilux
author_rank: Unknown
timestamp: January 17, 2021, 08:13:23 pm
content: Thanks for the replies. I hadn't really considered the orders of magnitude difference between frequencies where each solution is effective, but that definitely makes sense.Andreas, I'm aware of that thread and am basing much of the project on your design. For the time being my most precise voltmeter is a 5.5 digit Fluke 8840A, so I don't feel the need to go all the way with ppm tempco compensation, etc. It's more for the learning process, and to have a accurate reference on hand for other uses. To that end, I'd prefer to have a mains-powered device that I can leave powered on 24/7. Sealed lead acid might be nice, too, if I go for a battery powered solution. That would extend the run time quite a bit, eh?Anyway, thanks again!

author: KT88
author_rank: Frequent Contributor
timestamp: January 18, 2021, 12:15:33 am
content: @Dietert1: Common-mode chokes won't help if one side is floating as a filter consits of a passing- and a shunting element. The shunting element is missing here. There would be some capacitance against ambient though - this would unfortunately resonate at some frequency...The most viable measures to mitigate CM noise in this case is reducition of coupling capacitance, shielding and symmetry.CheersAndreas

author: dietert1
author_rank: Super Contributor
timestamp: January 18, 2021, 07:20:40 am
content: No, sorry, a primary side common mode filter is an established method to keep the guard/shield clean.As i wrote, a common mode choke helps in combination with guard. In that case it reduces excitation of the guard net by spikes from outside. Ideally guard would be a Faraday cage, with a very high resonance frequency depending on its dimensions. Below that frequency the inner of the cage would be well protected against excitation from outside. But in general it isn't. It consists of many partial guards in many instruments, connected to each other by (inductive) wiring. So the resonant frequency of the guard net will be somewhat lower. By the way introducing some 50 to 100 Ohm resistors into the guard net allows you to make measurements.Regards, Dieter

author: KT88
author_rank: Frequent Contributor
timestamp: January 18, 2021, 08:09:09 am
content: I agree on the placement at the primary side. I was referring to your first, more genral statement. I have seen a lot of examples where CMCs were placed on floating parts of a setup without the desired result of reducing EMI. I some cases during EMC tests some frequencies were even exaggerated depending on cable length...CheersAndreas

author: d-smes
author_rank: Regular Contributor
timestamp: January 18, 2021, 09:50:22 am
content: Quote from: branadic on January 14, 2021, 10:36:33 amIf you used conductive plastic, beware that this creates a shorted turn on the torroid. I suspect resistivity of the conductive plastic is high enough, or that contact resistance between the two halves is high enough, that the current in the shorted turn becomes meaningless.

author: Echo88
author_rank: Frequent Contributor
timestamp: January 18, 2021, 10:29:44 am
content: @d-smes: The shields are designed to be just conductive enough to fulfill the shielding while only dissipating neglegible power.Please readhttps://eprintspublications.npl.co.uk/1888/1/bemc99-7.pdfpage 3 Figure 6 discription.

author: exe
author_rank: Unknown
timestamp: January 18, 2021, 11:30:29 am
content: @branadic what plastic did you use for the shield?

author: ignilux
author_rank: Unknown
timestamp: January 18, 2021, 02:36:07 pm
content: What's the consensus on so-called "semi-toroidal" transformers likethisseries from Bel Signal? It looks to me like an R-core, but made with a laminated core rather than a secret sauce composite.

author: dietert1
author_rank: Super Contributor
timestamp: January 18, 2021, 08:25:43 pm
content: Have seen those in a Keithley 213 quad isolated DAC. They have low coupling capacitance as they are. And the advantage that you have access to the core. I remember bending up a corner of the uppermost core sheet and soldering a wire to it to connect the core to guard.Regards, Dieter

author: The Soulman
author_rank: Super Contributor
timestamp: January 19, 2021, 07:17:01 pm
content: Any drift for both the meter and the ltz1000 should be down in the noise for such a short time and temperature range.Did you let everything warm-up (for several hours) before starting the measurement/data-logging?

author: dietert1
author_rank: Super Contributor
timestamp: January 20, 2021, 07:15:04 am
content: Your reference output voltage increased by about 1 mV. In your own words this is bad. To get anywhere near the expected results, one would use voltage regulators between batteries and reference board. As far as i remember that HP 3458A reference board needs bipolar supply to work properly.Also i was wondering, whether that BNC cable on your black box is the reference voltage output.Regards, Dieter

author: TiN
author_rank: Super Contributor
timestamp: January 20, 2021, 08:16:49 am
content: Something is awfully wrong. Ditch the LiPo (it's there just waiting you to short something by accident and blow stuff up) and use 36313A PSU to test first.At first glance you seem to forgot connect heater supply or returns to the PSU, as that could be nearly only possible explanation to this insane instability. Tempco of most A9 boards is beyond what any of the 6.5d DMMs can measure in small temperature swings (<10°C).Why you don't show more photos showing exact setup , connections, wiring etc? It's not year 2005 when you had to pay $$ for every megabyte of internet data on dialup.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 20, 2021, 08:58:48 pm
content: The ground to the heater powermustbe connected to the main ground - just keep the output separate. It is just inside the DMM circuit that heater ground and power ground use separate paths, but they are still connected on the PCB.The negative voltage is only used to reduce the ground return current - this is not so much an issue here. For testing only the reference, one should get away without using it: one mainly misssed the extra heat source.It would help to have a regulated voltage for the main part. The heater may use the same battery pack with a separate regulator or even before the regulator.

author: dietert1
author_rank: Super Contributor
timestamp: January 20, 2021, 09:56:45 pm
content: Heater current 0.045 mA means the oven is inactive. The LTZ1000 datasheet recommends 0.1 to 0.3 W heater power. Depending on heater voltage it should take at least 10 mA or so.And the schematic in the image shows the heater minus separate from the reference Gnd, so in case of using two separate supplies one should make an external connection.Regards, Dieter

author: TiN
author_rank: Super Contributor
timestamp: January 20, 2021, 11:12:41 pm
content: Either LTZ chip is blown open now, or power is not properly supplied. Normal heater currents for A9 are about 25-29mA for heater and few mA for signal ground.As pointed out, ground MUST be connected as PSU end. Easy to check if heater works by looking at control temperature input (1/13k divider tap output) and control feedback (opamp output for oven control). Both should be same voltage around 0.4** V-ish. You should have four wires going to PSU which are +12...+15V for signal power (opamp, etc), signal ground return, +15 or so power for heater in, heater return to PSU-. Then short circult jumper between returns at PSU LO ports, if your PSU has isolated outputs (like E36313A does).Then you have your sense lines for zener+ and zener- from A9 to voltmeter.

author: MegaVolt
author_rank: Frequent Contributor
timestamp: January 21, 2021, 07:44:18 am
content: Quote from: View[+]Finder on January 20, 2021, 08:16:33 pmIf you click on the virtual display 6500 there is an option to save only the screen. In my opinion, this is more convenient than saving entire screenshots.

author: iMo
author_rank: Super Contributor
timestamp: January 21, 2021, 11:43:03 am
content: The first thing I would do - I would replace that cheapo LT1013 dil8 socket with precision one. I had many problems decades back with digital stuff not mentioning analog while messing with those sockets.. Unless the precision socket is not suitable there because of different plating..

author: branadic
author_rank: Super Contributor
timestamp: January 21, 2021, 05:02:06 pm
content: @ ViewFinderWould be great if you would either open a new thread or chose another one, as this thread is intended to address power supplies for voltage references and your specific problem has nothing to do with it, as far as I can see. *no hijacking*@ exeThe plastic I used for the shield is Proto Pasta, the shield itself is 3D-printedStill waiting for components, while I already build a second transformer.-branadic-

author: dietert1
author_rank: Super Contributor
timestamp: January 22, 2021, 08:20:21 pm
content: I think the engineering approach you propose is completely valid. Maybe the OP is a salesman and his thread title was really meant as "Look at this power supply for voltage references". Then people like you and me make it into a wild collection of personal taste, belief and knowledge. A true mishap.Regards, Dieter

author: Kleinstein
author_rank: Super Contributor
timestamp: January 22, 2021, 09:40:54 pm
content: The demand on the power supply is mainly low injetion of common mode signal, low capacitance and good EMI compatibilty. It is relatively hard to tel how a reference circuit reacts to EMI - there can be resonances. In addition the reference is not alone, but it would be connected to some meter or similar and these may to be sensitive.It is still a good question on how to quantify the quality - there are different frequencies involved and the capacitance from the output side to ground can make a difference. Chances are it would be more the current to a low impedance connection that would matter, not so much the open ciruit voltage, with little extra capacitance.

author: TiN
author_rank: Super Contributor
timestamp: January 22, 2021, 11:57:30 pm
content: Branadic hardly a sales person. And thread started for PSU design talk and sharing results, not for fixing particular altered and possibly damaged LTZ ref.Before going EMI rabbit hole it is better to apply requirements expected from voltage reference application. If its going to be measured by floating DVM and null-meters against other references then major required feature of PSU is low leakages and common rejection to avoid ground loops. If reference is used as a part of bigger system (like A9 3458A module) it have different requirements for PSU. Above shown A9 module is very sensitive for pickup and grounding. It is essentially useless as transfer reference on ppm level. Just turning common LED lamp with crappy SMPS will ruin your A9 output stability and noise. No matter if powered by nude virgin custom ppmium transformer or fire breathing lithium pack. And thats offtopic here, we already have excellent RFI\EMI test threadright here.I really don't understand View[]Finder's resistance to make own thread named "Experiments with DMMs and references" and post all stuff there, people will be much happier to help and suggest things when it's all in one dedicated spot, instead of guessworking thru 5 different hijacked threads on what is going in. Threads creation does not require payment or anythingPPMs also nothing to do with external data, that is completely wrong, just like saying percents are anything to do with calibrations. It's just a convenient tool to do the opposite - remove absolute values from measurands and outline the deviation of one set with arbitrary selected point(s) against another.Quote$200 standard will stay such, no matter what magical power is used to power it. Noise isleastof the problems in voltage references, compared to much harder issues like stability, temperature/humidity/pressure dependence, jumps and stress factors, robustness during transport, etc.

author: TiN
author_rank: Super Contributor
timestamp: January 23, 2021, 03:22:05 am
content: Threads are designed as tool to organize things. This one is about power supplies for voltage references. Not about references themselves or how to measure/analyze them as ultimate goal. It is nice and respectable to keep metrology section (and any other section same way) organized and structured, instead of having all threads go in random directions, also to help newcomers to find information easier. Branadic got into power supply design reference by study on modules he got, and its quite interesting direction. Your results with A9 board seem to be all over and so far unique to your setup/equipment/case/device. I can't speak for everyone here, but instead of trying to reply to your posts in every somewhat remotely related threads about this I would be more inclined to spend time helping you rectify issues in separate dedicated thread. From posts it is visible that you misunderstand some key concepts (nothing wrong with that!, we all learning here) so those warrant to be separated from power supply design or repair of my 3458A U2 as well. PPM means parts per million. It does not mean parts per ideal 10V, you don't subtract anything to calculate ppm deviation of value A to B.QuoteI've suggested to create your own thread twice in two different threads already. I think you just trolling now.Why make it hard on people to navigate thru multiple offtopic discussions? To save an entry in forum database?The reason why EEVBlog is one of the best forums about electronics is variety of topics and all different people who have shared engineering knowledge about those things in all those threads. Trying to keep it structured is least we can do before we drown in huge pile with everything mixed in a single thread. I'm done, sorry all.

author: View[+]Finder
author_rank: Unknown
timestamp: January 24, 2021, 07:31:27 pm
content: Quote from: TiN on January 23, 2021, 03:22:05 amPlease accept my apologies for the confusion this has caused--particularly to those who have offered me assistance in the past--and I assure you that I have no intent to "troll" on this or any other forum. I remain interested in contributing to the discussion centers on measurement, analysis and reduction of errors in voltage as measured by the kind of instruments I own: former premium-grade Keithley, Keysight and Fluke products. I believe that is consistent with the interests of the general readership. Of course I will be happy to start another thread. Would it be asking to much to request suggestions for a title and, please, may I place it under the Metrology section?Best regards,Donal B. Botkin

author: Gyro
author_rank: Super Contributor
timestamp: February 06, 2021, 06:44:24 pm
content: Quote from: Gyro on January 03, 2021, 05:49:54 pmJust a quick follow-up. I just posted a quick teardown of the charging base for a Lidl torch in Dave's original Braun toothbrush teardown discussion thread...https://www.eevblog.com/forum/blog/eevblog-284-braun-toothbrush-teardown/msg3451180/#msg3451180It seems to have similar power output capability to the Braun but operates at a higher frequency, in the region of 150kHz, and without the severe ringing that Dave observed. Surprisingly it's just a single transistor oscillator circuit. I was expecting something a bit more complex to cope with the inductive load difference between zero load (removed) and full charging load.Hopefully of some interest for a high isolation supply anyway if it is quieter. The higher operating frequency might be easier (or more difficult!) to filter out.

author: dietert1
author_rank: Super Contributor
timestamp: February 09, 2021, 07:04:18 am
content: Yesterday i found this image taken by ve7xen in a Datron 1081 multimeter. Those guarded transformers look like the solution i arrived at in my study of the "Art of electronics" proposal. Except each of those transformers includes a single turn winding (green wires). How does that work?Regards, Dieter

author: Vgkid
author_rank: Super Contributor
timestamp: February 09, 2021, 08:13:11 am
content: Those look like isolation transformers for the digital side. They were widely implemented before the use of opto-isolators Here is an example in the HP 3456A.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 09, 2021, 08:43:33 am
content: The circuit is like 2 transformers in series: First some 2x10 to 1 turn and than back 1 to 2x10 turns. One turn helps to reduce the capacitance of a single transformer and the 2nd transformer halves the capacitance and brings the signal back to a level were one wants it.The transformer with 2 cores close by and one coupling winding is a similar principle, just a few more windings for coupling and less distance. The closer coupling reduces stray magnetic fields, that can be more of an issue with a power transmission than with just a digital signal transfer in the old meters.

author: iMo
author_rank: Super Contributor
timestamp: February 09, 2021, 09:21:52 am
content: First time I can see TTL chips made in El Salvador.. Interesting..PS: the transformer solution looks better to me - no LED brightness degradation over time..

author: Kleinstein
author_rank: Super Contributor
timestamp: February 09, 2021, 04:22:08 pm
content: The transformer for digital signal transfer is relative bulky and needs extra circuit - however there are new transformer based couplers also in chip form.

author: KK6IL
author_rank: Regular Contributor
timestamp: February 10, 2021, 10:26:50 am
content: QuoteSome magnetic couplers, such as the NVE IL26x series, power up in an unknown state(BTDT). 5 pF isolation. The Silicon Labs Si866x does not require startup sequence, in to out C not specified. Note those devices are for data transmission, not power, as are the transformers in the photo Vgkid posted.John

author: Kleinstein
author_rank: Super Contributor
timestamp: February 10, 2021, 11:36:48 am
content: Those ring transformers shown by Vgkid may also be OK for low level power transmission, but in the 3456 and other old meters they were used to data transfer.The 2 transformers shown in the picture are one side only - so there are 2 more similar transformers at the other side. So the 2 don't belong together, they are 2 separate data channels.

author: iMo
author_rank: Super Contributor
timestamp: February 10, 2021, 12:47:56 pm
content: Quote from: Kleinstein on February 09, 2021, 04:22:08 pmThere are about 4-5 TTL chips at the both sides usually - no problem to put all the stuff inside a smallest cpld or an fpga. Thus you would need the 2 small FT37-43 toroids (an example) and maybe 2 resistors to limit the current into the transfromer winding. With two cores per channel (one is at tx, one is at rx side) the coupling capacitance gets to a half and the max isolation voltage doubles..

author: maxwell3e10
author_rank: Frequent Contributor
timestamp: October 23, 2022, 08:39:38 pm
content: I am wondering if anyone has suggestions for reducing small sharp spikes in a simple linear power supply that are associated with rectifier diode bridge. The spikes are synched to AC line and last 20-50 microsec. I am not sure how they propagate to the rest of the circuit, perhaps magnetic or capacitive coupling. They occur when the diodes turn on, so the exact phase depends also on the power supply load. Are there perhaps better diodes to use or putting some sort of filter to smooth out the turn-on transitions?

author: Kleinstein
author_rank: Super Contributor
timestamp: October 23, 2022, 08:55:28 pm
content: There can be short current spikes from diode reverse recovery. This can be reduced by using fast revery diodes and limiting the voltage slew rate, e.g. with capacitance on the transformer output. Usually the diode turn on part is not an issue, and diode turn on pretty fast and not much difference (e.g. a 1N4001 is about as fast as a 1N4148 in that respect).The other spikes come from the current pulse to the filter capacitor. Here some added resistance (could be just a fuse) or inductance can help to reduce the current pulsed and get some improvemen on the power factor. The fast rising current can otherwise be a problem and cause inductive coupling. The current puse may also be visible in the ripple voltage at the capacitors. 2 stage fitlering can reduce the shap pusle - it can also help to reduce the chance for a ground loop.20-50 µs sounds about like the rising edge of the current pulse to charge the main filter capacitor.

author: Echo88
author_rank: Frequent Contributor
timestamp: October 23, 2022, 09:02:02 pm
content: Heres an article that goes through the diode reverse recovery issue (if thats your issue in this case) in detail and shows how to solve it:https://www.desmith.net/NMdS/Data/Linear%20Audio%20-%20Soft%20Recovery%20Diodes%20Lower%20Transformer%20Ringing%20by%2010-20X.pdf

author: maxwell3e10
author_rank: Frequent Contributor
timestamp: October 24, 2022, 03:53:41 am
content: Quote from: Echo88 on October 23, 2022, 09:02:02 pmThis looks promising! Adding resistor-capacitor network across the transformer already helped reducing the transient, just need to find optimal values. I will also get some SBYV27-200 diodes. Does anyone have other favorite diodes for general purpose linear rectifier?

author: iMo
author_rank: Super Contributor
timestamp: October 24, 2022, 06:46:33 am
content: In past while building ham radios I put 4n7-10n ceramics in parallel with each diode in the bridge rectifier..

author: Kleinstein
author_rank: Super Contributor
timestamp: October 24, 2022, 06:50:12 am
content: The difference between the different fast diodes is rather small - it looks like hard to measure that accurate (e.g. mains can vary over time). For low voltages ( keep in mind the diodes see up to 2 times the AC peak voltage) schottley diodes are attractive. The choice depends on the current and availability. With only mains frequency there is no need to pick special or extra fast (e.g. SiC) didde - that is a point in the SMPS. The slow diodes have a Trr in the 2 µs range and even that is nearly fast enough (if mains is a clean sine) at mains frequency. Also series inductance (similar to PFC correction) can smoothen the current spike and thus make the reverse recovery less imortant.

author: David Hess
author_rank: Super Contributor
timestamp: October 24, 2022, 03:52:29 pm
content: Quote from: maxwell3e10 on October 23, 2022, 08:39:38 pmI thought this was caused by reverse recovery "snap off", which some slow diodes display. For 1N4004 series diodes, a couple hundred picofarads of capacitance directly across each diodes suppresses it. Some designs include a ferrite bead in series with each diode. Some standard recovery diodes have this problem and some do not. Fast recovery diodes seem less likely to have this problem, or maybe they never do.I first ran across this problem with a couple of simple fixed voltage power supplies that I had on my workbench for powering circuits during development. Later I noticed that many linear power supplies in audio equipment include suppression capacitors across the rectifiers. Tektronix included them in some but not all of their linear power supplies, implying that the problem was intermittent for them.

author: Andreas
author_rank: Super Contributor
timestamp: October 24, 2022, 05:42:37 pm
content: Quote from: maxwell3e10 on October 23, 2022, 08:39:38 pmHello,It has already been said that radios and analog TVs used (up to 47nF) capacitors across each of the 4 diodes of the bridge rectifier.some questions:- how did you measure the spikes and with what amplitude?(on the AC-Side it would be very difficult so I assume that you measured across the smoothing capacitor on the DC-side)- And what diode did you use for the bridge rectifier?For my "ageing" cirquits I am using Schottky diodes (MBR20100) together with a dual winding transformer (2*12V).Mainly to have not too much diode losses for the low drop regulator (15V).But up to now I never had the Idea to measure for rectifier spikes.with best regardsAndreas

author: maxwell3e10
author_rank: Frequent Contributor
timestamp: October 24, 2022, 06:27:43 pm
content: Thanks for the suggestions. The spikes do in fact appear at the time of the turn-off of the diodes. The circuit in question is an old HP power supply, it has a 50 nF capacitor across the transformer winding. The diodes used are 1N5059 which have maximum recovery time of 4 usec. I can probably do better with faster recovery diodes. The spikes are hard to see directly on rectified output, but they appear as ~0.5 mV spikes elsewhere in the circuit through some electromagnetic coupling.

author: kleiner Rainer
author_rank: Regular Contributor
timestamp: October 25, 2022, 09:59:20 am
content: Everything you want to know about snubber design:http://www.hagtech.com/pdf/snubber.pdfGreetings,Rainer

author: David Hess
author_rank: Super Contributor
timestamp: October 25, 2022, 04:27:14 pm
content: Quote from: maxwell3e10 on October 24, 2022, 06:27:43 pmWhen I had the same problem with my home built fixed output power supplies, I was working on audio circuits and the 120 Hz "buzz" showed up everywhere no matter what I did for shielding or filtering until I tracked the problem down to the standard recovery diodes and suppressed it at the source.I added capacitance across each diode until the noise stopped, and then doubled the amount of capacitance. The amount of capacitance needed seems to be proportional to the diode's capacitance, but it would not surprise me if it depends on the amount of stored charge.Not all diodes of the same nominal type produced this problem. It varied between batches of diodes from the same manufacturer, and different manufacturers.

author: dietert1
author_rank: Super Contributor
timestamp: October 25, 2022, 08:54:02 pm
content: It also depends on the straycapacitanceinductance of the transformer. I know that two chamber transformers with less perfect coupling between primary and secondary are much worse in this respect, independent of the type of diode.A good/complete snubber has two capacitors and one resistor, as described in the document linked above by "kleiner Rainer". And it sits across each diode, not across the transformer secondary.Regards, Dieter

author: miro123
author_rank: Frequent Contributor
timestamp: September 04, 2023, 08:44:43 am
content: Does somebody try gate driver dc-dc power supplies?All major manufacture offer then. I have not seen any problems when I use then in intended for use application - high side SiC or GaN bridge? The only limitation is that you can use then only as single output dc-dc converter.I don't have the equipment to measure 3pF parasitic capacitance.https://www.murata.com/en-eu/products/power/isolated-gate-drive-power

author: EC8010
author_rank: Frequent Contributor
timestamp: September 04, 2023, 02:50:08 pm
content: I see there's been lots of discussion about mains transformers and coupling. The key factor is CPS(capacitance between primary and secondary). A standard foil electrostatic screen between primary and secondary on an EI transformer will reduce CPSfrom 250pF on a 50VA transformer to around 5pF, perhaps 2pF. Adding capacitance to chassis from each end of the secondary creates a potential divider to common mode interference. Entirely covering the secondary with copper tape (without creating a shorted turn) reduces CPSto about 0.1pF. It's fiddly work and requires a split bobbin transformer. Topaz Ultra-Isolator transformers fitted individual foil electrostatic screens around primary and secondary to achieve <10fF. They also have a safety screening plate between the two windings capable of sinking enough earth current to blow a fuse quickly. That's what it's there for; safety, not screening. Adding a safety screen to an EI transformer kit plus totally enclosing foil E/S screen on secondary, I achieved CPS= 8fF +/-60% uncertainty. But what I really learned was that it's terribly easy to squander the improvement of a good transformer by sloppy external wiring. If you do something to prevent mains common mode interference getting in (low CPStransformer, mains filter) you have to totally screen incoming mains. Folded tin plate screens work well over incoming mains wiring, with self-adhesive copper tape (gardener's slug tape) over all joints. RF crawls through the tiniest gap.As has been mentioned, split bobbin transformers have high leakage inductance, needing an RC snubber across the secondary to damp their resonance when the diodes switch. Capacitors across diodes in the rectifier simply couple interference from the mains to your sensitive electronics. I know that's how it used to be done, but it was wrong. The Mark Johnson Quasimodo circuit works very well for determining the optimum snubber.

author: alligatorblues
author_rank: Regular Contributor
timestamp: September 13, 2023, 11:33:08 pm
content: If you want quiet, just get rid of the transformer. Take a metal box. Run some 120AC into it. Connect that to a 30W 500Ohm resistor with a thermostat in the circuit. Put the temp sensor in the metal box, set the thermostat for 60C, and put TEC chips in with the resistor. Put a cooler on the TEC chips, and you've got the cleanest power there is coming off the TEC chips: a thermoelectric generator. You just design the TEC pile to produce the voltage you need, and add the same configuration in parallel until you've got the current you need.TEC power is cleaner than batteries.

author: maxwell3e10
author_rank: Frequent Contributor
timestamp: September 14, 2023, 04:05:30 am
content: Interesting idea, never heard of it being practically used though. Why do you say it would be cleaner than batteries? There will be some low-frequency noise due to temperature fluctuations and convection. Also one can still have some capacitive coupling to the power line coming in.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 14, 2023, 09:56:14 am
content: Peltier elements / termolectric converters have pretty poor efficiency. Even if they would run rather hot (e.g. 100 K temperature difference) the Carnot limit is only some 30% and the TE converts are something like 10% of this. So one can expect an efficientcy of less than some 3%. A 30 W heater would thus hardly be good for some 1 W and one would need an hefty fan.Chances are the a combination of IR LEDs and photovoltaic cells could be more efficient. At some 800-900 nm both the LEDs (laser diodes) and solar cells are relatively effcient (e.g. some 40% for the solar cell and lasers diodes). There are systems that use power via optical fiber.Another not much know option for isolation is an acoustic transformer: piezo elements connected with a glass rod.For a voltage reference where size is not such an issue, there is also the option to power from batteries for the time of critical exleriments - normal standby over night could than use a conventional transformer. For those who run 24/7 the efficienty does matter a little.

author: maxwell3e10
author_rank: Frequent Contributor
timestamp: September 14, 2023, 05:28:51 pm
content: I remember seeing a home-built HV power supply that was powered by a rotating plastic rod connected to a brushless motor. One can have a large distance between two parts and a clean AC signal at a few kHz to rectify for power. And the efficiency of motor/generator can be over 50%.

author: miro123
author_rank: Frequent Contributor
timestamp: February 06, 2025, 11:11:14 am
content: I've just waking up old thread instead of starting new one.I'm considering to start journey in something different. Instead of following the traditional approach- tweaked push-pull, I am going with ZVS LLC converter. Back in the days such solution was quite complex.There COTS solutions nowadays. -https://www.ti.com/product/UCC25800-Q1At first glance solutions seems to the perfect match . Low capacitance transformers have big leakage inductance. Leakage inductance is desirable for LLC topology.I have few questions, just before start the PCB design and order components1. Does somebody see any drawback of such approach2. Why AD still propose push-pull configuration for modern design?My toughs are - careful push pull guarantee low capacitance and almost symmetrical noise spikes caused by hard switching. Therefore the integration ADC will make 0 of such noise.

author: David Hess
author_rank: Super Contributor
timestamp: February 07, 2025, 12:15:14 am
content: Quote from: miro123 on February 06, 2025, 11:11:14 amI suspect it is more complex than needed.QuoteAll of the old designs that I am aware of also use a push-pull configuration. Maybe the center-tapped primary provides less common mode swing because the center-tap is grounded?QuoteI would consider two other possibilities before a ZVC (ZVS?) LLC converter.High frequency linear analog sine wave drive has reasonable efficiency. The transformer turns ratio is selected to operate the linear power amplifier close to saturation. I might do this with an output LC notch filter and then tune the oscillator to match the notch.Alternatively a resonate Royer converter, like that used to drive a high voltage discharge lamp, can be driven from a constant current output linear or switching regulator. The secondary is now wound for low voltage instead of high voltage. Tektronix sometimes did this in various forms.

author: exe
author_rank: Unknown
timestamp: February 07, 2025, 07:36:32 pm
content: Quote from: David Hess on February 07, 2025, 12:15:14 amI'm not an expert, but I think, for the best result, the driving frequency should also match transformer's self-resonant frequency.

author: miro123
author_rank: Frequent Contributor
timestamp: February 08, 2025, 12:45:47 pm
content: Many thanks David for giving me alternative options.It seems that there are many solutions. The only way to know which is the best is to evaluate all of them.I will start with LLC. The reasons are :1. I want to start with easiest for me.2. Open loop LLC resonant converter sounds quite complex but thanks to advancement in power electronics we have single chip solutions. COTS transformers3. I know the fundamentals of this topology4. I think that this topology fits the my requirements the best.- Low intertwining capacitance comes at the cost of high leakage inductance.- leakage inductance in undesirable for most application. It crates many problems like EMI , high current/voltages loops. etc.- LLC operation relies on leakage inductance to achieve ZVS.5. I have the equipment toUCC 25800 circuit explanation.Capactiors at primary winding are used to settle the middle point. Uin/2Capacitors at the secondary defines the resonant frequency.Resonant frequemcy is defined by leakge inducant on secondary winding. The most popular LLC use ZVC capacitors at primary.I have to experiment between two solution - two diode doubler of full bridge rectifier. There are many shotcy diodes in that voltage range.As start point, I will go with Wurth COTS transformers -https://www.we-online.com/en/components/icref/texas-instruments/UCC25800-LLC-Resonant

author: Kleinstein
author_rank: Super Contributor
timestamp: February 08, 2025, 03:37:10 pm
content: With an open loop LLC one has to expect a relatively soft output voltage. So the output voltage would depend on the load. This may be OK, but could also need some care.A sine voltage drive would have short current spikes with a simple rectifier. One could also consider a more overdriven sine, with more haminics in the voltage but often less hamonics in the current.

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: February 08, 2025, 10:40:13 pm
content: Quote from: alligatorblues on September 13, 2023, 11:33:08 pmThe 30 Watts for the heater are not the biggest friends to nV offsets,esp. with a thermostat.> TEC power is cleaner than batteries.Strong opinion. It would have to beat these Panasonic/Sanyo NiCd cells ( 4 pcs. in series).18650 Lithium was about the same, I took no photos. Size matters.The strong 1/f is caused by the too small input coupling capacitor.(only 100 uF foil for 20 ADA4898 in par)0 dB = 1nV / rtHZ.more measurements of battery noise in<http://www.hoffmann-hochfrequenz.de/downloads/NoiseMeasurementsOnChemicalBatteries.pdf>and the cited NIST publication there in.Cheers, Gerhard

author: EC8010
author_rank: Frequent Contributor
timestamp: February 09, 2025, 12:02:33 am
content: Interesting stuff. Did you measure lead-acid batteries? I haven't made a pre-amplifier as quiet as yours (mine's about 0.7nV/root Hz), but a 12V 7Ah battery does not appear to noticeably increase its noise spectrum down to 0.1Hz, suggesting it's pretty quiet.

author: miro123
author_rank: Frequent Contributor
timestamp: February 10, 2025, 01:12:39 pm
content: Quote from: Kleinstein on February 08, 2025, 03:37:10 pmNo worry here I am familiar with this topology. In the mean time I have measured the parasitic capacitance of proto PCB with and made toroidal transformator. Parasitic capacitance in 3,5pF range. See attached. HF bump up above 10MHz is due to the probing. I think with beter layout I can make results better. Initial results are good but it is still work in progress. The next step is output pi filter design. If ready then DC-DC buck and CUK converters.

author: miro123
author_rank: Frequent Contributor
timestamp: February 25, 2025, 03:42:28 pm
content: Hello,I have start experimenting with few nanocrystaliine cores and i compare it to old low/MF frequency ferrite material 3C90. There are much better martials than 3c90 nowadays.Reading the parameters on simple rlc meters look ok, Thinks get worse when i start using Huoky and HP RLC meters at 1k,10k,10k,1MHz.Before i making final decision I want to ask you guys does somebody look at impedance of the transformer? - Magnetizing /leakage inductance vs frequency and other parametersDo you have datasheets of materials used on your design's? I am interested on graph representing the complex magnetics permeability real and imaginery part.Look at the attached pictures.My preliminary toughs are- material is not suitable for LLC converter using UCC25800- material is great for feritte beads or common mode chokes.- maybe material is good for old fashion push pull - all harmonics get absorbed in core losses. See at core loses graph attached- such material should mitigate the hard switching of push pull converters - anyway the parasitic capacitance should still play a role.-my experience with 3c90 is that it match the specified up to 400KHz application area. The losses at 100...400khz range are acceptable and matched the specified complex ur.

author: Overspeed
author_rank: Frequent Contributor
timestamp: February 26, 2025, 12:13:29 pm
content: HelloCould be nice to open a subject in the forum as your need / question do not look to be linked with the main threadRegardsOS

author: gamalot
author_rank: Super Contributor
timestamp: February 26, 2025, 06:45:23 pm
content: This is the USB-powered ADR1399 prototype board I made in May 2024. The isolation boost chip is TI's SN6505.

author: faraday
author_rank: Regular Contributor
timestamp: February 26, 2025, 07:24:05 pm
content: There is company iFi they make low noise power supply, model Audio iPower2 claim noise 1uV

author: miro123
author_rank: Frequent Contributor
timestamp: February 27, 2025, 09:53:10 am
content: Quote from: gamalot on February 26, 2025, 06:45:23 pmNice and neat design I fully agree that modern COTS solution works just fine or just beter than many handmade solutions. I have used SN6505 with wurth transformer like your solution in the past. I find the isolation performance excellent. The noise can be belter. I have one question what are the four pin devices at the bottom right corner. Devices that cross the isolation barrier.

author: miro123
author_rank: Frequent Contributor
timestamp: February 27, 2025, 09:58:19 am
content: Quote from: Overspeed on February 26, 2025, 12:13:29 pmMaybe my description is too cryptic. The design of the transformer is a major task in building a power supply for voltage references. I was wondering how other builders measured or characterized the performance of their solutions

author: Kleinstein
author_rank: Super Contributor
timestamp: February 27, 2025, 11:45:02 am
content: The nonocrystaline and amorphous cores are a thing for relatively low frequency (e.g. 15 kHz). There they allow higher magnetization than ferrites. At the high frequenicy as used with the UCC28500 or SN6505 the eddy currents cause extra loss and the usable magnitization may already be limited by the loss, not by saturation. A nice point for the nanocrystalline cores is that they have low magnetostriction, so little acoustic noise even of used at 10 kHz.The layout for the SN6505 output side looks good - kind of 4 wire contacts to the tantalum capacitorand a small area for the first diodes and capacitor.I would consider an extra common mode choke - it may help a little with the higher frequency CM part. The choke could be at the output or in the supply to the SN6505.

author: gamalot
author_rank: Super Contributor
timestamp: February 27, 2025, 02:08:11 pm
content: Quote from: miro123 on February 27, 2025, 09:53:10 amThey are HCPL-181 photocouplers, used to control analog switches to select different output voltages.The components that are not populated on the PCB are the nRF52805 MCU and its peripheral devices, including Bluetooth, temperature sensors, LEDs, and a push button.

author: gamalot
author_rank: Super Contributor
timestamp: February 27, 2025, 02:10:30 pm
content: Quote from: Kleinstein on February 27, 2025, 11:45:02 amI think I will try your suggestion if I attempt to make a new version.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 27, 2025, 03:40:10 pm
content: With only a single voltage, I would consider the CM choke on the output side. The CM choke would than also be used to have the filter capacitors on both sides as an addition normal mode filtering. An additional MLCC costs very little.The old PCB could still work OK. If needed there are clip on ferrites to go to the USB cable and also a few turns through a ferrite torroid could work.

author: exe
author_rank: Unknown
timestamp: March 29, 2025, 06:54:19 pm
content: Which components would you guys recommend for LC filter on output of SN6505?AN-70 recommends COILTRONICS CTX100-3, but it seems it's a transformer, not a single coil. Quite a big device. I wonder if I found a wrong device, or they do use a transformer and connect it as a T-filter, or they put two devices? I'm not sure.

author: Andreas
author_rank: Super Contributor
timestamp: March 30, 2025, 06:13:14 am
content: Hello,I think they used 2 single devices. (otherwise naming would be L2a + L2b).And it makes no sense to transform the current ripple from first to 2nd stage.100 uH and iron powder makes sense for low frequencies around 100 kHz.with best regardsAndreas

author: Kleinstein
author_rank: Super Contributor
timestamp: March 30, 2025, 08:04:09 am
content: The COILTRONICS CTX100-3 could be used as a common mode choke = current compensated inductors. However the inductance is still relatively low. How much CM inductance is wanted depends on the capacity to ground. Also the quality (symmetry in coupling capacitance, maybe shield windings) of the transformer used with the SN6505 makes a difference in how much common mode signal is generated. If more than a single voltage is generated, one could have the CM filter also on the primary side.The rectifier and filtering for the normal mode ripple could also use some inductor. However this would be a more normal inductor. Here also the layout on the PCB is important - learned that from a poor layout. Instead of an inductor one could also use RC filtering, especially if the power is relatively low.

author: dietert1
author_rank: Super Contributor
timestamp: March 30, 2025, 10:18:23 am
content: Yes those small parts may not be enough. They are specified at 100 KHz, yet the switching spikes are at a much higher frequency, where the parasitic capacitance of an inductor gets more important. I remember some Marco Reps youtube video where he studied a transformer with much larger parts, with air distance between primary and secondary coils and spacers between coils and core. Each coil had only 2 or 3 turns.Today i made some measurements on a 30 VA toroid mains transformer with shield between primary and secondary. Even when using a 100 mH common mode yoke on the primary side the shield injected high frequency noise of about 0.5 mA rms into the guard net. Capacitance between primary and shield is about 265 pF.The special switcher transformer mentioned before has a coupling of less than 5 pF.Regards, Dieter

author: Kleinstein
author_rank: Super Contributor
timestamp: March 30, 2025, 12:48:42 pm
content: The coupling capacitance gets larger with a larger transformer.An example for a ready made transformer for the SN6505A is here:https://www.we-online.com/components/products/datasheet/750315240.pdfThey give 18 pF of coupling capacitance.For the higher frequency SN6505B the transformer can be smaller and less capacitance is possible.The exact construction can also make some difference, but here the effect is somewhat limited.

author: dietert1
author_rank: Super Contributor
timestamp: March 30, 2025, 02:42:21 pm
content: The "low coupling" transformer has about 4 or 5 times less coupling. That difference doesn't matter, of course, if somebody adds another 10 pF or so by the PCB design.Regards, Dieter

author: EC8010
author_rank: Frequent Contributor
timestamp: March 30, 2025, 06:07:05 pm
content: Quote from: dietert1 on March 30, 2025, 10:18:23 amYes, toroids have high interwinding capacitance compared to EI, so you should expect significant current to the shield. But that doesn't matter because the shield is (should be) connected directly to chassis. What matters is the capacitance between primary and secondarywith the shield earthed. In general, a single crude E/S screen reduces CPSto <10pF. But it is possible to do much better (as I described in post #137); I have achieved CPS= 8fF on a mains transformer.But reducing CPSisn't the end of the story. You need significant capacitance from the secondary to chassis to form a potential divider. 4n7 from each end of the secondary directly to chassis works. Assuming CPS= 10pF, you now have a 1000:1 potential divider to common mode interference, constant with frequency. If you add 470 ohm from each end of the secondary directly to chassis, you add a lot of low frequency attenuation, but depending on your rectification etc after the secondary, that might not be feasible.

author: David Hess
author_rank: Super Contributor
timestamp: March 31, 2025, 04:17:11 am
content: That SN6505 is slick. Linear Technology had some parts which could be configured to do the same thing with voltage and current slew rate limiting, but they are not cheap.I have been thinking of trying a resonate design to reduce noise, and tuning the frequency to match an LC notch on the secondary side.Shown below is how Tektronix was doing it in the 1970s.The inverter is operating at 40kHz. The pot core transformer has special construction where the centertapped primary (left side red insulated wires) is led in through a fixture separate from the secondary leads, so I assume the primary and secondary are physically separated and electrostatically shielded from each other. The transformer is suppose to support 1500 volts of isolation.This particular one had some tantalum capacitor failures and I am messing with it.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 31, 2025, 07:41:59 am
content: The SN6505 (alt least the A) version has one detail that can be anoying. To make it easier with EMI regulations it has some frequency modulatoin. The speed of that modulation is at some 50 Hz (a bit hard to measure) in combination with mains ripple at the input side this can cause low frequency modulation at the output. For a precision measurement the modulation with some 50 Hz can be an extra issue If really needed one could feed it an external stable clock.For the SN6505 the idea to reduce ringing after switching seems to be a rather short dead time. So switching is well faster than the self resonance in the transformer and avoid ringing by driving it hard. This does not work well with a resonant secondary.

author: David Hess
author_rank: Super Contributor
timestamp: March 31, 2025, 12:09:25 pm
content: Quote from: Kleinstein on March 31, 2025, 07:41:59 amDoesn't the datasheet say that the frequency dithering is disabled when an external clock is used?Spread spectrum clocking reduces the measured EMI because of how it is measured, but the amplitude is the same.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 31, 2025, 01:14:06 pm
content: The datasheet tells that the SN6505 used spread spectrum, but I have not found the typical modulation frequency. Having this close to the mains frequency has pros and cons.

author: exe
author_rank: Unknown
timestamp: April 01, 2025, 02:39:11 pm
content: There are many similar push-pull ICs on the market. I'm gonna try NXP's equivalent of those. Anyway, my mini-research showed at least these devices. Prices are for single quantity, from my mouser account.- max5075 (€ 12.94, just driver, no mosfets, no spread spectrum, programmable frequency)- LT3439 (€11.5, slew rate control, wide supply voltage)- LT1533 (€ 13.91, all-you-can-eat, 20-250kHz, slew rate control, 2.7-23V, etc)- LT3999 (€ 5.32, no spread spectrum, wide supply voltage)- MAX253 (obsolete? no spread spectrum, 200kHz or 350kHz)- MAX485 (€ 4.73, no spread spectrum)- MAX13253 (€ 8.04, spread spectrum, slew rate 200V/us, 250kHz or 600kHz, 5V)- NXF6505 (€ 1.62, A and B versions, 5V, slew rate 57/155 V/us, spread spectrum, differ from corresponding TI parts)- SN6505 (€ 2.14, A and B versions, 5V, 48/152 V/us, spread spectrum)I like AD/LT parts for their versatility and voltage range, but NXP is cheaper...

author: exe
author_rank: Unknown
timestamp: April 01, 2025, 02:55:19 pm
content: Btw, what do you guys think of a snubber?I originally saw that in one of LT documents (don't remember which), and I found this:https://www.ti.com/lit/an/slla566a/slla566a.pdf.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 01, 2025, 03:20:18 pm
content: A snubber could make sense, but one could well get away without it. It would only be for the rather short break before make time. Other than that the input drive FETs and the output diodes would dampen any ringing effectively.

author: dietert1
author_rank: Super Contributor
timestamp: April 01, 2025, 05:35:08 pm
content: Don't quite understand the slope ratings, e.g. 48 V/usec for the TI part. If the converter could run at 20 KHz, that is a 50 usec period time, can't we assign some microseconds to switching? That would be about 1 V/usec slope. So another factor 50 less EMI amplitude, or 34 dB. The snubber studied in the TI paper gets you 7 to 12 dB.I think the TI part is designed for applications where efficiency matters. When it's about some 200 or 500 mW for a voltage reference one can do better.Regards, Dieter

author: exe
author_rank: Unknown
timestamp: April 01, 2025, 06:06:02 pm
content: Could it be because most of the noise comes from dead time? We'll see, I'm building a small proto board to play with. I'll try different parts with different slew rate.Question: does transformer's V-t product depends on the load? I mean, can I violate it if there is always a load? My logic is that some energy will go to the output, so less chances to saturate the core.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 01, 2025, 06:23:24 pm
content: The required V-t product does not change much with loading. Essentially all of the output current will come from the input side as additional input side current. It is only the small voltage drop at the swich and winding resistance that reduces the effective voltage.One can reduce the input side voltage if absolutely needed.

author: DeltaSigmaD
author_rank: Contributor
timestamp: April 01, 2025, 11:09:33 pm
content: Power supply with low leakage with LT1683I'm missing the LT1683 in the mentioned list of push-pull controllers. I developped a power supply with this LT1683 since it has a slew-rate control feature to minimise noise. My application is a supply for external test circuits, e.g. a controlled voltage reference, which are working inside a current acquisition system operating in the pA to uA range with fA resolution. The entity "leakage current" (LC) must be defined: this is not the differential noise generated by the power supply (which may be actually very low), but the current flowing through the ground connector (not protection ground) of the power supply when it is connected with the protection ground of the power line system. Many manufacturers do not specify this LC. A good value for LC is less than 1 uArms (broadband), but even this is much too high when you want to measure fA (current between Gnd of supply to Gnd of acquisition system).I tried a design with the double ferrite core topology (coupled by a short-circuit winding) as dc/dc transformer to reduce the coupling capacitance and hence the LC. It was hard to find a useful loop compensation. The slew-rate control of the LT1683 is working insufficiently with this transformer type. Therefore, the switching noise spectrum is unexpectedly high which can't be accepted. I had to reject this design.A dual core transformer has a considerable stray inductance, so that many push-pull controller ICs will have problems with this topology. In general, it is difficult to suppress the noise of push-pull controller ICs : the duty cycle and the switching frequency are not stable enough to attenuate the noise by digital filtering performed by the data acquisition. Any nonlinearities mix varying switching noise down to the base band. Spread spectrum is useless for these applications.Therefore, I'm just developping a power supply on base of a microcontroller. The focus is directed on low leakage by a dual-core transformer and constant frequency by a quartz. The duty cycle can be fixed to obtain a stationary noise spectrum, taking a worse power efficiency into account. I will report here the results, hopefully in a few weeks.

author: trobbins
author_rank: Frequent Contributor
timestamp: April 02, 2025, 01:34:16 am
content: Another option to suppress coupling capacitance between input source and destination supply is to use multiple small commercial dc/dc, such as from vintage Newport, but now C&D etc. For example a 5V to 5V dc/dc NMA0505S has 100mA output capability with 28pF coupling. Two to three of those in series, along with an isolated plug-pack, should suppress coupling to <10pF in a convenient, relatively cheap, off-the-shelf, space-saving manner.That style of isolation has many model variations to allow different input and output voltage rails, and power rating, to suit an application, and some have lower coupling cap. The final dc/dc in the chain may benefit from some L-C filtering to suppress residual hf ripple, and a final linear regulator IC. I haven't personally tried this to confirm no anomalous behavior from interaction between converters.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 02, 2025, 06:57:25 am
content: The LT1683 is not made to directly drive a transformer, but for external MOSFET switches and thus for higher power. So it would be an odd choice for a low power solution. The extra area and traces alone can be an issue with radiated EMI.The weak point with the push pull converters is the fast switching and related EMI issues at rather high frequencies (e.g. ~ 100 MHz in the SN6505 snubber appl. note). This part can be hard to predict and depend on resonances in the layout / wireing.For the capacitive coupling it is not only the capacitance that matter, but also the effective voltage at the transformer. Here symmetry can help and shield in the transformer can help with symmetry. An easy point to keep the voltage low is to use low voltages at the transformer. So more a 5 V to 5 V transformer and if needed a charge pump for higher output voltages. The overall capacitance to ground also has the case and cable capacitance for the outputs. The transformer is only a part of it.

author: dietert1
author_rank: Super Contributor
timestamp: April 03, 2025, 06:44:16 am
content: Yesterday i looked at a Wien oscillator with a TL082 that i made some years ago. I removed the jfet gain control, so it produces a clipped sine wave of about +/- 12 V from a +/- 15 V supply. Second opamp is used as inverter. Adjusted frequency to 25 KHz.At the time i wired it with four larger bipolar transistors as output buffers, but i think the opamp might handle the supply of a reference all by itself. At the time i hand made a small toroid 1:1 transformer with 2.2 mH coil inductance, coupling of 2.5 pF and a shield. It's impedance is low for use without the buffers.Then i found two pieces of VAC 4097X011 transformer in a drawer. Those are 1:1:4 transformers, roughly the size as the WE part mentioned above by Kleinstein, except the "4" coil has 200 mH inductance and 2 Ohm resistance. That appears to be a good solution. Coupling capacitance is 25 pF. A RRO opamp with good drive capability can improve the circuit.Looking to buy that transformer, i couldn't find an offer. Yet i found another interesting part: VAC 4099X011. That is a 1:1:1 transformer with 0.95 mH per coil and a low 2.5 pF coupling spec.Another useful part may be the black Schaffner 100 mH common mode choke. I measured a coupling of 21 pF. Two of those can be used: as transformer and as common mode filter.Regards, Dieter

author: Kleinstein
author_rank: Super Contributor
timestamp: April 03, 2025, 07:14:06 am
content: The idea with a clipped sine looks OK. As a driver I would consider a small audio amplifier like LM4871 (5 V brigde for 3 W). Full bridge drive would ideally keep the average votlage at the coil constant, which could help with the effective voltage for coupling.For the waveform one anyway has to compromise between the voltage waveform and current waveform. A sine voltage tends to cause pulsed current with a simple rectifier.

author: David Hess
author_rank: Super Contributor
timestamp: April 03, 2025, 10:42:28 am
content: Quote from: dietert1 on April 03, 2025, 06:44:16 amAlso look for gate drive transformers. They have about the right characteristics.

author: exe
author_rank: Unknown
timestamp: April 23, 2025, 05:36:17 pm
content: So I built the push-pull circuit. It supposed to boost voltage from 3.7 to some 21V. I think I made a mistake somewhere, as output voltage (unloaded) is 26V at only 3.1V input. Trying to increase the voltage I got current quickly increasing. I guess either diodes give up, or I overload the transformer's VT. I might miscalculated something. Circuit main components:- trafo: wurth 750317072https://www.we-online.com/components/products/datasheet/750317072.pdf- driver: NXF6505Bhttps://assets.nexperia.com/documents/data-sheet/NXF6505_Q100.pdfI think it's a moderate success, given the fact that trafo has quite some long leads inside the enclosure, and there is no snubber yet.But I'm struggling to measure the noise, because I got 6-7mV p-p noise (full scope BW, >100MHz), regardless of leads connected or not (measured at the output, after LC-filter).If I use 1X probe, and limit BW to 1MHz, I got 220-300uV p-p noise, according to my scope, regardless the probe is connected or not. Go figureQuestion: is it normal in trafo I got 4X the input voltage? If you look at the input, you'll see 12V (plus ringing) p-p, but supply is only 3.1. I kinda expected it to be 2X

author: Kleinstein
author_rank: Super Contributor
timestamp: April 23, 2025, 06:02:28 pm
content: If one used the full winding at the secondary 4 x the input voltage is normal for the peak to peak value. Half the winding at the primary side see both a positive and negative voltage. So 2 x the input voltage as peak to peak for the primary (half winding) and thus 4 x for the full secondary.The unloaded output voltage can be higher from the ringing part on top.

author: exe
author_rank: Unknown
timestamp: April 23, 2025, 08:04:41 pm
content: I put 220 ohm resistor on output, got voltage dropped to ~16V (72mA output current). Input current is 0.5A.The output noise because 36mv p-p (full bandwidth). I didn't investigate further, but spikes are ~1us apart, roughly matching switching frequency (~450kHz). Also the drop is much more severe than I expected, I lost 10V on the output.

author: EC8010
author_rank: Frequent Contributor
timestamp: April 23, 2025, 08:37:53 pm
content: I've just looked at the PCB photograph in post #183. This is to be a clean supply for a voltage reference, right? It won't be with all that flux left on the PCB. Trust me; deflux properly and you will see circuits work better. "Properly" means holding the board vertical and scrubbing with defluxer from top to bottom to release contamination, shaking as much liquid off as possible then immediately rinsing and scrubbing from top to bottom with isopropyl alcohol to dilute and lose the contamination, and immediately drying with a pre-warmed hair dryer. Then do the other side. Then the first side again. Then the second side again. Not only does it make your soldering look much nicer, but it all works better, and with low noise or low current stuff, that's important.

author: dietert1
author_rank: Super Contributor
timestamp: April 24, 2025, 06:49:43 am
content: I see only one buffer cap close to the driver IC while the original schematic in post #175 had two of them, maybe 100 nF and 10 uF. Also it appears like the two primary side snubbers vanished.Did the driver IC change from ..A (160 KHz) to ..B (420 KHz) due to transformer saturation?Regards, DieterEdit: I think the 16 V is roughly what one expects, as the turns ratio of that WE 75037072 is 6.17. So if you start with 3.1 V you end up with 19 V minus driver saturation and diode voltage loss. Note that the primary inductance is 200 uH but in your circuit that divides by 4 as you drive each half separately.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 24, 2025, 06:58:49 am
content: The higher frequency of the B version is kind of needed to avoid saturation with that transformer. The scope confirms the higher frequency.An asymetric load on the output side (one side rectification) can also drive the core slightly into saturation and this way increase (maybe double) the current needed. In the extreme case diode self heating may shift a rectifier to asymetric operation.

author: branadic
author_rank: Super Contributor
timestamp: August 03, 2025, 05:45:56 pm
content: Not sure anyone noticed Shahriar's videohttps://youtu.be/y3f1qwPo_TYand the Harmony Pi filter component that was presented byTransSiP? They were kind enough to send me their EVB and some samples free of charge. I will test them in an application, in which I need to create low noise power rails from USB using a DC-DC-converter. While the current version is rated for 6 V, a higher voltage variant is already in progress, I was told.-branadic-

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 03, 2025, 11:11:34 pm
content: I wonder why the Austin transformer has not been mentioned yet in this thread.They are designed for both high voltage isolation and low capacitive coupling, and they are used to couple mains voltage for lights and such into live broadcast masts.https://en.wikipedia.org/wiki/Austin_transformer

author: exe
author_rank: Unknown
timestamp: August 07, 2025, 08:17:01 pm
content: Quote from: Doctorandus_P on August 03, 2025, 11:11:34 pmAre they suitable for low-power equipment? Most pics from the internet show big installations.I see a problem assembling a small device. If two ring cores are used, one has to be split and the glued back together? The split kinda adds an airgap, which is, I think, not needed here.

author: David Hess
author_rank: Super Contributor
timestamp: August 07, 2025, 09:12:40 pm
content: Quote from: exe on August 07, 2025, 08:17:01 pmOh, so there is a name for those. I have an old (1960s?) plug-in high isolation power supply which uses transformer steel laminations to make that 90 degree double core arrangement. I think it was rated at 18 volts and 30 milliamps.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 07, 2025, 09:26:05 pm
content: I don't think there are 2 cores. It is one ring core with the primary wound directly on the core. Different from more normal toroidial transformers the ration of inner to outer diameter is relatively large - so quite some space for windings.The special point is the secondary that is with maximum distance and air for insulation.The way it is build the secondary windings are quite a bit longer than for a more normal classical windings directly on top. This causes increased resitance loss and with a small size this gets more of an issue. So mains frequency and small size would be rather poor efficiency (e.g. 40 % instead of 60% for a more normal shape). Larger size as on the photos the drop in efficiency would be not as bad (e.g. maybe 90 % instead of 95%). With higher frequency the winding resistance is less of an issue.A downside of the Austin transformer is that there is a relatively strong stray field.

author: wanghar
author_rank: Contributor
timestamp: August 08, 2025, 08:15:45 am
content: Trying to design a Type-C powered supply module for my newly designed FPGA-based MS-ADC.Need ±15V for the main analog stuff (op-amps, ADR1399 or LTZ1000 ref).Need +5V mainly for the flip-flops and analog switches.The +3.3V for the CPLD/FPGA will be stepped down from the +5V on the main board.Component Choices:Skipped the classic L7815/L7915 – not enough voltage headroom (dropout) for them here.Went with ADP7118/ADP7182 instead. Overall performance should be better... though admittedly pricier.First time using the SN6505B! Just sketched the circuit so far – haven't actually tested it yet.P.S. --obviously LCEDA uses a wrong 3D model for L7805, so just ignore this fault.

author: miro123
author_rank: Frequent Contributor
timestamp: August 08, 2025, 08:38:31 am
content: Quote from: branadic on August 03, 2025, 05:45:56 pmI really wonder from the results.I have one question. Does the filter attenuate the common mode noise? When I look at schematics they use a common ground symbol. On another circuit they shows connection of two grounds together somewhere.

author: iMo
author_rank: Super Contributor
timestamp: August 08, 2025, 09:16:06 am
content: Quote from: wanghar on August 08, 2025, 08:15:45 amNice..I would put a shielding over the parts - see below.Not sure where to ground it, however..PS: I would also add small caps, like 1-10nF for example, in parallel with those 4u7 tantalums (and to that 1/10uF around the L5 too) in all those CLC filters there..Also I saw somewhere doubling a single CLC filter helps a lot too..Why the R3 is placed there as-is?

author: Kleinstein
author_rank: Super Contributor
timestamp: August 08, 2025, 09:42:20 am
content: The HarmonyPI filters seems to be for the rather high frequency part. There a good CM filter can be relatively easy and work well.For an isolated precision supply the difficulty is not so much in the really high frequencies > 10 MHz, but more with lower ones (like 100 kHz or what ever the switching frequency of the transformer). Here the impedance at the output side is not necessary that low, as it is mainly the capacitance to ground that sometimes also is wanted to be not that large. So I am afraid no magicly improved CM filter here.@wanghar:The CM filter looks rather small, not sure if this would help very much at the frequency of the SN6505 and with a not so low impedance to ground.The SN6505 ideally wants some snubbers for the 100 MHz range - Ti has a AN on this. Ideally I would include a bit more capacitance on the input side of the DCDC converter - with USB there is however some limit (some 10 µF AFAIK) on how much is allowed there.The diodes at the rectifier would see a peak voltage of a bit over 2 x the DC output. So more like 40 V or even 50 V diodes needed.The filter design is somewhat sensitive to the layout, especially with the ground. It may help to separate the first filter stage with most of the AC currents from the rest with only a single ground link from there to avoid ground loops all the way to the output.I don't think 1 nF caps in parallel to the tantalums would make much difference. Some 100 nF or 1 µF MLCC could still make sense. At least have the option for a few more capacitors if really needed.P.S. the LDOs for the +-15 V looks rather small size. They may still need a bit more cooling, especially for the start up of a heated reference.

author: wanghar
author_rank: Contributor
timestamp: August 08, 2025, 03:30:31 pm
content: @KleinsteinI took a look at TI's application notes(slla566a, slla436 and sllu223) then made some modification on the schmetics:1) add more capacitance on the input side of the converter and follow the layout guide TI sugggested in ANs. Also add a few pcs of 104 mlcc on the output side of converter.2) Reserve snubbers place holders on board, the values of R and C need to be determined later.3) The currently placed CM filter has a impedance of 1000Ω @100MHz, almost be no effect for the SN6505B when it works at 420KHz. considering remove or short.4) You are right the diode need to be re-spected with a higher voltage. Changed to MBR0540 or 5819.

author: wanghar
author_rank: Contributor
timestamp: August 08, 2025, 03:52:32 pm
content: @iMOI do not think that is a good choice to use a shield covering both input and output portion of the converter(transformer). Taking reference from TI EVM board I also use 2 seperated copper pour on the PCB (before and after transformer) to serve certain shielding effect.The main function of R3（R5 inrevised sch ） is to offload a portion of the thermal load from the L7805. Assuming the FPGA/MCU current consumption is 30mA, the voltage drop across the 100-ohm resistor will be 3V. This reduces the voltage drop across the L7805, preventing heat dissipation from being concentrated solely on the L7805. I've changed the resistor package to 1206.

author: iMo
author_rank: Super Contributor
timestamp: August 08, 2025, 04:48:24 pm
content: Quote from: wanghar on August 08, 2025, 03:52:32 pmI would put there a 3.2V zener as the serial voltage dropper (HP does it in its high end meters, for example). But wired after that CLC low pass (like the R7 is wired)..