Power supply for voltage references

[Kleinstein]

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.

[Andreas]

Andreas, primary shield is tied to primary GND and earth / case,

Earth / 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.

secondary shield is tied to secondary GND.

I would have expected to a guard (inner housing)
Secondary GND does not make any sense for me. (will not reduce coupling capacity).

with best regards

Andreas

[fcb]

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.

[Echo88]

https://eu.mouser.com/datasheet/2/427/vom617a-1767067.pdf 0.3pF and no EMI from the Digital Isolators
There might be even better Optocoupler, i just found it after a bit of searching.

[fcb]

https://eu.mouser.com/datasheet/2/427/vom617a-1767067.pdf 0.3pF and no EMI from the Digital Isolators
There might be even better Optocoupler, i just found it after a bit of searching.

VOM617, great device, use on numerous projects.  But it can't do 200Mbps

[Echo88]

Sorry, was thinking about power supplies and feedback-coupling and not about your data transmission project.

[branadic]

Earth / 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.

If 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.

I would have expected to a guard (inner housing)

There is neither a guard nor an inner housing in this units.

Board design finished and order placed.

-branadic-

[branadic]

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-

[Kleinstein]

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.

[robert.rozee]

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.gif

are 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   :-)

[doktor pyta]

@robert.rozee

kind 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

[Kleinstein]

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.

[Gyro]

I hesitate to post this as it's a completely off 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.

[macaba]

I hesitate to post this as it's a completely off the wall idea.

As 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: 0.005uApp
Without charge base nearby/on: 0.4uApp
Without charge base nearby/on, inside thick steel box section grounded to PE through 1M resistor: 0.2uApp
Whilst on charge base: 1.5uApp

As 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)

[exe]

maybe the switching waveform is too noisy, but it might just trigger some ideas.

https://youtu.be/JJgKfTW53uo?t=1320 . This, of course, rerpresents only one model from one brand. Other may do it better.

[Gyro]

I hesitate to post this as it's a completely off the wall idea.

As 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: 0.005uApp
Without charge base nearby/on: 0.4uApp
Without charge base nearby/on, inside thick steel box section grounded to PE through 1M resistor: 0.2uApp
Whilst on charge base: 1.5uApp

As 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)

Wow, 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.

https://youtu.be/JJgKfTW53uo?t=1320 . This, of course, rerpresents only one model from one brand. Other may do it better.

Thanks 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.

[Kleinstein]

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.

[dietert1]

...
Edit: I'm unsure if there might be some interaction with the DMM6500's own leakage (0.14uApp)

That 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

[TexasRanger]

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.

[Kleinstein]

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.

[dietert1]

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

[robert.rozee]

@robert.rozee
kind 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

the 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/#msg139940

points to an actual implementation that someone has built here:

https://www.mikrocontroller.net/articles/Royer_Converter
https://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   :-)

[Echo88]

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-pumps 

https://www.buerklin.com/medias/sys_master/root/h5c/hea/9313902428190/8873768026142.pdf

[exe]

The next thread would be what type of oil is the best for volt-nutting.

[Gyro]

I think that one's already been done.     https://www.eevblog.com/forum/metrology/oil-for-a-leaking-tinsley-standard-resistor/