Author Topic: Current sense transformer: Pri:Sec coupling (Read 1855 times)

Faringdon · « **on:** July 26, 2022, 11:59:50 am »

Hi,
Please may i ask this small "pinpoint" question here, as otherwise i doubt it will see responses, due to "question saturation" of my other post?....

Doing Current sense transformer for 1000W Boost PFC with UCC28070A.
E20/10/6 core
Primary = one turn of TEX-ELZ 7/0.3mm
Secondary = 50 turns of 0.2mm TEX-E

Is the primary to secondary coupling good enough in the attached? (seen from four angles)

voltsandjolts · « **Reply #1 on:** July 26, 2022, 12:27:22 pm »

Quote from: Faringdon on July 26, 2022, 11:59:50 am

due to "question saturation" of my other post?....

pcprogrammer · « **Reply #2 on:** July 26, 2022, 03:42:44 pm »

@voltsandjolts, wow that must have taken a bit of time to write up. That's a lot of topics to come up with in 4 weeks.

@Faringdon, if it is your own design, should you not know what is correct yourself? I don't know what your deal is, but if you are trying to make a product to sell, all these reasonably simple questions you are asking do not build confidence in the quality and reliability of your product. Have you thought about that?

Don't see this as criticism, but as a genuine concern. To quite a bit of the questions you could easily have found your answers with google.

I also hope that you can distinguish the bullshit from the right responses to your questions, because some are making fun with you, and I'm sorry to say I, in some way, did too. Not in this thread though.

jonpaul · « **Reply #3 on:** July 26, 2022, 08:09:02 pm »

Farrington again a Bad design, seems a lack of understanding of current sense transformers theory or design.

easier solution Use stock toroid CT from Coilcraft, PE and other stock transformer vendors, perhaps $0.50...1.00$ @ 1K

It would be wiser for you to do your homework, research read and learn electronic design rather than your endless quest for free consultation on our forums.

Alternative, pay for a magnetics or power electronics consultant

Cordialement

Jon

j

T3sl4co1l · « **Reply #4 on:** July 26, 2022, 09:17:27 pm »

k ~ 0.99, I would guess.

Whether that's enough, I don't know, it's not my design.

Tim

Faringdon · « **Reply #5 on:** July 27, 2022, 10:53:58 am »

Quote

Farrington again a Bad design, seems a lack of understanding of current sense transformers theory or design.

Thanks, i once worked on a 300W Buck converter, 300v to 100v. It used a current sense transformer 100:1 for high side current sense......it used a split bobbin.....primary wound on one half, then secondary wound on the other half, on the other side of the separator. This Current sense transformer worked fine.
The pri:sec coupling that i have here is way better than that, so i cannot see why the attached (top post) should not work?

Quote

Whether that's enough, I don't know, it's not my design.

The application is like on page one of the UCC28070A datasheet
UCC280780A
https://www.ti.com/lit/ds/symlink/ucc28070a.pdf?ts=1658905447165&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FUCC28070A%253Futm_source%253Dgoogle%2526utm_medium%253Dcpc%2526utm_campaign%253Dapp-null-null-GPN_EN-cpc-pf-google-eu%2526utm_content%253DUCC28070A%2526ds_k%253DUCC28070A%2526DCM%253Dyes%2526gclid%253DCj0KCQjwxIOXBhCrARIsAL1QFCYQqiprVP8eos1f--9DXwNKmG7JbjFRVHQHzbp05_WWAKOtSdlECzoaAhOPEALw_wcB%2526gclsrc%253Daw.ds

Quote

Alternative, pay for a magnetics or power electronics consultant

Thanks, would your own consultancy be willing to do this work?, please could you possibly provide a quote/lead time?
Only 2 pieces are needed.
(Boost PFC, 220VAC in, 390Vout, 1kw, as per UCC28070A datasheet, will tell rest of spec after rough "ball park" quote?)

Quote

easier solution Use stock toroid CT from Coilcraft, PE and other stock transformer vendors, perhaps $0.50...1.00$ @ 1K

Thanks but sorry i should have said, we need this prototyping PCB to be good for a long time, and cannot be sure any offtheshelf part will be in stock whenever we come to order when we need to make one of these again...and it must fit the footprint.

Quote

Farrington again a Bad design, seems a lack of understanding of current sense transformers theory or design.

Thanks, had to say it because this is really interesting for us.....you do not have access to this transformer, neither do you have the full manufacture spec, so may we ask, please advise what is it from the above info/pictures that convinces you that this is a bad design?

Quote

your endless quest for free consultation on our forums.

OK my apologies, and so now, i contribute...here is a free SMPS course for anyone (305 people so far, after some 12 hours of the post) who presumably "googled into" this thread (from a google search using perhaps "current sense transformer").... for their learning...

SMPS course_Big folders
https://drive.google.com/file/d/17ij_pQLkAzzYeL_TRNci86sXHMirKHmD/view?usp=sharing

SMPS course_little folders
https://drive.google.com/file/d/1WvElhDNfHI0EBei4rouSNEdWWTO1oCtJ/view?usp=sharing

Mark · « **Reply #6 on:** July 27, 2022, 06:56:22 pm »

Have you seen the CT used in the UCC28070EVM?

https://www.mouser.co.uk/datasheet/2/87/eaton_corporation_coois08603-1-1734362.pdf

Perhaps an off the shelf toroidal inductor and add your own primary?

If you stick with your own transformer you could try making some measurements, inductance, coupling factor, frequency response etc.

mc172 · « **Reply #7 on:** July 27, 2022, 08:03:21 pm »

Quote from: Faringdon on July 27, 2022, 10:53:58 am

OK my apologies, and so now, i contribute...here is a free SMPS course for anyone (305 people so far, after some 12 hours of the post) who presumably "googled into" this thread.... for their learning...

SMPS course_Big folders
https://drive.google.com/file/d/17ij_pQLkAzzYeL_TRNci86sXHMirKHmD/view?usp=sharing

SMPS course_little folders
https://drive.google.com/file/d/1WvElhDNfHI0EBei4rouSNEdWWTO1oCtJ/view?usp=sharing

Curiosity got the better of me so I became one of those 305. It's not a course, it's a collection of articles, EEVBlog videos and application notes interspersed with Word documents full of rambling.

For example, there's a Word document containing the following:

Quote

There are all ders where a smps is in thermal test with in enclosed plastic (totally)

Please refer to below DER
DER805
DER803
DER600
DER804
DER856
DER820

How is this useful?

Here's another one:

Quote

HiperTFS-2 2TFC driver

FSW is either 66khz or 132khz
It is voltage mode.

Great.

There's also a really useful quote from a transformer company for the attention of -redacted- for some custom 10kW transformers that I'm never going to order.

jonpaul · « **Reply #8 on:** July 28, 2022, 10:26:05 am »

Farrington:

Nice white paper:

http://www.datatronics.com/pdf/current_sense_article.pdf

Stock vendors

http://www.datatronics.com/product.asp?cat=Transformers,%20Current%20Sense,%20SMT

https://www.coilcraft.com/en-us/products/transformers/power-transformers/current-sensing/

https://www.digikey.com/en/products/filter/current-sense-transformers/163

https://www.pulseelectronics.com/current-sense-magnetics/

No layout or design can assume the same parts of PCB footprint will be available indefinitely.

We use multiple sources or pad out for alternatives.

Bon Chance

Jon

Faringdon · « **Reply #9 on:** July 28, 2022, 06:58:03 pm »

Thanks very much Jon Paul.
For the next PCB i'll use one of those you recommend

I must admit that the positioning of the current sense transformer in the drain of the PFC FET sounds a bit off. (as is required with the UCC28070A) AYK, When the PFC FET switchs on, the SiC diode capacitance will get suddenly charged through the current sense transformers primary, and the leakage inductance will ring like mad with it….the ringing will get transferred to the secondary of the CST , and its the reset network that will be left with the extra job of clearing this up.
Must admit though, the same thing happens in a HV Buck with CST in its source, and the one I saw like that worked fine. I couldnt believe it when i took apart that CST......with the primary sitting all by itself on the one half of the bobbin separator, and the primary squashed into the other half. (300V to 100V buck at 300w)
(LTspice and PDF shows the problem with UCC28070A CST PFC)

jonpaul · « **Reply #10 on:** July 28, 2022, 08:03:15 pm »

Rebonjour Cher Farrington

for exactly the reasons mentioned, it is considered poor engineering to place any CT on the collector, FET drain, hot side, with 100s V fast voltage pulses,

CT interwindingS capacity is unspecified and difficult to design a shield with safety compliant creep, strike, insulation

Thus every PFC, SMPS, electronic ballast I ever have seen places the CT at or near ground or bus return rather than at a hot and fast leg of a converter.

One idea ...use a thin coax like RG/174U for the CT primary one turn, coax center conductor is CT primary at hot lead, ground ONE end of coax shield, leave other side of coax shield open cktmto,avoid shorted turn.
This technique could be used on the EE core design in your photos as well

Bon courage

Jon

PS...
My consulting fee...any Tektronix scopes 7000, plug-ins or 2465B, 2467B or parts,/spares....

uer166 · « **Reply #11 on:** July 28, 2022, 08:16:49 pm »

Quote from: jonpaul on July 28, 2022, 08:03:15 pm

it is considered poor engineering to place any CT on the collector, FET drain, hot side, with 100s V fast voltage pulses,

I believe this is a unhelpful generalization. By placing CT in source you're increasing the common source inductance between gate drive and the switching loop path, potentially causing issues with the gate drive. It's an optimization of what you actually need, and I would (by default) place the CT in the drain and deal with the dv/dt there, rather than compromising the gate drive.

T3sl4co1l · « **Reply #12 on:** July 28, 2022, 08:43:48 pm »

Putting it in drain circuit doesn't bother me; coupling of switching edges, through the tiny capacitance of a single wire link, is easily filtered given the bandwidth typically required for current sense (a few MHz). Better there than source for above mentioned reason, and the inductance can be used to help reduce EMI (at the expense of some snubbing power).

Shielding can also be done in the usual way, foil around the secondary, slightly more than one turn to overlap its ends, but insulated to not make a shorted turn. And grounding the core is probably helpful too.

Tim

jonpaul · « **Reply #13 on:** August 03, 2022, 11:29:56 am »

Rebonjour:

We found a A properly selected or designed CT with 1 T pri, will have neglible insertion R and L thus little effect on placement at the source electrode.

A toroidal CT with safety insulation will have a few pF from P lead to S.

Depending on the Vbus (normally 360V offline) and drain risetime, the placement in drain electrode is not optimal.

Just my memories.

Jon


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Author Topic: Current sense transformer: Pri:Sec coupling (Read 1855 times)

Faringdon

Current sense transformer: Pri:Sec coupling

voltsandjolts

Re: Current sense transformer: Pri:Sec coupling

pcprogrammer

Re: Current sense transformer: Pri:Sec coupling

jonpaul

Re: Current sense transformer: Pri:Sec coupling

T3sl4co1l

Re: Current sense transformer: Pri:Sec coupling

Faringdon

Re: Current sense transformer: Pri:Sec coupling

Mark

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mc172

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jonpaul

Re: Current sense transformer: Pri:Sec coupling

Faringdon

Re: Current sense transformer: Pri:Sec coupling

jonpaul

Re: Current sense transformer: Pri:Sec coupling

uer166

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T3sl4co1l

Re: Current sense transformer: Pri:Sec coupling

jonpaul

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