EI or C-Core

Design and use of the various types.
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julien591
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Joined: Tue May 28, 2019 8:58 am

EI or C-Core

Post by julien591 »

Hi everyone,

I love transformers and I always search proof why there is a claimed superiority of C-Core over EI.

In fact, I use EI with Finemet core and OCC silver everywhere, from my power transformer, to my PSU choke to my output transformer.

When I had to choose between C-Core and EI, I choose EI because it's told that unpotted C-Core are less stable than EI core.

Now I am searching why C-Core is really the holly grail.
AudioNote, Tribute, Noguchi, Tango, Monolith seems to prefer C-Core.

On paper, it looks like C-Core can beat EI, but what about the real life ? what about the sound ?

Thank you in advance,
Best
dave slagle
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Post by dave slagle »

Hey,

first... if you are using finemet, then it must be a C-core or toroid since the material is only available in thin ribbon form. There are three main differences between stamped lamination (EI) and ribbon based (C) cores.

Lamination Thickness:

The ribbon or tape based cores can be made with much thinner materials so cores with materials thinner than .006" can be easily fabricated. In the case of Nano and Amorphous where the materials are always 0.001" or less and very brittle the ability to stamp and stack a core is not economically possible which is why they are always found as tape wound cores.

Gap Size:

Toroids have the smallest possible airgap but cutting a tape wound core into a C-core increases the minimum gap size appreciably. The EI or other stamped laminations allow for stacking with an alternating patter which allows for a much smaller effective gap than possible with a C-core.

Grain Direction:

One of the huge benefits claimed by the C-core crowd is the ability for the grain direction to be in the same direction for the entire magnetic path. This is indeed important if you are using a grain oriented material like M-6. Both round loop 49% and 80% nickel materials are non-oriented so this benefit does not come into play for anything I do.

Since the Amorphous and nano are only available as tape wound cores and since all of the transformer I do with these materials have DC offset the only choice is C-core. For the nickel it becomes a choice fo smaller airgap for EI vs. thinner lamination thickness for C-core. When it comes to anything small signal, the smaller airgap dominates for me. Granted I could look to toroids but they come with their own baggage.

Anyone that tells you one core type is better than the other without complete application data is likely a member of the marketing department not the engineering department.

dave
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julien591
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Joined: Tue May 28, 2019 8:58 am

Post by julien591 »

Hello Dave,
Thank you for your explanation.
first... if you are using finemet, then it must be a C-core or toroid since the material is only available in thin ribbon form. There are three main differences between stamped lamination (EI) and ribbon based (C) cores.
I think it was a custom order for EI lamination in Finemet with thickness of 0,01mm.

My Power transformer, is in Z9 because of the low efficiency of Finemet at low frequency.

he EI or other stamped laminations allow for stacking with an alternating patter which allows for a much smaller effective gap than possible with a C-core.
Can you tell me the minimum DC gap in a C-core ?

Thank you again :D
dave slagle
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Post by dave slagle »

hey,

I have never heard of laminations being done in finemet and was under the impression that it was not possible. the glas in metglas represents the brittle nature of the material and think of the difficulty of stamping anything out of glass. I suppose one could laminate stack together and water-jet cut out a E and an I in the form of a solid. That seems unlikely since the amorphous material that finemet starts out as is then annealed in a field to give it the needed properties and this gives an oriented material so the anneal would need to happen after the E and I sections are assembled.

The minimum gap size of the C-core comes down to two things.... surface area and polish of the gap

I have found that E's and I's stacked with a butt gap will actually give a smaller gap than a well polished C-core. If you think of it, 3 points define a plane so in even a perfectly polished C-core, only 3 contact points are possible. Now take a similar EI core stacked from say 36 laminations. each E-I pair has the same three point contact rule but each one can operate independent of each other which leaves you with a smaller average gap size over the surface area.

When you include surface area into the mix, this is where the alternately stacked laminations really give an advantage over a cut core. the increase of gap surface area greatly reduces the effective gap size. the extreme example of this is the toroid core. Many people will insist that a toroid is gapless but this is incorrect. The gap in a toroid is actually the entire face of the ribbon which is so large that some say makes the core 'essentially gapless'. This is then incorrectly interpreted as toroids don't have a gap. Many people go beyond this and simply insist that since there is not an apparent physical gap, the core is gapless. Clearly put, Toroids have a gap but since it has such a large surface area many say it can effectively be ignored. In fact this is often a problem for power transformers since any DC imbalance on a power line can cause issues with the small gap inherent to the toroid. There is a patent on a process of winding a toroid with 4 discrete tapes each offset 90º. This means that for any magnetic loop to close is must jump 4 of the small gaps instead of 1 which makes for a larger effective gap without the need of the cutting process.

dave
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julien591
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Joined: Tue May 28, 2019 8:58 am

Post by julien591 »

Hello Dave,

Thank you for your reply.
I will search more information about the EI Lamination. I never heard about it before.

If I understood well, EI is really great when we need a minimal gap.

What do you think about the Hitachi FT-8K50D ? It looks wonderful for audio !
http://www.hilltech.com/pdf/Hitachi/Dat ... Series.pdf

Is it possible to find such material for a C-Core project ?

Best
dave slagle
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Post by dave slagle »

I do not believe those hitachi materials are available in cut cores since they must be field annealed and once annealed, cutting the core destroys the properties.

there is a line of the hitachi finemet C-cores it is the F3CC line.

dave
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