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Bobbin material rating DC vs AC voltage

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dtproff

Wed Feb 24 2016, 10:50AM

Registered Member #57929 Joined: Thu Nov 12 2015, 03:29PM
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Posts: 5

I notice that most of the materials for bobbins are rated at DC. I have had a hard time finding out what the effects of frequency are on the dielectric strenght.

Any insite here would be appreciated.

Sulaiman

Wed Feb 24 2016, 02:42PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

It depends upon the frequency and dielectric loss tangent of the material,
I found that hv at 100's kHz is enough to damage some plastics
but formers for ferrite transformers are intended for use at 100's kHz so should be no problem.

Patrick

Wed Feb 24 2016, 08:45PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Sulaiman wrote ...

It depends upon the frequency and dielectric loss tangent of the material,
I found that hv at 100's kHz is enough to damage some plastics
but formers for ferrite transformers are intended for use at 100's kHz so should be no problem.

what plastic is used for SMPS transformers Sulaiman ?

Sulaiman

Thu Feb 25 2016, 07:10AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

I don't know, I think it is some form of modified P.E.T.

dtproff

Thu Feb 25 2016, 03:14PM

Registered Member #57929 Joined: Thu Nov 12 2015, 03:29PM
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A little background on why I asked.

Originally I used Sumitomo E4008 (8 segments for the HV and a slot for isolation with the another slot for the primary on a EER core) with a 1mm wall between the core and the HV segments as well as segment to segment). I blew up 10 or 15 transformers using various potting materials and magnet wire coatings. The frequency was 30-200KHz, 300W, resonant half bridge with a low voltage input and a doubler output, a 25Kv peak output.

Next I tried Noryl (it was a black material)... same there. I switched from an EER32 bobbin core to a OP44130UC core and moved the primary away. I changed my wall thickness to to 3mm and moved the primary to the opposite leg. Still kept blowing them up.

I then changed to Sylgard 182 for the potting compound and used poly carbonate for the milled bobbins. Still no luck. But what I could see in the clear material was how I was breaking down. It was eating through the bobbin jumping to the core and running over to the primary side.

So I switched to UHMW-PE and had success. I also went back and annealed the milled PC and had success there also. Now that I have my isolation and creapage paths cleaned up some of the materials I used in the past are now working.

So that was 7 months of winding and experimenting before I came up with a build configuration I was happy with. I am in the process now of going back and revisiting the various build materials one at a time to find out which ones I keep and which I throw out.

So now you know the geneses of my question about the AC vs. DC specs in the material data sheet.

dtproff

Mon Feb 29 2016, 01:36AM

Registered Member #57929 Joined: Thu Nov 12 2015, 03:29PM
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Sulaiman wrote ...

I don't know, I think it is some form of modified P.E.T.

Most of the transformer bobbins I have designed used a Sumitomo E4008 or equivalent material. That's the same material used in most ATX power supplies and adapters. It is a LCP (Liquid Crystal Polyester) with a 40% glass fill. Good dielectric numbers but it broke down with 1mm of wall thickness between the core and the winding.

woodchuck

Mon Feb 29 2016, 02:36AM

Registered Member #39190 Joined: Sat Oct 26 2013, 09:15AM
Location: Boise National Forest
Posts: 65

Your experience with bobbins has been most interesting! Annealing PC...who woulda thought? UHMW-PE - marvelous stuff! But until now I knew it only as Spectra, the lines on the reserve canopy of my PPG.

Anyway, it seems that plastics with low dielectric constants (which correlates with covalent bonding) tend to deal with high frequency better. So one would expect, everything else being equal, for polyethylene and polypropylene to perform better than polycarbonate, polyester, or PVC.

radiotech

Tue Mar 08 2016, 05:14PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
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Posts: 1546

One aspect may be the effects of ozone on the plastic you choose. This was an issue in
high voltage parts of television sets with cathode ray tubes.

Flachzange

Fri May 19 2017, 05:21PM

Registered Member #61569 Joined: Sat Apr 15 2017, 05:12PM
Location: Germany
Posts: 28

I had the same problems with my bobbins for the construction of a 50 kV 10 kHz transformer.
In the end i used teflon since it has the lowest tan delta value of all insulating materials, that was still machineable.
All other bobbin materials i used failed in the same way as you described.

The dielectric constant of the plastic is not that important. Moreover the tan delta represents how much heat is dissipated within the dielectric.

jpsmith123

Sun May 21 2017, 03:58AM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

You were getting 50 kv @ 10 kHz from your transformer? I can see where that would be a challenge.

IIRC some simulations I did with "pie" type windings on ferrite cores with a square cross section predicted corona (i.e., e-field greater than 30 kv/cm in air) at the square edges of the core underneath the bobbin (with Vo = 12.5 kv peak @ 30 kHz).

On my ferrite cores, one of the things I did to try to minimize it was to file the sharp edges a little bit with a small diamond file.

Flachzange wrote ...

I had the same problems with my bobbins for the construction of a 50 kV 10 kHz transformer.
In the end i used teflon since it has the lowest tan delta value of all insulating materials, that was still machineable.
All other bobbin materials i used failed in the same way as you described.

The dielectric constant of the plastic is not that important. Moreover the tan delta represents how much heat is dissipated within the dielectric.

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