Bobbin material rating DC vs AC voltage
dtproff, Wed Feb 24 2016, 10:50AMI 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.
Re: Bobbin material rating DC vs AC voltage
Sulaiman, Wed Feb 24 2016, 02:42PM
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.
Sulaiman, Wed Feb 24 2016, 02:42PM
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.
Re: Bobbin material rating DC vs AC voltage
Patrick, Wed Feb 24 2016, 08:45PM
what plastic is used for SMPS transformers Sulaiman ?
Patrick, Wed Feb 24 2016, 08:45PM
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.
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 ?
Re: Bobbin material rating DC vs AC voltage
Sulaiman, Thu Feb 25 2016, 07:10AM
I don't know, I think it is some form of modified P.E.T.
Sulaiman, Thu Feb 25 2016, 07:10AM
I don't know, I think it is some form of modified P.E.T.
Re: Bobbin material rating DC vs AC voltage
dtproff, Thu Feb 25 2016, 03:14PM
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, Thu Feb 25 2016, 03:14PM
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.
Re: Bobbin material rating DC vs AC voltage
dtproff, Mon Feb 29 2016, 01:36AM
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.
dtproff, Mon Feb 29 2016, 01:36AM
Sulaiman wrote ...
I don't know, I think it is some form of modified P.E.T.
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.
Re: Bobbin material rating DC vs AC voltage
woodchuck, Mon Feb 29 2016, 02:36AM
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.
woodchuck, Mon Feb 29 2016, 02:36AM
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.
Re: Bobbin material rating DC vs AC voltage
radiotech, Tue Mar 08 2016, 05:14PM
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.
radiotech, Tue Mar 08 2016, 05:14PM
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.
Re: Bobbin material rating DC vs AC voltage
Flachzange, Fri May 19 2017, 05:21PM
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.
Flachzange, Fri May 19 2017, 05:21PM
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.
Re: Bobbin material rating DC vs AC voltage
jpsmith123, Sun May 21 2017, 03:58AM
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.
jpsmith123, Sun May 21 2017, 03:58AM
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.
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.
Re: Bobbin material rating DC vs AC voltage
Flachzange, Sun May 21 2017, 07:23AM
Well, i also rounded the edges but furthermore the entire transformer is emerged in shell diala x3 and degased in a vacuum for 10 hours. If you are interested i can post some pictures in the next few days.
The transformer is made for a voltage multiplier and so far it is working great.
Flachzange, Sun May 21 2017, 07:23AM
Well, i also rounded the edges but furthermore the entire transformer is emerged in shell diala x3 and degased in a vacuum for 10 hours. If you are interested i can post some pictures in the next few days.
The transformer is made for a voltage multiplier and so far it is working great.
Re: Bobbin material rating DC vs AC voltage
Finn Hammer, Sun May 21 2017, 08:13AM
Flachzange,
You have my attention, keep those pictures coming.
Cheers, Finn Hammer
Finn Hammer, Sun May 21 2017, 08:13AM
Flachzange,
You have my attention, keep those pictures coming.
Cheers, Finn Hammer
Re: Bobbin material rating DC vs AC voltage
jpsmith123, Sun May 21 2017, 10:17PM
I vacuum encapsulated all of my secondary coils in epoxy, and I shielded them from the ferrite core with some copper foil. They seemed to work when I briefly tested them, but I haven't really run any of them for a long time.
I take it that the failures you experienced happened before you put the transformer in oil?
jpsmith123, Sun May 21 2017, 10:17PM
I vacuum encapsulated all of my secondary coils in epoxy, and I shielded them from the ferrite core with some copper foil. They seemed to work when I briefly tested them, but I haven't really run any of them for a long time.
I take it that the failures you experienced happened before you put the transformer in oil?
Flachzange wrote ...
Well, i also rounded the edges but furthermore the entire transformer is emerged in shell diala x3 and degased in a vacuum for 10 hours. If you are interested i can post some pictures in the next few days.
The transformer is made for a voltage multiplier and so far it is working great.
Well, i also rounded the edges but furthermore the entire transformer is emerged in shell diala x3 and degased in a vacuum for 10 hours. If you are interested i can post some pictures in the next few days.
The transformer is made for a voltage multiplier and so far it is working great.
Re: Bobbin material rating DC vs AC voltage
Flachzange, Sat Jun 03 2017, 08:32AM
Hi, here are the promised pictures of the first transformer that i build.
The housing is not that great and i am just going to finish the second one that should perform better.
On the first picture you can see the bobbin which is machined from Teflon. It has a 30mm hole in the middle for the ferrite core (100mmx27mmx27mm and the corners are rounded). The individual sections are not cut to the same depth. The first one is the deepest, and the last one on the left the flattest. This is also the high voltage side of the winding. Therefore the distance to the core is bigger and the electric field stress is reduced. Additionally the parasitic capacities are reduced which is also achieved by the winding in these sections.
To get the wire from one section to another i have just sawed a small groove diagonally in the walls between the sections.
On the second picture the transformer is shown in the aluminium housing filled with diala x3. The left copper sphere is the high voltage output and the sphere on the top of the picture is grounded to act as a safety gap.
The third picture shows the multiplier arranged for an output of 300kV. I have used this multiplier with 8 Stages up to 750kV however corona discharges get a major issue (Even with 1m toroids).
the multiplier on the picture works corona free up to 250kV then i get discharges from the small toroid on the upper right.
Flachzange, Sat Jun 03 2017, 08:32AM
Hi, here are the promised pictures of the first transformer that i build.
The housing is not that great and i am just going to finish the second one that should perform better.
On the first picture you can see the bobbin which is machined from Teflon. It has a 30mm hole in the middle for the ferrite core (100mmx27mmx27mm and the corners are rounded). The individual sections are not cut to the same depth. The first one is the deepest, and the last one on the left the flattest. This is also the high voltage side of the winding. Therefore the distance to the core is bigger and the electric field stress is reduced. Additionally the parasitic capacities are reduced which is also achieved by the winding in these sections.
To get the wire from one section to another i have just sawed a small groove diagonally in the walls between the sections.
On the second picture the transformer is shown in the aluminium housing filled with diala x3. The left copper sphere is the high voltage output and the sphere on the top of the picture is grounded to act as a safety gap.
The third picture shows the multiplier arranged for an output of 300kV. I have used this multiplier with 8 Stages up to 750kV however corona discharges get a major issue (Even with 1m toroids).
the multiplier on the picture works corona free up to 250kV then i get discharges from the small toroid on the upper right.
Re: Bobbin material rating DC vs AC voltage
Patrick, Sat Jun 03 2017, 09:35PM
nice.
jpsmith did something similar
but yours has diagonal channels, winch i think is safer.
Patrick, Sat Jun 03 2017, 09:35PM
nice.
jpsmith did something similar
but yours has diagonal channels, winch i think is safer.
Re: Bobbin material rating DC vs AC voltage
jpsmith123, Sun Jun 04 2017, 02:33AM
Nice project you've got there.
As per the picture Patrick linked to, I put intermediate grooves in the bobbin to try to keep the wire away from the "sides" of the windings. So once one section is complete, the wire goes from the top of that section through a vertical slot and down to the bottom of the intermediate groove, then after a few turns the wire goes through another vertical slot and is introduced at the bottom of the next winding section, and so on.
All I wanted out of my coils was 12.5 kv peak. I think if I needed 50 kv peak (or more) I might try to use an air core transformer.
jpsmith123, Sun Jun 04 2017, 02:33AM
Nice project you've got there.
As per the picture Patrick linked to, I put intermediate grooves in the bobbin to try to keep the wire away from the "sides" of the windings. So once one section is complete, the wire goes from the top of that section through a vertical slot and down to the bottom of the intermediate groove, then after a few turns the wire goes through another vertical slot and is introduced at the bottom of the next winding section, and so on.
All I wanted out of my coils was 12.5 kv peak. I think if I needed 50 kv peak (or more) I might try to use an air core transformer.
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