How to glue ferrite SMPS transformer halves together?

[Flyback]

Hello,
We are doing a 60W isolated offline flyback (120W peak).
We are using an ETD44 ferrite core with 1mm gap. The transformer winding company say that they cannot glue the ETD44 ferrite core halves together because it is not a robust method of adhesion. Is this true?
They tell us that they must instead use the core clips to hold the core halves together and then varnish the cores so that they stick together.
As such, do you agree that glue cannot be used to glue the core halves together?

The 1mm gap is in the centre leg (only) of the ETD44 core, and I believe that the core halves can be held together while the glue sets in the outer core arms by squirting “hot melt” glue into the air gap to hold the core pieces together whilst the outer ferrite arms are glued together. (obviously ‘hot melt’ glue cannot be used to glue all the surfaces of the core together because it goes soft at 85 degC.)

So do you agree that “hot melt” glue should be squirted into the core gap to hold the core halves together while the proper glue in the outer core “arms” drys off?

I presume you agree with me that the permeability of air and “hot melt” glue is similar enough to mean that the inductance of the coils will not be affected by having “Hot melt” glue in the gap instead of air?

ETD44 core datasheet:-
https://www.italtras.com/filePDF/etd_44_22_15.pdf

(Sorry to ask, though I felt many could benefit from this knowledge)

 

[jpanhalt]

A 1-mm gap in a ferrite core is huge. It is like having separate cores. You might ask, why hold them together at all? (That is a small exaggeration.) If you just want to physically hold them together in some loose approximation to each other, hot melt or an epoxy will work.

If you want to couple them closely, consider that ferrite is like a glass, so I would treat it like glass. Polish the parts that need to fit tightly together. I can't suggest anything for high volume production, but for my low volume experiments, I use 3M abrasive papers (actually, they are not paper; they are on a polyester film). They come in grits <6000 to fractional microns. I lap the parts with water (a drop of soap in 100 mL helps) on a flat glass plate. Then the parts almost stick together like lapped measuring blocks. A very tiny drop of the most liquid CA adhesive you can find will bond them forever.

John

 

[Flyback]

Thanks, you say its a huge gap, but its only 3.1% of the overall centre leg length. And also, it's an off-the-shelf gap provided by epcos, so it surely cant be that unusual?..otherwise they wouldn't sell such a thing as an off the shelf part?

Your method is of interest, though as you say, for volume production, we would need a quicker method...I am wondering if just squirting hot melt into the gap to hold them toghether whilst the glue sets in the outer "arms" is a feasible method?

 

[jpanhalt]

Hot melt will certainly do what you suggest and it sets rather quickly, but it remains flexible. An acrylic or epoxy (mixed at application) can have a similarly short setting time, be gap filling, and be more rigid. They also probably have more peel strength. You might even consider some newer silicone polymers. However, if this thing is going to be mechanically attached, then the rigidity and peel strength may be a non-issues, so hot-melt might be the cheapest and easiest way to go. It has a very long shelf life compared to the other adhesives I have mentioned.

John

 

[Flyback]

thanks, I think in that case hot melt it is, as it only has to hold the cores together long enough for the long-time-setting glue in the outer arms to set.
I wonder how different the hot melt permeability is than air...I mean, its supposed to be air in the gap....certainly I suppose the permeability can be no less than air..pity the datasheet doesn't say it for hot melt.....then again, the magnetic permeability of glue is usually not of interest...but it is in this case.

 

[ronsimpson]

I have used tape, clips or varnish to hole the cores. I tried super glue for a while.
Some Hot Melt flows at 185C. I don't know when it gets soft. Some data sheets say 80C. My power transformers run hot.

 

[Mosaic]

Hmm, rather than lapping, how about doing basic filing for fitment and then mixing the resultant ferrite powder with the CA glue and make a ferromagnetic adhesive? You can separate the magnetic powder from any other 'dust' with an electromagnet.

 

[ronsimpson]

The core comes from the factory with the middle leg shorter. You don't need to file.
Don't add much of anything between the two pieces. Super glue is fine but anything thick will change the gap.

 

[alec_t]

anything thick will change the gap

Presumably if the centre leg has a 1mm gap, a few extra microns (or even tens of microns) won't make much difference to the overall reluctance? So lapping or grinding etc seems unnecessary.

 

[Flyback]

well according to our transformer winder company, all the above are of no use, and they must be held together with clips, and then varnished to hold them together. I cant see how varnish can hold them properly when subject to vibration.?

 

[Mosaic]

Well, when I mod a microwave transformer, I grind off the laminate E/I welds, pull off the windings and shims, rewind and reassemble the E/I laminations with JBweld. Being a core/copper starved MOT, efficiency ain't so good to start with anyway. JBweld reduces the vibration noise considerably, got 6 years out of my eldest mod so far. None have failed.

 

[jpanhalt]

well according to our transformer winder company, all the above are of no use, and they must be held together with clips, and then varnished to hold them together. I cant see how varnish can hold them properly when subject to vibration.?

I don't have much experience with varnish as an adhesive. I do have experience using shellac. It is a nice, early form of hot-melt adhesive. I learned to use it from an uncle who was a watchmaker of the old type. I suspect the varnish (BTW, you could consider modern polyurethane adhesives as varnishes) will do fine.

John

 

[kubeek]

I don´t undetstand your problem, my ETD cores look just like this https://newimg.globalmarket.com/PicLib/770/706770/prod/20_1308553723938_l.jpg The two halves are inserted into the coil former, and each one is held and tensiond frimly by its metal spring clip, so you cannot have any airgap on the outside posts. Varnishing is a nice touch, but the spring clip is the key to success.

 

[chemelec]

I just use an Aluminum Plate on each end, drilled on each side for a screw on the sides.
two Long Stainless Steel Screws with Nuts holds these nicely.
The Aluminum and Stainless Steel won't affect the transformers performance.

No Glue and it can be taken apart if ever needed to do so.

 

[ronsimpson]

This is what Flyback is talking about. The bobbin is such that tape will not work. There is no place for welding or screws.
Use the clips! There really is no other choice with this bobbin.
I like varnish or what ever they dip the transformer in. You know that clear dip that holds the parts from vibration and keeps the water out.

 

[chemelec]

This is what I use.
Also on those types of cores with the Pins.

.

 

[Mosaic]

Mebbe a couple zipties?

 

[tunedwolf]

well according to our transformer winder company, all the above are of no use, and they must be held together with clips, and then varnished to hold them together. I cant see how varnish can hold them properly when subject to vibration.?

I would say they are correct in telling you to use Transformer Varnish, however it won't be for holding the core halves together, that's what the clips are for. The manufacturer designed and manufactured them specifically for that purpose, so I suggest you use them. The Varnish will be for vibration damping and likely flashover protection. I find that when I go to professional winders for something, even if it is just to pick up some enamelled wire, that their advice is worth listening to, after all, they deal with those problems all day every day

 

[ronsimpson]

Mebbe a couple zipties?

Zips are good for a hobby.
In a production product (power supply) heat from the transformer and time will make the plastic brittle and will break.