UD2.7C Coilcraft Slot 7 (Seven) Phase Lead Inductor

[faraday]

I purchased a few 7M3 inductors from Coilcraft some time ago for use with my SM UD2.7C driver board.  The driver works fine, except for the inductors.

If I try to turn the slug down very far (not even close to bottoming) into the inductor, either the slug or the ceramic tuning tool will fracture.

I've been in contact with Bryan, the Coilcraft Product Quality Supervisor over the course of the last two months.  To summarize -

Coils have been in production since the 60's.
They couldn't get the old coil form material starting a few years ago, and had to switch to a glass filled material.
The glass filled material isn't as smooth as the old material, so the ferrite sticks to it.
They've also had to switch suppliers on their tuning tools.
The most recent tool supplier has been doing a poor job.
They re-sent me 3 tools to try.  All three were shaped differently, and only one was close to fitting the slug.
Even with the best of three tools I still fractured the ferrite slug, AND the tool tip.

They were supposed to get back with me with another set of replacement coils, and tools, but it's been 3 weeks and I haven't heard.

With the glass filled form material problem, they didn't sound like they would have a solution anytime soon.

I'd suggest staying away from these inductors on new designs, and maybe redesigning existing PCB's for something else.

OR, does anyone have a suggestion for a footprint compatible inductor, still in production and available through Digikey or Mouser?

[profdc9]

I recently had good luck winding my own with these coil forms I bought on ebay.  They fit into the SLOT 7 inductor format:

https://www.ebay.com/itm/20-Pcs-Inductor-Shell-Skeleton-Empty-Ferrite-Core-No-Inductor-Coil-25-100MHZ/172915617600

I have pictures of the empty and wound forms.  I used about 70 turns to get up to 50-60 uH inductance, which is adjustable by about 20 to 30%.  Still, the ferrite slug is a bit fragile, and I did crack a little bit off of one of them.  For 10 cents a piece, it's hard to go wrong to try it, and if you crack a slug, you can swap in another from another form.

Dan

[faraday]

Many thanks Dan, I just ordered the eBay part you suggested, as a temporary solution.

If anyone redesigns the UD2.7 again it would be nice to have a footprint compatible in production coil.

Also, I don't know if shielding is important, but the eBay solution won't yield a shielded coil.

But for now, maybe this solution will get me by...

[profdc9]

I resdesigned the UD2.7 with both a slot 7 and a screw terminal connector for a hand-wound inductor or other inductor that might not fit into the slot.  It is a redesign that uses all through-hole parts.  It may not be what you want, but it provides flexibility to use alternate inductors.

https://highvoltageforum.net/index.php?topic=283

Dan

[Garyf]

In addition to the slug problem, i also had a failure of the 7m3 that reduced performance. I only found the cause when i jumpered the 7m3 out of the circuit and the performance returned to normal. This happened after an igbt failure and replacement.

[Mads Barnkob]

I have also destroyed my Coilcraft Slot-7/10 inductors as well, I also had some really huge ones I found on ebay long ago, they are not as nice but they certainly do not break that easily.

[faraday]

Dan,
Have you tried using an external inductor on the screw terminals?

A few weeks ago, when I first ran into this problem, I took a physically larger inductor, cut it down winding wise till the inductance was correct based on an LRC meter, and soldered it as tightly to the board as I could.   Because it was dangling there it seemed to be picking up enough EMF from the tesla coil that the signal going to the comparator got corrupted.  I could be wrong on this, but that's what I remember.

So, have you, or anyone else gotten a UD2.7 variation working with a dangling inductor?

If not, it may be better if the board is ever redone again, to find a reliable alternative board mount inductor.  I'm in a real time bind now, and can't do it as we've sold the house and moving soon, or I'd spin it myself.

BTW, Coilcraft is sending me another set of tuning tools to try.  They had to custom hand grind them to try to get the fit right.  Coilcraft has been really great in trying to help with this (their problem), but unfortunately, even if they fix the tuning tool, they still have inductors where the slug is too tight in the wrong material coil form.  Sigh.  And they are definetly not going to redesign the slot 7 style inductor as the demand is too low to justify it.  One thing I may try on a last go around is to run a flat bottom tap of the appropriate thread into the coil form to open up the form a bit so it won't be that tight.  What a mess.

So, dangling inductor work?  Yes / no?

[Hydron]

If it helps, I have used the following two parts for my own UD2.x based driver:

https://www.digikey.com/product-detail/en/toko-america-inc/A119ANS-T1038Z/TK2410-ND/25271
https://www.digikey.com/product-detail/en/toko-america-inc/A7BRS-T1040Z/TK2417-ND/11220

I have not measured the adjustment range, but I can confirm that I was able to adjust for both TO-247 and CM300 based coils (possibly using the larger inductance for one, smaller for the other, though I do not remember).

I picked these as they are available from digikey rather than requiring a separate coilcraft order. If used in sockets (i.e. without soldering them down) then you need to be careful not to over-adjust the ferrite slug, as it will pull the cover off the base. This doesn't do any permanent damage, and is only a problem because they are not soldered down as intended on my board (to allow for swapping).

[faraday]

Thanks for the tip.  Is the Toko coil easily adjustable without breaking the slug or tuner?  What tool do you use to tune it?

[profdc9]

I did not try the dangling inductor because I got the ebay inductors to work, but I built a variable inductor by taking several smaller fixed axial lead inductors and wiring them in series, each in parallel with a jumper block so I can short out each inductor to vary the inductance.  This could be done on the board to prevent any noise pickup, so the inductance would just be modified by setting or removing jumpers in a binary-like pattern.  I might try this at some point.  If you find a particular inductor that is useful, let me know, and also the kicad project is included with the gerber files in case you want to add it yourself, and the kicad layout software is open source software.

Dan

[Hydron]

Thanks for the tip.  Is the Toko coil easily adjustable without breaking the slug or tuner?  What tool do you use to tune it?

Yes, and any old flat-head screwdriver works (using a steel rather than plastic screwdriver didn't seem to matter either).
I would suggest checking the Toko footprint/pinout, it may be different (I can't remember if I had to change it or not).

Also if a wider range is needed then something like the jumper-shorted fixed inductor could also be put in series with an adjustable one.