EMRFD Message Archive 14212

Message Date From Subject
14212 2017-08-17 09:04:36 mosaicmerc Gapping an Impedance Transformer to adjust AL
Hi All:
Still working on the LTspice model of a 4Mhz,  200W class E amp. getting around 90% Eff in the model.
Placing parasitics now to finalize expected LC values.

Selected a salvaged 1500 u, AL 2150 flyback core for the 80uH RFC. Might be mix 31. Lossy at around 25KΩ per inch of core material length.=> MnZn

Anyway, I am using a salvaged CRT yoke ferrite AL 730, u =800 (seems to be mix 43) as the final transformer.
Very high material resistance so it's NiZn

To do the (50Ω) 1:4 impedance match the core AL needs to be about 500,  to deliver a 4Mhz, single turn, 12.5Ω XL loading.

I added such an air gap using a folded transparency strip.   I now have a 500 AL  for a 3/4" copper bar being the primary.

Is this an acceptable approach to modify a core?

Another question: With around 6A RMS expected in the 465pf series tuning capacitor (up to 1.8KV across it)

Do I need one of those hard to get double ganged air variables? Or can I go with fabricating a variable cap. out of  copper clad PCBs. I note the Q (I measured it  - Mastech MS5308 LCR) is around 70 for the copper clad DIY units. It's not like I plan to change the base frequency so tuning adjustments are limited.

The 5 mil kapton dielectric in the flex PCB is good to > 3KV.
Once the capacitor is set I can  paint it with liquid electrical tape to secure and insulate.

I don't expect to run the amp for more than 5 min at any one time. it's for RF CW calibration purposes.

14214 2017-08-20 09:28:11 kb1gmx Re: Gapping an Impedance Transformer to adjust AL
The bigger issue is core heating.  Lossy cores will heat considerably.
Gapping to lower mu/al helps saturation and with tuning but not losses.
Most transformers are gapped to improve or avoid magnetic saturation.

Myself I'd be looking at maybe something like with a u125 liket 61 material.


14215 2017-08-20 18:39:08 mosaicmerc Re: Gapping an Impedance Transformer to adjust AL
Alas, the core material is limited to what I can salvage. I'll have to experiment with the type 43 results, a bit of heating isn't too much of an issue as the operation of the CW is for a few minutes at a time.

On a related matter, the tuned circuit of a Class E CW amp behaves much like a loop antenna with a couple KVpp across the capacitor.

My Spice design requires around 465 pF for the series tuned  4Mhz unit.

So  I have a requirement for an HV variable capacitor on the cheap, which isn't easy.

I've looked at the DIY 'trombone' tubular copper tubing designs etc, but they are way too bulky.
I've experimented with using regular copper clad PCB with mylar, kapton, PTFE and LDPE (sandwich bags) dielectrics.

The LDPE dielectric seems the best given it's availability, cost, DF and HV properties. The Q factor is >2K (tested) and a 4.5 sq inch of capacitor plate area (PCB) delivers the capacitance. A compression approach appears to allow for sufficient tuning. About an 8:1 tuning range with quality soldered connections and flat copper braid conductors. On installation the copper braid becomes solder filled for rigidity and improved conductance & corrosion resistance.

I have another trial to do....etching the square cut PCB copper capacitor plates (perfectly round), and building a laser cut compression (acrylic) housing with polystyrene or rubber gasket compression pads that is sealed from humidity (and contains a bit of dry silica gel)  to preserve the copper from any skin effect losses due to tarnishing.

The application is low HF so skin effect shouldn't be an issue.

All the materials mentioned are cheap and readily available.

The copper resistance is only around 1 milliohm (combined both plates)  which provides for a very good current handling capability. The inductance will be low as well.
14232 2017-08-29 13:32:55 Sandeep Lohia Re: Gapping an Impedance Transformer to adjust AL
For gapping in Ferrite core can we use paper or LDPE? 
Or must be air only ...

14233 2017-08-29 14:11:22 Sandeep Lohia Re: Gapping an Impedance Transformer to adjust AL
How much inductance do I need with  LM2576 T-5V SMD 100 uh or 330 uh , 12 volt is input & 5 Volt out max .8 Amp max.

Ferrite transformer's performance is reduced permanently if it gets saturated,
 or too hot to touch because of thin wire used?

Does it make any difference in skin depth in between litz wire or simple multistan Teflon wire ( not varnished ) ?
Copper braid from coax R also not varnished & all multi wires R DC short from one another but at RF?

On Aug 30, 2017 2:02 AM, "Sandeep Lohia" <sandeeplohia12@gmail.com> wrote:
For gapping in Ferrite core can we use paper or LDPE? 
Or must be air only ...

14234 2017-08-29 18:32:28 mosaicmerc Re: Gapping an Impedance Transformer to adjust AL
That inductance is freq. dependent. Higher freq= lower inductance required as XL goes up. A common core used with that buck reg is 47uH with the necessary 25% headroom on the max current.