Author Topic: What ferrite toroidal core should use for 160-210 kHz 2-4 kW power rating  (Read 372 times)

Offline ScottE

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I will be using litz wire (composed of 4000 strands of 0.07mm (41 AWG) wire with Teflon outer insulation (4.75 mm diameter) good for 80 amps at 200kHz.
I want the power that it can handle 2-4 kW.
What type of ferrite core I should use and what physical size.
Have any ideas of how many turns I should with 1:1 ratio. Most likely I will use a different ratio.
I have never done anything ferrite transformers so I don't know the answers to these questions.

Offline T3sl4co1l

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For a typical e.g. N87 material, at that frequency and size you will want Bmax ~ 70mT, maybe more or less.  Expect a core around 70mm o.d. (toroid) or ETD50 (E-E style, various cross sections).  This should be a good starting point; vary size larger or smaller depending on how much winding area is needed.

Flux density Bmax gives the volts per turn:
Vpk / N = 4*Bmax*Ae*Fsw
for a square wave with full duty cycle (if this is a full-wave forward converter or the like).  Ae is in the datasheet, and N is the number of turns.

What leakage inductance do you need?  Litz has the regrettable property that, because it's transparent to magnetic fields (that's how it works), all the space within as well as around the cable counts towards leakage inductance.

For example, if you're making a welder with DC output, you'll need leakage low enough that it doesn't burn out your diodes.  That would be, uh, well 0.5*LL*Ipk^2 is the energy in the leakage, so the power dissipation (as diode avalanche or burned in a snubber) is that times Fsw.

If you use about a meter of cable for the winding length, with no interleaving, just overlap typical of a toroidal transformer say, then expect around 0.6uH of leakage.  Which would give, at 80A and 200kHz, 384W of dissipation.  Indeed, 0.6uH would drop 60V at 80A and 200kHz -- this is actually so much leakage, compared to your load impedance (less than an ohm!), that such a transformer would actually be practical in a resonant topology!

If low leakage is a concern, consider using alternating layers of foil on a rectangular style core (EE?).  A planar transformer is also not unreasonable, but gets a bit challenging to design at this scale.

Tim

Offline Mads Barnkob

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You should visit the websites of ferrite core manufacturers, many of them have excelent guides on choosing the right core for your application. F.ex. here is a short guide from mag-inc.com: https://www.mag-inc.com/Design/Design-Guides/Transformer-Design-with-Magnetics-Ferrite-Cores
http://www.kaizerpowerelectronics.dk - Tesla coils, high voltage, pulse power, audio and general electronics

Offline ScottE

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I like the mag-inc site. Thanks for the info. I am a novice in high frequency power electronics and i am trying to get up to speed. I know somethings that I want to do, but i am finding out that thing can get complex quite quickly. I made a vacuum tube tesla coil in the early 1970's with 3kV transformer and 1 kW tube pats that I got from a scraped broadcast transmitter. But I have not done anything like this since then.

I am planning on building a different type of double resonate tesla coil. First I am going to make a switched full bridge tuned resonate RF power system (around 200 kHz) at about 4 kW using UnitedSiC SiC cascode MOSFETs. This part of what I needed RF transformer for. I want the secondary of the transformer to go to low impedance twisted pair transmission cable made up litz wire.(I want to do this some I want be away from the primary and secondary of Tesla coil.) This cable will connected to the primary of another RF transformer that will link the primary of the Tesla coil. The primary will be a tuned magnetic loop antenna (with many turns of litz wire) with vacuum variable cap.(This tuned antenna have very high Q so it will have quite high voltages and currents within it.) The secondary of the tesla coil will also be wound with litz wire.
I am planning on trying using different configurations of the primary to see what works best. Changing the coupling factor using different diameters of primary coil.

I have the litz for both the primary and secondary and a heavy pyrex glass coil form the secondary. So I have acquire a lot of stuff before I can build it.

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