[Amps] Real time tests to see if an RF transformer is saturating?

[Chris Wilson]

Hello Manfred,


Thanks  for  your  usual  very  comprehensive and clear reply, this is
superb  info  and  should help me a lot on a practical and theoretical
level,  although  I  sometimes  think  theology has more to do with RF
calculations :)  Please see my reply to Alan Ibbetson where I go into
a bit more detail of how I hope to progress this. With a better choice
of  ferrite  and the additional turns I will post in due course how it
works  out. I would welcome opinions on NOT using a transformer on the
combiner,  but  instead changing the output transformers of the 2 amps
to  match into 100 Ohms. Someone else mooted that suggestion. pros and
cons? Thanks again, I learn a great deal from all your posts here!

On Sunday, May 14, 2017,  you wrote:

> Chris,

> it's very easy to calculate the flux density. Easier than measuring it.
> So, let's do the maths:

> You have 2kW applied to a 7 turn winding. Assuming the impedance at that
> point is 50 ohm, 2kW is 316V.

> You have three FT-240-77 cores. That core has an effective cross 
> sectional area of 1.57cm². Three of them are then 4.71cm², or 0.000471m².

> This is all we need to apply equation 4 from my web page

> http://ludens.cl/Electron/Magnet.html

> 316V / 7 turns / 4.44 / 0.000471m² / 136000Hz = 0.159 tesla

> That's the peak flux density in your cores, meaning that the total flux
> swing is between 0.159 and -0.159 tesla. This is comfortably below 
> saturation, which begins roughly at 0.3T, but it's in a range where core
> loss is high.

> Let's consult manufacturer's data for core loss. Extrapolating the data
> given in a graph in the material's datasheet, it looks like the 
> volumetric loss is around 600mW per cubic centimeter of ferrite. Each of
> your cores has a volume of 22.6cm³, so you have a total of 67.8cm³, and
> that means a power loss of slightly over 40 watts in those cores! That's
> clearly FAR too much.

> So, saturation is no problem, but power loss in the cores is excessive.
> You need to either add more turns, or design a different transformer, so
> you get lower core loss.

> You might start looking at the material. Type 77 is really not so good
> at 136kHz. There are newer materials that have lower loss at that 
> frequency, at the roughly the same permeability and other data. The 
> question is what you can buy...

> If you want to keep the same cores, you need to increase the turns 
> number. At 136kHz the wavelength is enormous, so you won't run into 
> trouble from excessive wire length.

> Let's take 5W as an acceptable maximum core dissipation. That would make
> it warm, but probably not too hot. So you need 8 times lower core loss,
> which is roughly 75mW/cm³. Consulting the loss graph given by the 
> manufacturer, the allowable peak flux density is roughly 50mT. You have
> to triple the current number of turns to achieve this: 15 turns primary
> and 21 turns secondary should work.

> A lower-loss ferrite would allow you to get away with fewer turns for 
> the same core size, but never as few as you have now!

> And a pot core, RM core or even EC core would probably be better than 
> stacked toroids - if you can find one large enough!

> And at that frequency a bundle of thin wires has lower loss than a 
> single solid wire of the same diameter, and is far easier to wind.

> Manfred







-- 
Best regards,
 Chris                            mailto:chris at chriswilson.tv