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

[Roger D Johnson]

If you can find it, you're better off using Litz wire.

73, Roger


On 5/16/2017 8:52 AM, Chris Wilson wrote:
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> Hello Manfred and Alan,
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> I stacked 3 off 78 material cores in the same size as the 77 material
> toroids and wound them with 180 strand speaker wire with a conductor
> diameter of about 2mm. I used 15T primary and 21T secondary. It now
> runs about 40 degrees Celcius with no fan! The drain waveforms are not
> as spiky now either, so many thanks for the excellent advice :) I am
> toying with buying the much bigger core that I linked to earlier as
> there is an error on Fair-Rite's site, and the new 98 material is not
> available in toroidal form at all. Worthwhile or over kill and a waste
> of money? Thanks again!!
>
>
> http://uk.rs-online.com/web/p/ferrite-rings/4956317/
>
>
> On Sunday, May 14, 2017,  you wrote:
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>> 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
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