If you can find it, you're better off using Litz wire.
73, Roger
On 5/16/2017 8:52 AM, Chris Wilson wrote:
Hello Manfred and Alan,
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:
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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