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