-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Dick
Sent: 13 June 2006 14:34
To: [email protected]
Subject: LF: PA4VHF/p propblems with core/high voltage etc
Aperently the N30 core seems to be not the best solution as an step-up
transformer.
Luckely I do have another 3c85 core (50-60mm) lying around.
Is it better to use this one instead of the N30 material??
Dear Dick, LF Group,
The peak flux density in the core is given by
B(peak) = Vrms/(4.44.f.n.Ae) (for a sine wave voltage)
Where
B(peak) is peak flux density, T
Vrms is the voltage across the winding, V
f is frequency (Hz)
n is number of turns
Ae is effective cross-sectional area in m^2(nearly the same as actual
cross-sectional area for a toroid)
(Be careful of the units - the manufacturer's data is often given as mm^2,
cm^2, square inches, etc...)
The flux density does not depend on the type of ferrite - so if you are
using cores of similar cross-section, and the same number of turns, the flux
density is similar also. What does change is the specific core loss, Pv,
which is a function of type of ferrite, flux density and frequency. For 3C85
at 137kHz and 100mT (for 500W TX and so Vrms = 158V, and 16t primary), Pv is
somewhere around 160kW/m^3, according to the graphs in my old Philips data
book. The effective volume of a TX51 size toroid is 21500mm^3, (i.e. 2.15 x
10e-5 m^3 - careful of units, again), making the total power loss in the
core about 3.4W. Obviously I am just guessing about what size the core is,
but it should be about right. To this you will need to add some watts of
losses in the winding resistance. Overall, this will be a substantial
improvement on the N30 material, but will require good ventilation all the
same.
There is a trade-off involved in the number of turns - increasing the number
of turns reduces the flux density and so the core loss, but increases the
loss in the windings. Also, having a large number of turns makes the leakage
inductance higher, and a small number of turns makes the winding inductance
too low.
Cheers, Jim Moritz
73 de M0BMU
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