For a sinusoidal waveform, the minimum number of turns on a core is given by
:
V(rms) = 4.44. F . N . A . B where
V is the voltage across the winding
F is the frequency in Hz
A is the core cross sectional area in m^2
B is the peak flux in Teslas
To avoid excessive dissipation, for modern ferrites if you keep B to a value
of 0.2T or less you will be OK.
As an example, for a core of 15mm diameter (approximately an ETD49 core)
used in a 700 Watt PA the 50 ohms winding is calculated as follows :
V = SQRT(700 * 50) = 187V(rms)
This corresponds to a cross sectional area of 177mm^2, or A = 177*10^-6
m^2
V = 187, F = 137000, B = 0.2, A = 177E-6
N(min) = 187 / (4.44 * 137000 * 177E-6 * 0.2) = 8.7 Turns
Taking to the nearest integer above, you will at least need 9 turns for this
winding. You could use more turns to get B even lower, but now the copper
losses begin to increase as the wire length rises so adding a couple of
turns to get B even lower is the most you really need to do, so 10 to 11
turns is about right for this core.
Note that the type of ferrite is not important, all transformer ferrites
saturate as you get near 0.3T, althoug h modern ones MAY allow a bit higher.
This equation seems to suggest that any size of core is acceptable for any
voltage provided enough turns are used, and in a way this is true, but
getting higher power through the core needs larger cross-sectional area.
The calculations here are more involved, and it is easiest to resort to the
data sheets and look at what the manufacturer claims is the maximum rating
for any particular core.
As a guide line, on my switchmode PA that generated RF direct from 340 Volt
rectified mains (see QEX November/Dec 2002) I found an ETD49 sized core
using 3C85 material would happily carry 700 Watts with B kept to below
0.18T. The primary winding operating at 29 ohms load impedance was made
with 3mm diameter plastic covered Litz wire, and the secondary winding for
matching into 50 to 150 ohms antenna resistance from 1mm diameter enamelled
wire. At 700 W continuous operation for several hours in UK summer
temperatures, the transformer core sat around 60 - 70 degrees - comfortably
within its rating
Andy G4JNT
-----Original Message-----
From: Marek SQ5BPM <[email protected]>
To: [email protected] <[email protected]>
Date: 2005/03/29 14:46
Subject: LF: More on cores...
Hello all,
I noticed that people are using much more turns than me in the output
transformer, so I would appreciate any help in calculating the correct
number of turns for the cores I am using here. Messing around the trafo
would probably allow me to achieve more power than now (450W). Trying to
go higher causes heavy ringing + FET heating. I've tried almost
everything to reduce this (different combinations of Zobel network,
decoupling, inductances in series with gates, screening, grounding
etc.). In every case around 400W things start to go bad, 450W is a
border of manageable heat and ringing. I am thinking about making a new
FET driver in a way that allows regulation of the time of the FET
on-state, leaving some rest period in-between.
Here are some details about my configuration: the power supply is 45V,
dropping to 38-39V at around 12A (depending on the 230V mains voltage).
I am using two R40/24/16 rings made of K2006 material, stuck together,
for detailed info see
http://www.kaschke.de/english/framesets/frame_ringkerne.html . After
experimenting with different configurations, currently there's 2+2 turns
in the primary winding, 5 or 6 turns in the secondary winding. I
remember that 4+4 primary didn't work good, but this was just a short
try with low power - maybe there's a way to make this work with that as
well. Cores are slightly warm but never get hot. The power supply is
driven through a coil made of a single core (the same type), consisting
of 5 turns. This one gets hot only when there's too much ringing in the
PA. Adjusting turns in this one does not change a lot.
Thanks in advance for any help, I hope to make the TX again from scratch
this summer: bigger, better & more powerful ;)
73! Marek SQ5BPM
P.S. Don't be surprised seeing another special callsign on LF, these
days we are celebrating 25-th anniversary of the SP5ZCC Scouts Amateur
Radio Club. I am planning some LF operation with this call on April
9-10th. For the time being, I hope to see some of you in the SP DX
Contest CW/SSB on April 2nd-3rd (HF). Check
http://www.contest.spdxc.org.pl/en/index.php for more information.
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