Chris,
Nobody is answering so I'll have a go. If I muck up the arithmetic
others will be quick to cut me off at the knees :-)
I make the max flux density around 160mT (1600 Gauss).
If you can't be bothered to do the calculation from scratch there's an
online calculator here
http://www.daycounter.com/Calculators/Max-Flux-Density-Calculator.phtml
The data sheet for type 77 material says it has 300mW/cc loss at 100KHz,
the upper freq limit recommended by Fair-rite
http://www.fair-rite.com/77-material-data-sheet/
It's probably around 400mW/cc or even more at 136Kz. Your cores have a
mag volume of 22.8cc according to the data sheet
http://www.fair-rite.com/product/toroids-5978003801/
so 68.4cc for your stack of three. That makes 27.4W dissipation for
brick on the key WSPR operation. I'm not really surprised the cores get
hot. Fair-rite want you to use type 78 material, which has 1/3 the loss
at these frequencies. This was the experience of the guy who's combiner
design you copied
http://www.w1vd.com/137-500-KWTX.html
where he says
"At the kilowatt level issues developed with the output transformer. The
FT-240-77 cores that worked well in the 500-watt deck began to show
signs of 'stress' while testing the prototype 137 kHz kilowatt deck. The
core ran noticeably warm to the touch, transition ringing on the drain
waveform was difficult to tame and efficiency was less than expected.
Additional core losses at the increased power level and excessive
leakage reactance were to blame. After a study of the ferrite
literature, the best solution appeared to be a switch to 78 ferrite
material which was designed specifically for the 100 - 200 kHz frequency
range. As luck would have it, 78 cores were available in FT-240 size and
readily available through several distributors! A few days later
FT-240-78 cores were being tested in the prototype 137 kHz deck. The
cores just warm to the touch, ringing was much easier to tame and
efficiency was back in the mid 90% range".
To answer your specific question, a scope on the drain will probably
show flat topping if the cores goes into saturation as its inductance
collapses. But in your case it seems unlikely you are saturating the
core, it's just that the AC loss is too high.
Mouser and RS in the UK stock the type-78 part: Fair Rite 5978003801. RS
is cheaper.
Hope that helps,
Alan G3XAQ
Date: Sat, 13 May 2017 19:06:59 +0100
From: Chris Wilson <chris@chriswilson.tv>
Hi. Two off 1kW quasi Class D push pull FET amps on 136khz feeding a
Wilkinson combiner via the amps own output transformers designed for a
50 ohm load. Combiner built from plans for one to combine two 500 Watt
amps. Combiner's output transformer is three stacked FT-240-77 ferrite
toroids, 5 turns 12 AWG enameled (2mm OD) wire primary. Secondary is 7
turns of the same wire. The combiner feeds a big LPF bank. At full
power the combiner's toroids get hot quite fast with WSPR 2 signal
applied. Needs a small fan to stay within the realms of sensibility,
even then it creeps over 70C if left too long. How can I tell in real
time, with measuring instruments, if it's saturating, and if so what
cores might be more suitable please? Hopefully the schematic of the
combiner is linked. Many thanks! Chris 2E0ILY in the UK.
http://www.w1vd.com/137-500-500WCombiner.pdf
--
Alan Ibbetson
alan@g3xaq.net
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