Hello Alan,
On Sunday, May 14, 2017, you wrote:
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
Hi Alan, I cannot thank you enough for the reply on the "AMPS" forum
to my questions. I did use 78 material for the two amps built to
W1VD's specs i have built, and guessed he may not have updated his
pages to reflect this better material for use in the combiner, but i
thought i wouldn't be to clever and change anything. a subsequent
correspondence with Jay, W1VD showed he intended the combiner to be
used with 2 of his 500W amps, so I am pushing the limits presumably
with whatever material. I see Fair-Rite lit a 98 material which is
supposedly better agin than 78, but getting info on what sized toroids
are available in 98 is proving difficult. Also, Fair-Rite USA charge
insane postage to the UK making small orders very costly.
Even with the combiner gate and drain waveforms seem good, with a
slightly more spiky edge to the drain ones when combining, so i guess
the saturation level hasn't been exceeded, but material choice is
poor, and maybe turns counts are sub optimal for this power level.
I will try two approaches, a copy of my existing combiner transformer
using spare 78 compound FT-240 sized toroids I have to hand, and
another with the tripled turns count Manfred kindly suggested. The
problem with LF for a 2E0 is the guys running "down there" are all
very experienced and their replies are often a bit over my head,
especially as I foolishly chose to lark about in maths classes, much
to my regret when I took up amateur radio at over 60 years of age.
My teacher's words never rang so true, "Wilson, one day you'll wish
you had paid more attention, as maths is often needed to do things
that interest you, rather than just at a purely academic level that
you seem to find so unimportant and boring". Damn, he was dead
right.... :(
Much appreciated, and thank you again Alan!!