[Amps] Real time tests to see if an RF transformer is saturating?

[Alan Ibbetson]

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 at 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 at g3xaq.net