Calculating Bmax for stacked toroids? - Page 1
Forum section: electronics / rf-microwave

author: Chris Wilson
author_rank: Super Contributor
timestamp: May 08, 2017, 03:39:13 pm
content: I want to (try) and work out the Bmax for three stacked FT-240 ferrite toroids in 77 mix, fed with 137khz. the maths to me appears daunting as it's by far my worst subject. Is there any sort of calculator an idiot like myself might not get into too much trouble with? i am trying to work out if the output transformer of a Wilkinson combiner is near saturation when combining two 1kw amps running 137kHz. the combiner is shown here and was really designed to combine two 500 watt amps:http://www.w1vd.com/137-500-500WCombiner.pdfThanks!

author: Richard Head
author_rank: Frequent Contributor
timestamp: May 08, 2017, 08:33:33 pm
content: ChrisIt's very straightforward. B = v/{4.44fnAe}. This is for sinewave excitation.Where B = peak flux density in Teslasv = winding voltage (RMS)f = frequency in Hzn = number of turnsAe = minimum core cross sectional area in m^2So for your three cores Ae = 3 x 12.7 x 12.7 mm = 484 x10^-6 m^2If Po = 1kw into 50 R then v = sqrt{1000 x 50} = 224v rms.But since you don't know n change the equation to solve for n and use a B value of 90mT. (Reasonable figure for 137khz)So n = 224/{4.44 x 137000 x 0.09 x 484 x 10^-6}s= 8.45 turns. (Round it to 9 turns)As it turns out the transformer is loss limited not saturation limited. This is common above about 50khz.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 09, 2017, 04:16:19 am
content: You don't need two stacks, as the combiner part uses. Transmission line construction also isn't necessary (though it is helpful to keep in mind with many applications).A zero-degree combiner is simply a tapped inductor, where the two amplifiers connect to the two ends, and the output connects to the centertap. The core only needs to handle the flux of the phase/amplitude mismatch, so the better matched the amplifiers are, the smaller it can be. (Of course, the smaller the combiner is, the more current will be drawn if the amplifiers accidentally end up out of phase and start fighting each other...)If you place the combiner at the inverter output, rather than after the filter, you can also leave off the termination resistor -- the increased impedance (the impedance of the combiner winding, which will be quite high when wound on ferrites) will simply reduce power delivered by the amplifiers. (A full sized combiner, that'll handle the full flux of both amplifiers being out of phase, will draw very little current when mismatched.)As for flux density, simply multiply A_e by the number of cores.Tim

author: Chris Wilson
author_rank: Super Contributor
timestamp: May 09, 2017, 03:22:20 pm
content: Thanks for the replies both of you. If I go for two primary windings do these have to be centre tapped or can I use two separate primary windings with a separate secondary? What should the turns ratio be please?I was thinking of using the three stacked FT-240 toroids in 77 material that the output transformer of the combiner is wound upon as the core, does that sound reasonable?I will test on a big variable voltage, variable current bench supply that goes up to 60V at 50 Amps and ramp up slowly whilst monitoring current and drain wave forms, and the waveform after the LPF bank, which I will put after the new transformer, with fingers crossed. I appreciate your help! Thanks again.