From: jeroen@nospam.please   
      
   On 2021-08-25 23:21, Phil Allison wrote:   
   > Jeroen Belleman wrote:   
   > ==================   
   >>>>   
   >>>>> ** Typical iron core transformers intended for 50/60 Hz can be used to   
   20kHz and beyond with no such issue.   
   >>>>> As the operating frequency rises, core magnetisation falls cancelling   
   any rise in losses.   
   >>>> Why isn't laminated iron good for RF transformer cores then?   
   >>>   
   >>> ** You have misunderstood my post.   
   >>>   
   >>   
   >> It's true I assumed constant Bmax. I should have said so.   
   >> Constant voltage, as you assumed, is indeed more natural.   
   >>   
   >   ** Well, I deal  lot with audio transformers  - from mic input to hundreds   
   of watts.   
   >   
   >   
   >> I measured the frequency response of a few iron core   
   >> transformers: A 75VA rectangular-core worked well up   
   >> to 40kHz,   
   >   
   > **  Yep.  Audio output types go to about 60kHz or more.   
   >   
   >   
   >> while a similar sized toroid went up to only   
   >> about 10kHz (-3dB), measured between the two independent   
   >> 15V windings of each.   
   >   
   > ** That is odd,  toroidals are usually the best with -3dB responses to   
   100kHz.   
   >     Just the fact the secondary is wound all over the primary does the trick.   
      
   It surprised me too. I also measured an inter-winding capacitance of   
   2nF, which strikes me as high. The drop-off was a resonance dip.   
      
   >   
   > What cannot be done is having a tiny laminated iron core running at 100kHz   
   and high power.   
   >   
   > As well as ferrite there are "powdered iron" cores and toroids that will.   
   > Another material is  "amorphous steel" which as very low losses.   
      
   I use transformers for RF. I've used ferrite, of course, but also metglas   
   and similar materials. The cores are mostly there for the low end of the   
   frequency range. Beyond a few MHz, you really want to keep the flux out   
   of the core, which is done by using transmission lines for the windings.   
   Some of my transformers are good to 9GHz, but those bear little resemblance   
   to a traditional wound transformer.   
      
   Jeroen Belleman   
      
   --- SoupGate-Win32 v1.05   
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