XPost: sci.electronics.design   
   From: tiwill@seventransistorlabs.com   
      
   "Piotr Wyderski"  wrote in message   
   news:rapefq$1p6jh$1@portraits.wsisiz.edu.pl...   
   >> When mu_r >> 1, the path is essentially all in the core (or gaps between   
   >> core pieces), so is the mean circumference of the core. l_e is almost   
   >> exclusively used with cores, since it isn't very meaningful elsewhere...   
   >   
   > And for the same reason you read it from the datasheet of the core, not   
   > measure. One can also resort to FEM sims, but I believe it is pretty rare   
   > outside of academia.   
      
   In my experience, l_e and A_e are very close to the expected mechanical   
   dimensions -- i.e., cross section of the wound limb(s), mean circumference   
   of expected path.  I don't think that's necessary, and is in part a   
   consequence of conventional shapes being well behaved -- compact,   
   symmetrical, optimized for cost and performance.   
      
   Also, v_e ~= l_e * A_e, which I'm not sure has to necessarily be true.   
   (There could be vestigial core features that don't magnetize, so the core   
   volume is greater than the active volume; but then, it's _effective_ volume,   
   so that wouldn't be counted anyway?).   
      
   And when you bring nonlinearity into things... As magnetization rises:   
   mu_eff falls, A_e rises some (fringing fields), l_e rises some (because the   
   inside track saturates first, especially inside corners, pushing the active   
   volume outwards).   
      
   The changes in mu_eff and A_e partially oppose, so it's not immediately   
   obvious how to separate them; since they're both effective parameters, we   
   might just assume one or the other remains constant instead, and measure the   
   other as the combination.   
      
   These are hopefully effects we can ignore... which for power application,   
   yep, no problem.  For signals, well obviously you want to keep the   
   magnetization low to avoid distortion, frequency shift, etc.  Some airgap   
   helps ballast changes in core mu, which would otherwise be rather sensitive   
   (not to mention, to temperature as well as signal level).   
      
   Tim   
      
   --   
   Seven Transistor Labs, LLC   
   Electrical Engineering Consultation and Design   
   Website: https://www.seventransistorlabs.com/   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)