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14th September 2012, 02:55 PM  #1 
diyAudio Member
Join Date: Nov 2002
Location: Hungary

Determining the core cross sectional area, Afe, for SE OPT
Hi!
How to calculate the minimum required core cross sectional area, Afe, for a SE OPT? I know some formulas, but all are different and gives also different results. e.g.:  Afe (in sq.mm) = 450 x sqrt Po  Afe = 2*sqrt Power  Afe = 20* sqrt (Power/Fd) fd is the lowest frequency  Afe = 20* Power/Fd  Afe = sqrt (power*20)  Afe = (sqrt Power)*2 etc. Is there any reliable formula? Greets: Tyimo 
16th September 2012, 09:05 PM  #2 
diyAudio Member
Join Date: Apr 2007

Hi
I don't know of any "all in one", simple formula that always works. I've used the  Afe (in sq.mm) = 450 x sqrt Po  as starting point and it comes close. However this is a starting point, then you have to calculate your windings and your air gap. It might turn out that you cannot reach your desired primary inductance whitout risking saturation. Then you have to go with bigger Afe...and so on it is an iterative process. /Olof
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17th September 2012, 06:43 AM  #3  
diyAudio Member
Join Date: Nov 2002
Location: Hungary

Hi Olof!
Thanks! Quote:
Greets: Tyimo 

17th September 2012, 07:03 AM  #4 
diyAudio Member
Join Date: Jul 2004
Location: Ardeche

A reliable formula (if it exists) should include not only Pmax and lowest frequency but also the DC current and the maximum induction the core can accept for a given distortion.
Geometry of the core has a role to play allowing more or less room for more or less copper . . . Also, the need for interleaving consumes some of this room for insulation layers. Interactions are complex and I second Olof about using an iterative process. Yves. 
17th September 2012, 02:57 PM  #5 
Account disabled at member's request
Join Date: Dec 2001
Location: Hickory, NC

A properly gapped SE core loses half of its core area to the DC flux. So for 2X the area to compensate, this would require 1.414 X the side length, and that would make for (1.414)^3 = 2.828 the volume. The usual power estimation formulas are for power transformers at 60/50 Hz operating to Bmax, so you have to figure in another 3X area for say 20 Hz. Then for E/I laminations, quality audio usage is more like 60% of the Bmax (as used by the power case), so another 1.666 X area (to stay in linear flux region). Then SE pushes into saturation even faster than PP, since it operates on one side (flux polarity) only (ie, it already starts out at 1/2 Bmax for zero signal) so some more adjustment is needed. (actually a lot would be needed to reach full PP equivalent core linearity, see bottom) Typically, the SE xfmr will weigh around 3X to 4X the equivalent PP case for a "quality" design, requiring around a 1.44 to 1.6 X larger side dimension (cube root of 3 or 4).
But if you really wanted to get totally equivalent core flux linearity to the PP case, it would take more like a 10X volume (for E/I, a cut core could redeem most of that due to it's higher linear flux level). Obviously this doesn't get done though, since the air gap linearizes the flux curve considerably. Last edited by smokingamp; 17th September 2012 at 03:26 PM. 
17th September 2012, 03:34 PM  #6 
Account disabled at member's request
Join Date: Dec 2001
Location: Hickory, NC

Then you have to consider the winding inductance too. The air gap will typically reduce that by 1/4, the (minimum) doubled core area brings that up by 2, but the core material Mu is lowered around 1/2 due to the DC. Net result is that the 3X to 4X heavier core is still inductance challenged with a limited bottom end and higher winding resistance with more turns around the bigger core (poor damping factor, but then zero feedback is still the main problem there).
Last edited by smokingamp; 17th September 2012 at 03:48 PM. 
17th September 2012, 03:49 PM  #7 
diyAudio Moderator

I'd be interested in trying to design an example output transformer here, look up formulas and so on. What are the specs you want from your transformer?

17th September 2012, 06:12 PM  #8 
diyAudio Member
Join Date: Nov 2002
Location: Hungary

O.K. I understand.
Thanks for everybody! Greets: Tyimo 
17th September 2012, 06:14 PM  #9  
diyAudio Member
Join Date: Nov 2002
Location: Hungary

Hi Ikoflexer!
Quote:
Greets: Tyimo 

17th September 2012, 06:21 PM  #10 
diyAudio Moderator

You, or anyone else. But you probably have a vested interest, being the original poster.

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