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-   -   Determining the core cross sectional area, Afe, for SE OPT (http://www.diyaudio.com/forums/tubes-valves/219711-determining-core-cross-sectional-afe-se-opt.html)

 Tyimo 14th September 2012 02:55 PM

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

 hemgjord 16th September 2012 09:05 PM

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

 Tyimo 17th September 2012 06:43 AM

Hi Olof!
Thanks!
Quote:
 However this is a starting point, then you have to calculate your windings and your air gap.
Yes I know, but these formulas gives very different results and I was looking for a reliable stating point.

Greets:
Tyimo

 Yvesm 17th September 2012 07:03 AM

Quote:
 Originally Posted by Tyimo (http://www.diyaudio.com/forums/tubes-valves/219711-determining-core-cross-sectional-afe-se-opt-post3164796.html#post3164796) . . . Is there any reliable formula?
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.

 smoking-amp 17th September 2012 02:57 PM

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 P-P, 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 P-P equivalent core linearity, see bottom) Typically, the SE xfmr will weigh around 3X to 4X the equivalent P-P 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 P-P 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.

 smoking-amp 17th September 2012 03:34 PM

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).

 iko 17th September 2012 03:49 PM

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?

 Tyimo 17th September 2012 06:12 PM

O.K. I understand.
Thanks for everybody!
Greets:
Tyimo

 Tyimo 17th September 2012 06:14 PM

Hi Ikoflexer!
Quote:
 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?
Do you mean me?
Greets:
Tyimo

 iko 17th September 2012 06:21 PM

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

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