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Old 27th August 2012, 12:43 PM   #1
ISTPYRO is offline ISTPYRO  Portugal
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Join Date: Aug 2012
Default Help with SMPS Transformer

Hello guys, I am new around here and I need some help to build my SMPS transformer.

I was thinking designing a transformer to use on a forward converter. My goal is to, first, learn how to do a transformer and second, how the forward converter works.

I was wondering if there is someone who can help me out here with the design of that trafo.

I will wait for some answers and see if someone can help me. After that, I'll post the specifications and the calculations that I have done so far.

Kind Regards.
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Old 27th August 2012, 07:07 PM   #2
ISTPYRO is offline ISTPYRO  Portugal
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Join Date: Aug 2012
Hi again.

Maybe would be better to post directly the specifications.

Specifications

Input

Vin = 230VAC
Vin(nominal) = sqrt(2) * 230VAC = 325V
Vin(minimum) = 300V
Vin(maximum) = 350V

Output
Vout = 30V ; Iout = 5A ; Pout = 150W

Expected efficiency = 80%

Pin = 150W / 0,8 = 190W

Iin(maximum) = 190W / 300V = 0,65A

Transformer specifications:

I got this transformer from an old power supply. It is an EI type core. I don't know what kind of core material and because of that I do not intend to strive the transformer that much. The specifications are:

Bmax = 1600Gauss
Ae (center leg cross-section) = 1,21cm2
f = 100kHz

TRANFORMER CALCULATIONS

Just before start, I have a couple of questions here. From what I have read, in forums, books, applications notes, there are two methods to calculate the primary transformers turns.

The first one is: Np = ( Vin(min) * Delta(max) ) / (Ae * Bmax * f )
Ns = Np * ( (Vout + Vf * Delta(max) ) / ( Vin(min) * Delta(max) ) )
The second one is: Np = ( Vin(max) * 10^8 ) / ( k * f * Ae * Bmax )
Ns = Np * ( ( Vout + Vf ) / Vin(min) )

The Ns, from the first method I saw it on a paper, but I have seen too the Ns from the second method, applied on the first.
I have calculated the turns with both methods and both turns gave me completely different values. Here they are:

Using the first method I got: Np = 70T ; Ns = 17T;
Using the second method I got: Np = 45T ; Ns = 5T;

Now my question is, which method should be used? Why they give so different values?
The method that doesn't use the Delta, is less accurate than the other?

I would appreciate if someone could help me out.

Kind regards.
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Old 29th August 2012, 03:37 AM   #3
dtproff is offline dtproff  United States
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Join Date: Oct 2008
Location: Plano, TX, USA
Quote:
Originally Posted by ISTPYRO View Post
Hi again.

Maybe would be better to post directly the specifications.

Specifications

Input

Vin = 230VAC
Vin(nominal) = sqrt(2) * 230VAC = 325V
Vin(minimum) = 300V
Vin(maximum) = 350V

Output
Vout = 30V ; Iout = 5A ; Pout = 150W

Expected efficiency = 80%

Pin = 150W / 0,8 = 190W

Iin(maximum) = 190W / 300V = 0,65A

Transformer specifications:

I got this transformer from an old power supply. It is an EI type core. I don't know what kind of core material and because of that I do not intend to strive the transformer that much. The specifications are:

Bmax = 1600Gauss
Ae (center leg cross-section) = 1,21cm2
f = 100kHz

TRANFORMER CALCULATIONS

Just before start, I have a couple of questions here. From what I have read, in forums, books, applications notes, there are two methods to calculate the primary transformers turns.

The first one is: Np = ( Vin(min) * Delta(max) ) / (Ae * Bmax * f )
Ns = Np * ( (Vout + Vf * Delta(max) ) / ( Vin(min) * Delta(max) ) )
The second one is: Np = ( Vin(max) * 10^8 ) / ( k * f * Ae * Bmax )
Ns = Np * ( ( Vout + Vf ) / Vin(min) )

The Ns, from the first method I saw it on a paper, but I have seen too the Ns from the second method, applied on the first.
I have calculated the turns with both methods and both turns gave me completely different values. Here they are:

Using the first method I got: Np = 70T ; Ns = 17T;
Using the second method I got: Np = 45T ; Ns = 5T;

Now my question is, which method should be used? Why they give so different values?
The method that doesn't use the Delta, is less accurate than the other?

I would appreciate if someone could help me out.

Kind regards.
As to your question above, please check out this app note... http://www.fairchildsemi.com/an/AN/AN-4134.pdf

It will go a long ways in explaining things. After you have read through it we can chat some about the important design considerations around a forward converter.

Tony
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Old 29th August 2012, 09:18 AM   #4
ISTPYRO is offline ISTPYRO  Portugal
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Join Date: Aug 2012
Thank you Tony. I'll read that.

Regards.
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Old 29th August 2012, 03:29 PM   #5
dtproff is offline dtproff  United States
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Join Date: Oct 2008
Location: Plano, TX, USA
Here is a fairly thorough treatment of a forward converter that I wrote about 10 years ago but it still applies today.

Tony
Attached Files
File Type: pdf 150W 3A 48V Forward PQ2625-PC40.pdf (66.4 KB, 78 views)
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