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27th August 2012, 12:43 PM  #1 
diyAudio Member
Join Date: Aug 2012

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. 
27th August 2012, 07:07 PM  #2 
diyAudio Member
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 crosssection) = 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. 
29th August 2012, 03:37 AM  #3  
diyAudio Member
Join Date: Oct 2008
Location: Plano, TX, USA

Quote:
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 

29th August 2012, 09:18 AM  #4 
diyAudio Member
Join Date: Aug 2012

Thank you Tony. I'll read that.
Regards. 
29th August 2012, 03:29 PM  #5 
diyAudio Member
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 
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