Toroid Transformer Design HELP

[muddy]

Hi Folks

Well I appoligize If i beat the horse here, but ive searched the archives and couldnt find an answer to my question, so here is goes.

Im trying to design a step-up transformer for a SMPS. (unless you guys no of an off the shelf one) Im basing my toroid selection off the following equation.

(Wa)(Ac)=(PoCx10^8)/(4eBFK)
Wa=window area
Ac=core area in cm^2
Po=power output
e=tranformer effiency (90% for transformers)
B=Flux density(in gauss)
F=Frequency
K=Winding factor (.2 for toroids primary)
C=Current capacity in cm^2/amp

Ok Question one
I need some equations for calculating B(flux density). Seem to have lost the one text i need to answer this, but either way if i member correctly there is a function that that allows me to go between(algebra please, dont feel like dusting of my Calc, shudder)

L=(m)(N^2xA)/l x 1.26X10^-6
L=inductance(henry)
m=permeability
N=# turns
A=Squar meters
l=lenth meters


Any help?

2nd Question Once I have Is my topology for the transformer will be primary and secondary bolth centertapped. will my Flux be a sum of the total of all 4 inductive coils? Im thinking just 2 because they are inducing the secondary, but like i said Ive never build a coil :P

and Last Question
I can here you all breath a sigh of releif :P Anyone got advice on How to actually wind the bloody thing?

Either way Im attempting to design a very effient Transformer That I will have specs on and have predictable operation. Once I spec out the core (Im getting from Magnetics) and design a few protos I will share that info with you all.

Best Regards

[jackinnj]

if you stop by the library out there in NM chances are you will find an "American Radio Relay League" Handbook -- usually they have several copies -- in one of the recent issues they go into great detail on how to wind the transformer.

If you want I can email you the ATX article -- it's for a pp-buck converter, but all the math is there for winding the transformer.

[jcx]

hand winding toroids is a pain unless you're at such a high freq that single layer coils are all that is necessary

for hand prototyping swps xfrmr pot cores w/split bobbins are easier to handle - some EI ferrite cores come as "systems" with matching bobbins that meet lline isolation standards

swps xfrmr design is highly techinically challenging if you're at all interested in efficiency (power or material) - Linear Technology and National Semi sites have some useful simplified info - the magnetic core manufacturers also have extensive info but you really need to know what to look for

[muddy]

The bain in my life, to know where every little electron goes :P.
K, So I take it Toroid is not the way to go, but single wind, after a week of looking at cores :P. hehehehe. I will look at nat semi, and magnetics(core manufacture is was looking at). Still, I Know that the First Eq can be used on core selection, and still looking for Flux(B) eq. I will continue to check in :P More research this weekend. One thing is for certian Im am learning more than i ever wanted to about inductance :P