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[SOLVED] Forward transformer question

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bowman1710

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Hi Guys


I am going through a forward design and have came across two different equations for it and was unsure about why there is a difference between the two, or what I am doing wrong, my specs are as follows


Bflux:0.1T
Vin min:170
Vinmax:400
fosc:310KHz
Dvsec or Dmax=0.49
Vout:24
core Ae:55mm^2


The first one is:
transformer core 2.PNG

mathcad2.PNG


http://cds.linear.com/docs/en/datasheet/3752fb.pdf

page 39



Second one is
transformer core 1.PNG

mathcad1.PNG


http://www.google.co.uk/search?q=fo...former+design#q=Forward+Converter+Design+Note

(top PDF link (at page 5))

Why am I getting such a difference in primary turns between the two equations, is it a unit issue or something? To get 48T and 116T in difference is a big difference
 

You seem to be handling units fine. Both formulas are based on the equation V*t=ΔB*A*N. But these two approaches suggest different ways of estimating V and t. For example, the LT version refers to the secondary side, and uses the assumption that the converter is operating at steady state and continuous conduction mode, so Vs*t=Vs*D/f=Vout/f. The second formula takes a much more conservative approach, which assumes that the maximum duty cycle and input voltage may happen simultaneously (plausible under transient conditions, implausible at steady state). So the second equation will give a much higher number of primary turns than the first.
 
I just noticed that and just thought I had found a typo, but yes I guess at transients it could be possible, thanks for your help confirmed it for me then that this is the reason behind it. Thread closed
 

Closed i know but...some would say dont bother fussing to cater for the transient case of "max vin AND max duty cycle" occurring together...just put a small 0.2mm gap in the ferrite core, and then the onset of saturation flux density will be more gradual and more easily catchable by any overcurrent limit.

It is that "Nuisance" case of "max vin AND max duty cycle" occurring together that makes the little gapped ferrite core more favourable. If you dont gap then you can get runaway saturation..or at least are more likely to.
 

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