# Ferrite Transformer design calculation

1. ## Ferrite Transformer design calculation

I am planning to design a DC-DC converter in push-pull config

and as you all know Transformer formula as Vs/Vp=Ns/Np

For example 3/4 =6/8=12/16 .... How we can take which ratio is best?

Vp=12 volts DC Chopping at 20khz in push pull config
Vs=310 volt DC after rectification.
As per calculation formula says Np is 3-3 in bifilar Fashion .
and Ns around 200 Turns .

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2. ## Re: Ferrite Transformer design calculation

you need to learn about how the flux in the core rises with applied volts - otherwise you will saturate the core - and short the pri drivers

For example V / N Ae = dB/dt, where V = pri applied volts on one winding, N = number of turns on that winding, Ae = effective cross sectional area of the core, B = tesla, t = time

Such that; for a core of 15mm diameter, 180mm^2, 12V pri, a B swing of -150mT to +150mT, a max on time of 10uS ( 50kHz) the Npri must be > 2.222 Turns

thus the min pri turns are 3 + 3 for bifilar under these conditions

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3. ## Re: Ferrite Transformer design calculation

Hi,

Ae as used by Easy Peasy is obtained from the datasheet of an already properly selected core size. You have to be able to select a proper core first before you proceed to calculating the number of turns. One good method to select a core size is the geometrical constant Kgfe for transformers. The Bmax too has to consider the core material.

Search the internet and you'll make some hits.

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4. ## Re: Ferrite Transformer design calculation

Your question about the actual numbers of turns is a very common one.
Turn it round, and tell yourself, you can have as many , or as few turns as you jolly well like as long as…

1…..The number of turns for a coil generally fits across the bobbin length (even if you do multiple layers) …you generally get better coupling if you use the full bobbin width.
2…you can get your turns ratio……for example, if you want a turns ratio of 3.5, then you cant pick a single turn primary because you cant have a 3.5 turn secondary.
3….You don’t saturate the core
4….you don’t suffer too high core losses
5…the turns fit on the bobbin space
6…you can terminate a coil to the former pin (eg a 3mm thick wire wont be easily solderable to a small former pin)
7…you dont have too many layers, as if you do , you may get proximity effect problems.
8…you dont get too much winding loss
9….you get leakage inductance reduced to your requirement
10..the layers of coils actually fit on the bobbin.
……………..
So make out a spreadsheet and carry it through….when you see you violate some condition, just change it till it converges to the right solution.
If you don’t know what number of turns to start with…pick any number out of your head and try and work it through…if its not viable your spreadsheet calculations will soon flag this up to you and you will be able to see if you need to reduce or increase it.

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