# Flyback converter design. Problems with the required gap and number of turns.

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#### student92

##### Newbie level 3
Flyback converter desing. Problems with requied gap and number of turns.

Hello,
At first, I am sorry, but i have to post all my calculations as images.
I have problem with requied gap for my core and number of turns on both sides. I don't know how to calculate primary inductance too.
I was trying to design flyback by using this website: http://schmidt-walter-schaltnetzteile.de/smps_e/spw_smps_e.html but i don't know how the autor gets the numbers of turns for both sides.

Core: ETD44
Ident. 1.5
Manufacturer:Siemens
Al/nH: 194
Ain/mm2: 17
lin/mm: 103
Amin/mm2: 172
Wmax/µWs: 6862
Bmax/mT: 281
N1: 5
N2: 6668
N1 - number of primary turns,
N2 - number of secondary turs,
Wire Data:
d1 ≥ 2.38 mm
A1 ≥ 4.45 mm2

d2 ≥ 0.08 mm
A2 ≥ 0 mm2
How to calculate wire diameters?

I also did calculations in Mathcad so i am posting my results as jpgs. I am stuck at required gap and all the next calculations goes wrong.
http://obrazki.elektroda.pl/4731388300_1444160177.jpg
http://obrazki.elektroda.pl/2866997200_1444160178.jpg
http://obrazki.elektroda.pl/5119057900_1444160180.jpg
http://obrazki.elektroda.pl/9450762900_1444160181.jpg

Re: Flyback converter desing. Problems with requied gap and number of turns.

i don't know how the autor gets the numbers of turns for both sides.
.............
N1: 5
N2: 6668
N1 - number of primary turns,
N2 - number of secondary turs,

Your step-up ratio is 1000x. The calculator added a few more to make up for inefficiencies in real hardware. As for number of turns, you might increase it to 10 primary and 13000 secondary for example, but you would then need to reduce the switching frequency. The reason is because more turns would give the primary a greater inductance, and a longer time constant.

The wire diameter must be large enough to carry current without overheating and without offering hampering resistance. Ampacity tables tell you what wire gauge is suitable.

Re: Flyback converter desing. Problems with requied gap and number of turns.

Hello Good Friend,
Here is a list of the equations that you need to know to design any hard switched SMPS.
Including the ones you need.
You calculate gap empirically, (iteratively), do a gap, then calculate how many turns gives what inductance.
You must use reluctance to get the gap into the calculation.

Remember v=L.di/dt
AND
V = Nd(phi)/dt

So L.di = N.d(phi)

So I(sat) = B(sat).A.N/L

I assume you know AL = L(NH)/N^2

AL is the allowance, and is "kind of" the reciprocal of the reluctance..(proportional to reciprocal of reluctance)

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