winding multiple secondaries on a toroid???

[Puggie]

I need to wind my own toroid for a SMPS I need to go from 12v to 18V and ± 25V.

the 3 secondaries (18V and ± 25V) do I need to wind these each all the way around the toroid or can I wind the primary and 3 secondaries over a 1/4 of the toroid each. I'm guessing I can't but had to ask.

EDIT: how many turns would be required for the primary, roughly, are we talking 5, 10, 20 or more?

thanks

Bill.

 

[richie00boy]

For best performance the windings should be evenly spread over the whole core. I would do the 18v one first, then bifilar the two 25v ones.

 

[Puggie]

thanks, thought that might be the case.

 

[N-Channel]

as far as the number of primary turns, this would be dependent on the frequency through the transformer. For the supplies I have done, I use the following equation:


N = [Vin (min) x e^8]/[K x Fs x Ae x Bmax)], for

N = number of turns
Vin(min) = 11V
K = 4.0 (Sq wave), K = 4.44 (sine wave)
Ae = Effective cross-sectional area of the toroid
Bmax = Max flux density in Gauss (@ switching frequency)

So, at 50kHz (100kHz oscillator), using a #77 material ferrite toroid (FT-140-77), my primary is 4.26T + 4.26T, at 11V minimum input, rounded down to 4T + 4T for the primary. Figuring out the secondaries number of turns ratios is the same as for linear supplies. This equation is good for center-tap push-pull, half-bridge and full-bridge topologies, because their cores swing in opposite (and hopefully equal) directions of the B-H graph, and have better core utilization than uni-polar topologies (single-ended forward and flyback).