This is my first time designing a transformer based power supply. I have wound a couple of experimental toroids so far with varying results and am still not sure exactly on how to calculate the number of primary turns.

Using a T94-2 core as an example (because I have one sitting around, datasheet link: micrometals T94-2) i'd also like to know the process to determine if a specific core is suitable or not.

Do I select Bpk based on an acceptable core loss, maybe 5-10% of rated load (method 1), or do I calculate the required primary inductance and then check that Bpk is within an acceptable range (method 2). Does the primary inductance matter?

Method 1:

Vprimary = 8.5Vrms

Iprimary = 0.470A

power ~= 4W

core loss 10% = 400mW / 2.16cm^3 = 185mW/cm^3 (not sure if i'm converting this right, using Ve from the datasheet?)

switching freq: 50kHz

Bpk ~= 1000G

Nprimary = (8.5*10^8) / (4 * 0.362 * 1000G * 50000) = 12 turns

Now, i'm not sure if the H - DC graph in the datasheet is purely for a DC analysis, or if I can plug my values above into it, which gives me 1.2Oe, well within the saturation limit of the core.

Method 2:

Vp = 8.5Vrms

Ip = 0.470A

power ~= 4W

switching freq: 50kHz

So I need an inductive reactance of 8.5 / 0.470 = 18ohm

Inductance of the coil is :. Xl / (2 * pi * f) = 18 / (2 * pi * 50000) = 57300nH

Using the value Al from the datasheet, number of turns for the required inductance is sqrt (57300 / 8.4nH/N^2) = 82 turns

Bpk = 143G, core loss of 51mW.

H - DC more saturated than method 1, but still >99%.

Building two similar cores using method 1 and method 2 as an experiment (T68-1 cores under different load conditions), the second method produced a better output. The low turns core appeared to saturate.

If it's not obvious at this point, I don't know what i'm doing! I've read countless guides on how to calculate the number of primary turns, but they always seem to pick the Bpk value out of no where. There is probably something fundamental I am missing so any guidance on determining the number of primary turns and the suitability of a core would be appreciated (not including inductive, capacitive and resistive losses also associated with a transformer, one step at a time).