The inductances are far too high for the stated input voltage and frequency, you need to re-design from 1st principles:
P = 0.5 L Ipk^2 freq, dIpk/dt = V/L, => dI = t . V /L
L ~ Uo Np^2 Ae / Lg (air gap), Bmax = 80mT = V .dt / Np Ae ( Ae = centre cross section of core), Uo = permeability of air/ free space.
From the output power required (divided by 0.75 for eff losses ) and desired freq you can determine I^2 L, work out the peak current you can draw from your source, e.g. 1 amp
this gives you L, now you can determine air gap and Np^2 (knowing Ae in m^2), for a given air gap, say 1mm, you can determine Np.
Check that V . dt / Np. Ae gives less than 80mT for 200kHz, knowing Np you scale for Ns
Calc wire lengths for core bobbin diamter, calc R of each winding, knowing the power, and therefore current, calc I^2.R to see if wires will be too hot
Repeat for larger core if wires too hot.
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I'll start you off, for 260VDC and 75mA, this is 19.5 watts, lets say ckt eff will be 80%, thus need 24.4 watts at the input.
for 7.4V in min, the average current will be 3.3 amps, if we build a fully DCM flyback at 50% duty cycle max (to help us with the o/p diode and RFI noise) then the peak input current will be 4 x 3.3 = 13.2A,
At 200kHz, the max ON time is 2.5uS, thus 7.4 volt ( assuming 10 milli ohm fet = 132mV lost across fet) allows a max inductance of (V/L=di/dt) 1.4uH for the current to build up to 13.2 amps in 2.5uS
For a Bmax of 80mT (AC peak flux swing of 40mT at 200kHz) we see that 0.08 = V.dt / Np.Ae, Ae = 47mm^2 (no centre hole) for this pot core, giving Np = 0.09 turns
This is obviously too small - so we set the pri Turns to FIVE turns.
To get the same L, 1.4uH, we need a suitably large gap, from the formulae in the panel above we see than the gap will be 1.05mm (in practice a little larger - due to energy stored in core itself).
and now the Bmax will be, 78.7mT, keeping core losses low.
Ns will be 5 x 260/7.4 = 175.6 say 178 turns to allow for the output diode, the flyback volts on the pri side fets will be 7.4 x 2 plus leakage spike...
wind half the sec, then the pri, then the rest of the sec to lower the leakage and help with efficiency, happy building ...
the output diode will need to be ultra fast recovery (Trr < 35nS) and 600v rated.