Hi Rob,

sorry for URL but you found the right part.
I just rewind this core with 300 to ?? windings. I assume it was 22 from the test before. More than 300 windings were not possible because of the E ferrit.
The result was better but not good enough.
I work with just 1 primary winding. The clock is generated separately. This is much more easier to vary the frequency. The switch is a MosFET with very low resistance and should be OK.
Looking on the oscilloscope the voltage raise up very rapid to about 100V, then the curve becomes very flat.
That's still my challenge

Later this week I will reduce the primary no. of windings and see what happens.
Next step will be to remove the "walls" of the core in order to get more space and to wind the primary on top of the secondary winding.

Best regards
Uwe