The bias voltage is too high

In this case, you can try to put the bias winding as close to the secondary winding as possible.

Thank you PI-Scotty.
Could you please justify me this kind of decision? I could send the technical information to the transformer builder.

what is the range of the load power?
-is it always on max. load of 5V, 1A?

If you go down to no-load, you can expect the bias winding to drop out.........the only way round it is to wind too many turns, then use an r/zener to regulate it down when you are on max load.

The cross-coupling between bias windings and secondary windings is typically poor and what is happening to you is the norm.

Thank you.
Yes, the maximum load is 1A, but normally the load is <= 500 mA.
The bias voltage is 55-60 V, even at small loads (a few milliampere).
So, is a new transformer the only solution ?

oh sorry i misread you, it looks like a simple case that you have wound too many turns on your bias winding.........what is the ratio N(sec)/N(aux) ?

For ideal transformer and ideal diodes, if your turns ratio is correct, your bias voltage should be always proportional to your output voltage. However, the existing of the leakage inductance cause the cross regulation issue so that your bias voltage will be away from the ideal value. By put the output and bias winding as close as possible, the coupling between bias and output winding will be very good, and the positions of both winding in the magnetic field will be similar, thus similar leakage, which makes bias voltage following output voltage closely. It will be helpful if you can send me the transformer data and construction diagram.

Good day PI-Scotty and thanks.
Please, see the attached file with the transformer data.
Unfortunately, I need a new PCB prototype If I change the position of the bias winding.
Consider that the bias winding (Pins 4-5) has been changed from 7 to 9 turns in order to change the voltage between 12V to 15V.
Sorry, but I have only an italian version of the document.

Changing of winding arrangement will only change the coupling and will not change the pin connections and layout.
From my reading, you winding order is primary, secondary, bias from the bottom to top of the bobbin, is that right? From your transformer data, it seems the turns for bias winding is about right. Please make sure that Pin 4 is the pin where you will get positive bias voltage after rectification according to your diagram. What is the input voltage when you got that high bias voltage?

Sorry, the transformer scheme doesn't show the names next to the windings. Here is the windings position: 1-2 primary, 4-5 bias, 7-9 secondary.
Changing the input voltage between 16V to 64V the bias voltage is always about 55-60V.
Can I send you the electrical scheme to a private e-mail address? Maybe it could help you.