TOP246YN 30W Flyback (Fig. 41) does not start in a few samples

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I have a TOP-246YN on a design closely similar to Fig 41 on page 20 of TOP242-250 data sheet Nov. 2005 (see attached).
In series production, occasionally it happens that a few (1%) power-supply boards does not start (no output voltage).
By looking at the signals it seems as the TOP-246 continuously restarts the soft-start cycle (as described on page 6 of the manual); that is to say it seems as U2 (in Fig. 41) does not feed the Control pin.
Actually, by shorting for a brief moment the photo-transistor of U2, the power-supply starts running and operates properly.
Once this has be done, the power supply will continue to start-up properly after any recycling of the main power supply (independently on the duration of the off-state, even in the range of several days).
I did not find a reliable and repeatable solution to this problem. In fact in a few cases the problem was fixed by replacing both U2 and C10/C11; but this solution fails on other samples.
I also tried to "regenerate" C10/C11 as they could have increased leakage current due to possibly long-term storage, but this did not seem the cause at all.
Any suggestions will be welcome.

Hi VEFA,

Are you using the same opto-coupler? LTV817A?

Also have you noticed any output overshoot at startup? It could be at startup this can cause auto-restart. You can try adding 47uF capacitor (soft finish capacitor) across VR2 to eliminate output overshoot.

Here is an example design with similar output power. https://ac-dc.power.com/sites/default/files/PDFFiles/der95.pdf

Regards,
PI-Pike