We have designed 50W smps using TOP271JX. In some smps we observed problem that smps output gets on & off continously(once in second). Most of the time this problem observed @ hige side voltage i.e. above 230Vac.
Our observation is smps which are going in thermal shutdown that have flickering problem. i.e TOP switch IC derating. when we replaces TOP switch and fitted it with clip on heat sink the same smps works fine.