I used VIPer53 in my product,I found that some of them entered into overheat （about 65℃） protection mode after running 70~180 mins at out power was 17W, and my design output power is 30W at Vin=220Vac. Change another VIPer53 the circuit can work very well.
We use viper53, about 4 years, there has been no problem.However, starting in April, chip has not work properly. We compared the difference between old and new chips.Find chip printing appear to change.the old chip( "ST@ CHN")has a space between "ST"mark and "CHN".but the new chip don't have the space.
The VIPer53 with something wrong need more 4~5 Watts than the one without problem at the same output load.