VIPER16LN thermal shutdown happening in 2mnts at 3.75W

[SRedd.6]

Hi Team, we are using VIPER16LN in one of our project. we are facing thermal shut down happening within 2mnts time. the below are the testing condition

Input voltage: 230VAC

Output Voltage : 15V

Output current: 250mA

But viper16LN can handle around 6W power as per the datasheet. and as per the edesign suit also at the above power the temperature rise of VIPER is 30 degrees only.

verified the circuit as edesign suit as well. but there is no improvement.

Please share your feedback.