I designed a power supply using PI Expert 9 about four years ago for TNY290PG. The design file is attached herewith. We observed a failure today when the supply was in use in our product connected to 240 V AC fairly stable supply with a load of 0.6 A. The load was steady and after an hour the TNY290 IC burst. The image of the IC is attached. The power supply was successfully tested at 100% of rated load for eight hours at 110V AC and 8 hours at 260 V AC before using it on our product.
The change to the original design is we have increased the clamp as below to compensate the range of the leakage inductance of the transformers and to keep the Vds below 500 V:
1. The original R1 is 27K, R1 in use is 11K
2. The original C3 is 2.7 nF, C3 in use is 4.7 nF
3. In addition, a TVS diode P6KE200A is used in parallel to the above two components.
The transformer from the failed power supply is recovered. The measured Nominal Primary Inductance is 1035 µH and the Primary Leakage is 59 µH.
I checked the same design working power supply on oscilloscope with a load of 9 Ohm resistor at 110 V AC and 260 V AC. The images are attached herewith. The switching frequency seems half of the designed frequency. Don't understand why it is so. Is it correct?
Could you please help solving the failure issue?
With Best Regards,