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UCC28600 OUT pin weird job

Other Parts Discussed in Thread: UCC28600

Hello,


I've designed a wide-range 100-260Vac to 24V, 60W ac/dc converter using UCC28600 IC. Initially first PCB's (2 pcs) worked fine, with proper voltages  on CS, OVP, FB, OUT and circa 20Vdc at VDD during full load.

But after several days of testing something weird happened to OUT pin that changed its "ON" voltage from about 14Vdc to appr. 8Vdc. More, this IC begun to heat himself to a point of completely switching itself off. This happens only with a load at the output.

It happened to both PCB's and one of them has now even trouble starting to work under load.

Where is the problem here ?