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UCC28730: UCC28730 start-up failure.

Part Number: UCC28730
Other Parts Discussed in Thread: UCC24650,


i'm having some problems with a 10 W power supply which was built using the UCC28730 controller and UCC24650 voltage monitor. The power supply schematics is basically the same as the example build UCC28730EVM-552 from UCC28730's user guides.

After switching the supply on, there is no voltage on the VDD capacitors. The capacitors are fine, the controller itself seems to be working as well (controller was also replaced but the problem did not persist). Additionally there is current flowing from the bulk capacitors to the HV pin of the controller (there is a voltage drop on the 100k resistor connecting bulk capacitor and HV pin), so the controller never exits start-up mode. I also found the TI PSR controllers troubleshooting guide but after checking the VDD voltage signal on a osciloscope, none of the described causes fit this case. Correct me if i'm wrong but the current flowing to the HV pin which doesn't charge VDD capacitors would suggest a short on the board, but there is certainly no short.