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UCC28710: Only Starts Switching at Higher Input Voltage

Part Number: UCC28710

Hello,

I am trying to get our UC28710 design to work with a new transformer, Wurth 750312170, but I can only get the chip to begin switching when Vin_Bulk > ~180V. I would ideally like to use input voltages down to ~100V. I used the values in the screenshot below for the design. I am also powering Vdd externally at 22V to isolate that we are not having issues with Vdd dropping below Vdd_UVLO. (I removed the diode from the aux winding to Vdd as well).

I do believe that we might have too high of a turns ratio at a 100V input based on equations (12) and (13) of the datsheet (screenshot below). I have attached screenshots of the start up sequence voltages on the flyback AUX pin for 120V input (chip failed to start) and 200V input (switching properly started). I believe the screenshots look very similar, so I am curious what fault the chip might have detected with the 120V input that caused it to stop switching after its initial 3 pulse start up sequence described in section 8.4.4 Start-Up Operation of the datasheet. 

Thank You again!