UCC28780: Brown-in/Brown-out Measurement

Hello Connor,

Thank you for your interest in the UCC28780 ACF controller.

This controller samples the VS signal for the magnitude of negative current (current flowing out of the VS pin) each switching cycle, basically at the falling edge of the PWML signal.  This means that Ivsl is detected just before the low-side MOSFET turns off.  Presumably the Fet has had enough time to turn on fully and the full bulk voltage is impressed across the primary winding, reflected to the auxiliary winding and applied to Rvs1 to generate the current.    

Since this issue occurs at "Brown-in", the controller is looking for 3 sequential occurrences of Ivsl > Ivsl(run) current to meet the condition to proceed with start-up.  The controller will cycle through UVLO until this condition is met.  If your input voltage changes significantly (downward) during these tests, you'll need to increase Vin until all 3 cycles meet Ivsl(run), or recalculate Rvs1 (and then Rvs2 to reset OVP) to accommodate the Vin behavior at the level where you want it to start.

Note: After start-up, "Brown-out" measurement is done at the same instant each cycle.  Reflected output voltage is measured continuously during the demagnetization interval for other purposes, but the moment for output-OV detection is just before the end of that interval.   

Regards,
Ulrich