UCC28070A: Updating kVFF value and resetting VINAC Peak Detector

Hello Martin, 

Thank you for your question on the kVFF function of the UCC28070 PFC controller. 

At every initial power-up (VDD exceeds turn-on threshold), the kVFF factor in the IMO equation is set to 3.857V^2 (level 8) regardless of actual VINAC voltage.  This is to ensure that the IMO current-reference gain is minimum at start-up.

The VINAC peak detector is "reset" each time that the instantaneous VINAC voltage falls below a 0.7V "zero-crossing" threshold.  It is not a complete reset to zero, but the internal kVFF level is set based on the last previous peak voltage at VINAC captured prior to the zero-crossing.  Therefore, the kVFF levels of low-going input voltages are updated only during the down-slope edges of the rectified input with some time lag after the peak. 
Note: If VINAC cannot fall below the 0.7V threshold for some reason (such as charge held on Cin at light load, or DC input) then the kVFF level cannot be updated even if the input voltage falls.  Current-loop gain will be low and full output power will not be attainable until kVFF can be updated to a lower level. 

On the other hand, rising input voltages change the kVFF factors to higher levels immediately as the rising thresholds (shown in the table above) are crossed.   
Falling thresholds have a 5% hysteresis below each of the rising thresholds, to avoid chatter around a level boundary. 

This information is discussed in the datasheet. 

Note: the first three columns (and hysteresis) of Table 6-1 (above) are fixed values in all UCC28070x controllers.  The values in the fourth column are based on a VINAC divider ratio of 400V/3V.  If the PFC output voltage is programmed for higher or lower voltage other than 400V (like 385V or 410V, etc.), the corresponding input voltage (Vin) peak ranges for each kVFF level will differ from the values in Table 6-1, based on Vinac(pk) x (Vout/3V).  

Regards,
Ulrich