SLUSD37E October 2017 – November 2019 UCC28056
The controller applies Line Voltage Feed-Forward to the COMP pin voltage (VCO) before it computes the TON and TDCM durations. This sequence ensures that COMP voltage represents input power regardless of Line voltage and ensures that Burst operation occurs at the same level of output power for all Line voltages. It also ensures fixed gain between the COMP pin voltage and input power simplifying compensation of the voltage control loop.
For ease of computation, UCC28056 employs seven discrete GFF levels, the most appropriate value being selected by a series of comparators monitoring the peak input voltage level. Hysteresis is built into each comparator to avoid repetitive changes in the selected GFF value and the step change in Line current that would result. The comparator thresholds and GFF levels are selected to ensure that the demand to input power gain (PIn/VCO) does not vary by more than ±20% over the full Universal Line voltage range (between 90 and 264 VRMS).