Buck converters duty cycle
As per the datasheet:
https://www.ti.com/lit/ds/symlink/ucc28911.pdf?ts=1591886894954&amp;ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FUCC28911


  During voltage regulation, the controller operates in frequency
  modulation mode and amplitude modulation mode. The internal operating
  frequency limits of the controller are 115 kHz maximum and 420 Hz
  minimum. The transformer primary inductance and turns ratio sets the
  maximum operating frequency of the converter. The output preload
  resistor and efficiency at low power determines the converter minimum
  operating frequency. There is no external compensation required for
  the UCC2891x devices.


as long as the loading isn't too light and the voltage conversion range is too large that the lower frequency limit is hit, the controller shall regulate the switching frequency according to the operating case e