Duty Cycle Factor represents the change in Duty Cycle, over the range of full load to the load at which the converter is at the brink of going discontinuous (DFULL LOAD / DMIN LOAD), at the minimum specified input voltage. This change in Duty Cycle is due to circuit parasitics, including DPA-Switch Drain to Source voltage (VDS) during conduction, winding resistance, leakage inductance and PCB trace inductance.
The lower the value of DFACTOR, the more the parasitic resistances and inductances are detracting from the power train’s ability to deliver power to the load. Its value is always less than 1 and is typically within the range of 0.8 to 0.95. A DFACTOR value of 0.9 means that full-load, low-line duty-cycle will be 11% (1 / 0.9 = 1.11) more than light-load, low-line duty-cycle (assuming that the converter is operating in the continuous conduction mode).