The actual control technique and circuit used are largely irrelevant to the calculation of losses in UPF boost preregulators, and are not discussed in this paper.
The basic AC-DC boost preregulator power circuit is shown in Figure 1. The operation of this circuit is generally well known; the duty cycle of main switch Q1 is controlled by logic (not shown) to boost the rectified line input voltage "Vi" to the output voltage "Vo", while forcing the short term average input current (= L1 current) to be proportional to the instantaneous AC line voltage. Since the AC mains voltage is (ideally) sinusoidal, the line current is also sinusoidal.