As discussed in Control challenges in a totem-pole PFC, large reverse current is generated when input AC voltage drops, and the power stored in the output capacitor is discharged and the hold-up time can no longer be assured. Another condition is the light load operation, when CCM operation changes to discontinuous conduction mode (DCM), in order to improve the light load efficiency. Thus, in either condition, it is required to fast detect the reverse current and then shut down the sync switches, or all the switches for simplicity, as shown in Figure 1-5.

Figure 1-5 Reverse Current Control