The PFC control block modulates the boost converter so that the input current of the power supply will be in phase and have the same wave shape as the input sine-wave voltage. As the wave shape and phase are aligned, the supply is said to achieve near unity power factor. The DC/DC converter, usually a step-down converter, steps the high dc voltage down to output voltages required to run the system’s electronics. The DC/DC circuitry also provides  primary to secondary isolation while the output filter stage blocks switching noises from the power supply.