Firstly all switched mode converters will have ripple on their output when supplying a load. The controller can control the long term average output voltage but within a cycle the current supplied through the inductor must vary and hence if the load current is constant the voltage on the output capacitor will also vary.
The magnitude of this ripple will depend on a number of factors including switching frequency, load current, capacitance of the output capacitor, inductance of the inductor and parasitic losses in the switching components.
Secondly lets look at the case of active power factor correction. A traditional switched mode mains power supply without active PFC looks like.
Input -> Rectifier -> primary capacitor -> Flyback converter -> output capacitor.
The problem with this design is that it's power factor is shit. Current is only drawn from the input on the peaks of each voltage waveform.
So we change our design to add power factor correction.
Input -> Rectifier -> small primary capacitor -> PFC boost -> Main primary capacitor -> Flyback converter -> output capacitor.
The controller for the PFC boost converter is trying to satisfy two goals.
- On short time-scales, keep the input current as close as possible to proportional to the input voltage and hence the input power factor as close as possible to 1.
- On longer time-scales, maintain the desired voltage (usually 380V) on the primary capacitors.
So we have two sources of ripple on the output of the PFC boost.
- The switching frequency ripple that every switched mode converter has.
- Ripple at twice the input frequency caused by the varying input power through the cycles of the input waveform.
This ripple doesn't really matter because the output of the PFC boost is not the final output of the power supply and it is most likely smaller than the ripple on the primary capacitors would be if you did not have the PFC stage.