There are two types of surge currents in PFC boost converters. The first type is the initial turn-on inrush current, which is at a line frequency (50 Hz), as shown in the waveforms on the left. This inrush current  is present when the PFC control IC has not been enabled to full operation and the output voltage of the PFC boost converter is still below defined values. As soon as the PFC boost converter output voltage reaches a certain level, the control IC enables and this “non-repetitive surge current” dissipates. After the control IC has been enabled, the PFC controller IC will drive the main power switch to the maximum duty cycle to regulate the output voltage to the defined level. The high speed switching generates switching surge current, as shown in the waveform on the right in the first few line cycles. The secondary surge current repeats continuously until the PFC boost converter output voltage is the within regulation window or reaches steady state.