This applet explores the transfer of power from the AC side to the DC side of a PFC, ignoring the switching action of the converter. As is shown the PFC is treated as a black box.

When unity power factor occurs, in-phase sinusoidal currents are drawn from the mains. This results in an instantaneous power flow from the AC side to the DC side that is not constant. When the mains voltage is zero then the power transfer is also zero. The AC side power has a squared sine waveform shape as shown. The average power transferred from the AC side, which is positive, is also shown.

The current flowing to the DC side is not constant as shown by the ‘i2’ waveform. A large enough capacitor must be used to store sufficient energy during the time interval when the power from input side is less than output power, especially at the time when the input voltage is zero. The shaded yellow areas above the output current ‘iout’ represent when the capacitor is being charge and the area below represents the capacitor being discharged. Since the capacitor is being charged and discharged over one half mains cycle the output voltage is not constant and has a voltage ripple on it.

• Adjust the load resistor value and observe the influence on output voltage level and ripple.
• Adjust the reference current magnitude and observe the change in the power flow, output voltage and current.