The same boost PFC is used as in the discontinuous conduction PFC applet, but in this case the generation of the switching signal is different. The inductor current must still start from and return to 0A as in the discontinuous case but the switch is turned on as soon as the 0A level is detected. This is known as critical conduction since it is the mode between discontinuous and continuous conduction.

The output DC voltage is controlled by adjusting the on-time of the switch. By operating in critical conduction mode a higher value of average inductor current is produced. However the disadvantage is now the switching frequency is not constant, only the on-time of the switch is constant. When the input voltage magnitude is high the switching frequency is low since the inductor current reaches a higher value for the given on-time. Therefore the inductor discharge time is also longer.

• Move the red timeline to observe the current paths in the circuit.
• Change the switch on-time using the red dot and observe the output voltage and inductor current. Observe the change in the average switching frequency.
• Adjust the inductor, input voltage magnitude and load resistor values and observe the changes in the waveforms.