The active rectifier, which is also known as the boost power factor corrector (PFC), is constructed from a full bridge rectifier and a boost converter. The objective of the circuit is to produce a regulated output DC voltage while drawing a unity power factor, sinusoidal current from the mains supply. To achieve this, the boost converter must be switched in such a way to produce the sinusoidal current. Therefore the average inductor current must have the waveform shape of a rectified (absolute) sinusoid. The next three applets demonstrate 3 different control methods to generate the switching signal to produce the sinusoidal current. These are the discontinuous conduction, critical conduction and continuous conduction methods.

In this applet the discontinuous conduction method is investigated. Discontinuous conduction means that the inductor current must always start from and return to 0A during each switching interval. If this occurs then the peak value of the inductor current is then dependent on the magnitude of the voltage at the output of the bridge rectifier, which is a rectified sinusoidal. Therefore the average inductor current also has a rectified sinusoidal shape, which leads to a sinusoidal input current. In a practical implementation of the active rectifier the filtering of the input current would be performed by an EMI filter (not shown) to produce the smooth average current waveform.

The circuit values and operating parameters, such as switching frequency, must be calculated to ensure that discontinuous conduction is maintained for all operating conditions. If the circuit goes into continuous conduction then the average inductor current does not have a sinusoidal shape. With this applet the parameters can be changed such that continuous conduction is occurs and the resulting current waveform can be viewed.

The switching frequency of the discontinuous conduction PFC is constant, and therefore it is more straightforward to design an appropriate EMI filter. The output voltage level is controlled by adjusting the duty cycle of the switching, however if the duty cycle is increased too much then continuous conduction occurs.

• Use the red timeline to shift the operating time point and view the current paths.
• Change the duty cycle by moving the red dot on the switching signal. Increase the duty cycle and observe the output DC voltage. What happens to the average inductor current when continuous conduction occurs?
• Adjust the switching frequency and the inductor value and observe the influence on the inductor current waveform.
• Change the input AC voltage magnitude and the load resistor value. Observe the change in the inductor current.
• Note the output DC voltage level. Now adjust the load resistor value and then use duty cycle to return the output DC voltage level back to the original level. Is the boost PFC still operating with discontinuous conduction?