This class of rectifiers are called Active Rectifiers since they contain a switching device to actively control the input current so that it has a sinusoidal shape which is in phase with the source voltage. These rectifiers can also be called Unity Power Factor Rectifiers. This applet gives an example of one particular type of active rectifier, called the Vienna Rectifier (VR). The VR has 3 transistor switches and 12 additional diodes compared to the uncontrolled bridge rectifier. The VR is a boost rectifier, which means that the DC output voltage is at level that is higher than the peak of the line-to-line voltage. In this example it is assumed that the output voltage is being maintained at a constant value and is equally split between the two capacitors. 

Since each switch can either be on or off and that there are 3 switches in the circuit, there can be 8 different switching combinations. The 8 possible values are given in the vector diagram where each vector is given by a switching function where a 1 represents the switch being on and 0 the switch being off. The switching function is also shown graphical in the bottom right figure. By moving the red line in the switching function graph it can be seen how the switching pattern produces a voltage vector. The controller for the VR selects the appropriate switching pattern to ensure that the mains current remains in phase with its associated voltage. 

Since the direction of the phase current has an effect on what the switching function achieves, the selection of vectors must change for different portions of the fundamental waveform. By moving the red line on the fundamental voltage and current graph you can see how the operating sector, in the vector plot, changes. In one fundamental period there are 6 different sectors. 

By adjusting the input voltage up and down it can be seen how the choice of switching vectors changes.

• The magnitude of the input voltage can be adjusted for all 3-phases by using the red dot on the R phase input voltage. Adjusting this magnitude results in different switch combinations and on times being used to generate the constant output voltage.
• Using the red line in the switching figure, the switches will open and close. See the effect that this has on the current path direction in the rectifier and on the vector being used in the vector diagram.
• Move the red line in the fundamental voltage and current plot to see how the different vector sectors are used. Determine what causes the new vector segment to be used?