Circuit Operation

A three-phase fully-controlled bridge circuit is a much more suitable circuit to be used for generating a variable dc output voltage than the single-phase fully-controlled bridge circuit, on account of two reasons, which are:

a. reduced ripple content in its output and
b. much higher ripple frequency.

Both these factors lead to an LC filter which is relatively small and economical. This page describes how such a power supply can be built and controlled.

An inductor in the dc link reduces ripple in the output current of the bridge circuit, whereas the capacitor absorbs the ripple in output voltage. The inductor has to be designed such that it does not saturate even when it carries the maximum current. This means that it should have an airgap in the path of flux. The ripple current through the capacitor can also be significant. Hence it needs to be checked from the datasheet that the capacitor chosen has the required ripple current rating. For such an application, an electrolytic capacitor is normally chosen and its voltage rating should also be adequate.

We can have a block diagram to describe the operation of this dc power supply obtained using a three-phase fully-controlled bridge rectifier. The output voltage V_o is varied by varying the firing angle a. The firing angle in turn is controlled by voltage V_C, which is the output of a PI controller. The inputs to the PI controller are a voltage named V_ref representing the desired output voltage and the output voltage V_o of the bridge circuit. If V_o is less than the desired output voltage, the resultant error causes the output, V_C, to increase, which in turns should advance firing angle. As the firing angle is advanced, the output voltage of the bridge circuit increases. The next section describes how the block diagram can be analysed, leading to simulation of the system.

 
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