When full wave rectifier with filter is used, current on ac sides contains spikes. THD for this current is very high. This harmonics needs to be eliminated. This can be done using full bridge. Current on ac side of full bridge can be controlled easily.
Here, voltage of output capacitor is measured and subtracted with reference voltage. This difference is passed to PI controller which gives the value of power to be drawn from AC side to maintain the capacitor voltage. In steady state, this value is equal to power drawn by DC load. From the value of power, reference current in phase with input AC voltage is calcuated whose magnitude determined by the value of power to be drawn. In this way, reference current is calculated. This current is drawn from AC side where current being controlled by hysteresis band current controller. In this way, harmonics of current in AC side can be mitigated.