Control Strategy for Discontinuous Conduction Mode Boost Rectifier with Low Total Harmonic Distortion and Improved Dynamic Response

Due to its simplicity, the discontinuous conduction mode boost rectifier is potentially the least expensive active line-harmonics reducing circuit. Problem statement: The line current however, shows considerable distortion when the peak input voltage is close to the output voltage, As a result, the input power factor is poor. This study proposes a simple, low-cost method to reduce the line harmonics. Approach: A periodic voltage signal was injected in the control circuit to vary the duty cycle of the boost switch within a line cycle so that the third-order harmonic of the input current is reduced and the THD is improved. The proposed technique eliminates the additional harmonic generator, phase detecting and phase-locking circuits, which was proposed in the literature. Instead we can utilize the bridge rectifier's output voltage of the boost converter to modulate the duty cycle of the boost switch. As a result, the injected signal is naturally synchronized with line current. In addition, to obtain nearly constant harmonic content over a wide range of load variation, a modulation index m is used to update the injected signal with a fraction of duty cycle which reflects the load changes. Results: The results proved that third-order harmonic, which was the Lowest Order Harmonic (LOH), can be attenuated by adjusting the modulation index of the injected signal. Moreover, the rectifier shows a good transient performance where the converter's output voltage overshoots during load and input voltage transients is reduced. Conclusions/Recommendations: The proposed circuit can be used as a front-end converter for DC/DC or DC/AC converters in order to improve the power factor of the input current. Also the proposed control circuit could be integrated in a single chip in order to reduce the cost of the industrial implementation.