Control Schemes for Reducing the Second Harmonic Current in Two-Stage Single-Phase Converter: An Overview from DC-Bus Port-Impedance Characterization

The instantaneous input and output power of the single-phase converter are imbalanced, resulting in second harmonic current (SHC). This paper provides an insight for reducing the SHC in two-stage single-phase converter from the perspective of the dc-bus port-impedance characterization. It is found that the dc-dc converters in the two-stage single-phase converter can be categorized into two types. One is operating as a bus-voltage-controlled converter (BVCC) whose dc-bus port-impedance is approximately inverse-proportional to the voltage loop gain. The other is operating as a bus-current-controlled converter (BCCC) whose dc-bus port-impedance is a negative resistor. Based on the dc-bus port-impedance characterization, critical points for SHC reduction are summarized, indicating that, for reducing SHC in BVCC, proper control scheme needs be incorporated for increasing the dc-bus port-impedance at twice the input frequency $(2f_{\text{in}})$ or output frequency $(2f_{\mathrm{o}});$ concurrently, the dc bus voltage ripple should be limited for reducing the SHC in the dc source or load; by contrast, for reducing SHC in BCCC, the dc bus voltage ripple should be reduced; simultaneously, the loop gain at $2fi_{\text{in}}$ or $2f_{\mathrm{o}}$ should be high enough for eliminating the SHC in the dc source or load. Thereafter, proper SHC reduction approaches are recommended for different types of two-stage single-phase converters, and pros and cons of different SHC reduction schemes are carefully reviewed. Finally, potential challenges and issues related to this research topic are discussed.