High-Bandwidth Control of a 20-kW Single-Stage Unfolding-Based AC–DC Converter Using the Extra Element Theorem and Current Emulation Technique

In this article, a thorough analysis is presented concerning the challenges associated with achieving high-bandwidth control in high-frequency dc–dc converters used within single-stage 3-$\phi$ unfolding-based ac–dc systems. High-bandwidth control in these ac–dc conversion systems is crucial for effective grid-side power factor correction and appropriate output power regulation; however, its implementation can pose substantial challenges that impact converter stability. The main issue is identified as an undesirable resonance between the grid inductance and dc-link capacitors, and the resonance phenomenon is investigated using the extra element theorem (EET) as the analytical framework. The plant transfer function and EET parameters for the dc–dc conversion system are determined through phasor transformation-based small-signal modeling, which has been validated by multitone analysis and hardware experimentation. In response to the identified issue, a control-based methodology is proposed to address the $LC$ resonance. This approach involves the emulation of current sources/sinks to actively damp the resonance, enabling high-bandwidth control, and eliminating the need for passive physical damping networks typically used in unfolding-based ac–dc systems. The validation of this proposed technique is conducted through simulation-based analysis and hardware testing of a 20-kW prototype, which comprises a grid-tied 3-$\phi$ Unfolder followed by a T-type bridge-based dc–dc conversion system. The hardware results demonstrate a significant tenfold increase in achievable closed-loop bandwidth compared to operating the system without damping. In addition, stable system operation with a low grid current total harmonic distortion of under 2.29% is achieved, even at bandwidth levels higher than the $LC$ resonant frequency.