Down-sampled Repetitive Control Design for a Four-Active-Switch Isolated Bridgeless Inverter

This paper proposes an isolated bridgeless inverter that operates with only four active switches. By eliminating the H-bridge, the presented inverter enables direct DC–AC single-power-conversion (SPC) and operates in Zeta or Ćuk mode depending on the output voltage polarity. Through this asymmetric operation and bridgeless structure, both step-up and step-down voltage characteristics are achieved with a reduced number of switches. Furthermore, galvanic isolation is realized with a single transformer, significantly reducing the component count and cost. However, when the inverter is connected to the grid, two major control issues arise: the right-half-plane (RHP) zero in its transfer function, which limits control bandwidth and stability, and high sensitivity to grid disturbances. Conventional PI or feedforward controllers cannot guarantee precise waveform quality under these conditions. To address these challenges, this paper introduces a down-sampled repetitive controller (DRC) that dramatically reduces computational and memory burden compared to conventional repetitive control, while a phase-lead compensation method mitigates the phase lag induced by the RHP zero. As a result, the proposed control scheme achieves high output current tracking accuracy and robust performance against grid disturbances.