Analysis of three-phase/three-level rectifiers at low load and discontinuous conduction mode

A prominent boost-type three-level topology, which proved to represent a cost effective and highly efficient solution for switched mode rectifiers is inspected towards its operation at discontinuous conduction mode (DCM). This mode of operation occurs not only at high input voltage in conjunction with low load currents but even at medium loading in the vicinity of mains voltage zero crossings. Since the requirements on THD conflict with the actual behavior of the circuit when operated at DCM, measures are needed for optimization. After reviewing basic measures to avoid DCM the resemblance to three-phase/single-switch boost rectifiers is utilized to control the circuit. Novel measures to optimize the modulation scheme are proposed, facilitating the design of even smaller inductors, thus lowering costs. Selected simulation and measurement results prove the enhanced modulation scheme.