Control of three-phase buck-type rectifier in discontinuous current mode

In the three-phase buck-type rectifier, the current on the dc-link inductor becomes discontinuous under light load condition, at which point the current ripple is larger than the dc current value. Traditional control algorithms and modulation schemes do not work consistently well in discontinuous current mode (DCM), causing input current distortion and output voltage ripple. In this paper, the three-phase buck-type rectifier is modeled and analyzed in DCM. The DCM transfer function is derived and compared with the one for continuous current mode (CCM). It is shown that the pole and gain of the DCM transfer function changes significantly compared to that of CCM. A new modulation scheme for DCM is then proposed, which places the space vectors in such way to keep the dc-link current continuous during the active states. A digital controller is then used to eliminate the sampling error caused by the large current ripple, successfully controlling the rectifier in DCM. Simulation and experimental results are used to verify that the input current distortion and the output voltage ripple are dramatically reduced under the proposed DCM modulation and control strategy.