Optimal regulator based control of NPC boost rectifiers for unity power factor and reduced neutral point potential variations

Neutral-point-clamped boost rectifiers (NPCR) are suitable for high-voltage systems because of their circuit structure. The NPCRs, however, have a problem in that the neutral point potential (NPP) varies when the current flows into or out of the neutral point. The variations cause voltage deviations in the input waveforms as well as unbalanced voltage stress in the devices. Accordingly, reducing NPP variations is very important. This paper describes a controlling method for NPCRs based on a state-space model. The neutral point current is treated as one of the inputs. The controller is designed based on the optimal regulator theory in order to achieve the control of DC voltage, power factor and NPP, simultaneously. The validity of the proposed method is demonstrated by experimental results.