A Quasi-Unipolar SPWM Full-Bridge Transformerless PV Grid-Connected Inverter with Constant Common-Mode Voltage

The unipolar sinusoidal pulsewidth modulation (SPWM) full-bridge transformerless photovoltaic inverter with ac bypass brings low conduction loss and low leakage current. In order to better eliminate the leakage current induced by the common-mode voltage, the clamping technology can be adopted to hold the common-mode voltage on a constant value in the freewheeling period. A full-bridge inverter topology with constant common-mode voltage (FB-CCV) has been derived and proposed in this paper, two unidirectional freewheeling branches are added into the ac side of the FB-CCV, and the split structure of the proposed freewheeling branches does not lead itself to the reverse-recovery issues for the freewheeling power switches and as such superjunction MOSFETs can be utilized without any efficiency penalty. The passive clamping branches consist of a capacitor divider and two diodes, is added into the dc side of the FB-CCV, therefore, the weakness of active damping branch has been overcome, and the better clamping performance has been achieved in the freewheeling period. In the dead time between the main switches and the freewheeling switches, the antiparallel diodes of diagonal main switches of the FB-CCV form the freewheeling path to clamp the common-mode voltage at a constant value, and a quasi-unipolar SPWM strategy is presented. The operation principle, differential-mode, and common-mode characteristics of the FB-CCV, Heric, H6, and HB-ZVR topologies are analyzed and compared in detail. Finally, the viability of the FB-CCV is verified by a universal prototype inverter rated at 5 kW.