Quasi-resonant DC-Link transformer-less structures for grid-connected PV systems

This paper presents the quasi-resonant DC-Link transformer-less structures whereby the efficiency of the grid-connected PV systems could be improved. Transformer-less PV inverters as a critical part in power conditioning units are more advantageous to the user. However leakage current that is resulted from omitting bulky transformers can compromise their higher efficiency. So the low leakage current and high efficiency are two crucial points in transformer-less PV systems. By incorporating Quasi-resonant DC-Link as a method of soft switching into the H6 structures two new DC-decoupled soft-switched transformer-less PV inverters are proposed by which the switching losses of power devices as well as high di/dt, dv/dt and EMI could be avoided. A new auxiliary circuit is extracted from available structures. Proposed auxiliary circuit features lower voltage stress and saves one gate drive circuitry as well. Using only two auxiliary switches, ZVS turn on/off for high frequency DC bus switches and ZVS turn on/off of the antiparallel diodes of all grid frequency main switches are realized. Soft switching condition for auxiliary switches and their antiparallel diodes are also provided. Finally a 20 kHz 1-kW universal prototype is built and Operation characteristics, soft switching conditions and efficiency improvement of proposed QRDCL transformer-less PV inverters are verified.