Photovoltaic DC-DC module integrated converter for novel cascaded and bypass grid connection topologies — Design and optimization

Grid connected Photo Voltaic (PV) inverters fall into three broad categories — Central, String and Module Integrated Converters (MICs). MICs offer many advantages in performance and flexibility, but are at a cost disadvantage. Two alternative novel approaches proposed by the author — cascaded dc-dc MICs and bypass dc-dc MICs — integrate a simple non-isolated intelligent dc-dc converter with each PV module to provide the advantages of dc-ac MICs at a lower cost. A suitable universal 150W 5A dc-dc converter design is presented based on two interleaved MOSFET half bridges. Testing shows Zero Voltage Switching (ZVS) keeps losses under IW for bi-directional power flows up to 15W between two adjacent 12V PV modules for the bypass application, and efficiencies over 94% for most of the operational power range for the cascaded converter application. Based on the experimental results, potential optimizations to further reduce losses are discussed.

I. The Growth of Grid Connected PV

The installation of grid connected photovoltaic (PV) systems is growing at a staggering rate, driven by a number of factors including growing concern about global warming and energy security, and improvements in technology and subsequent decreasing costs. Growth rates of approximately 40% each year are reported in the member countries of the International Energy Agency Photovoltaic Power Systems Programme (IEA PVPS) (Fig. 1). The total reported installed PV capacity in 2004 is over 2500MW with over 80% of that grid connected [1]. Note that these numbers do not include the rapid recent growth in non-member countries China and India, among others. Grid-connected systems typically cost about 5 to 7 US$ per watt, with PV modules accounting for 50-60% of system costs [2].