A novel soft-switching modular inverter for photovoltaic PCU with high efficiency and reduced size

In recent years, the concerns for environment and available natural resources have ever grown to a worldwide agreement of developing and promoting the usage of renewable energy sources. The household photovoltaic power system is one of the most promising approaches to utilize the solar energy. The power conditioning unit (PCU) used in conventional household PV power system with a centralized grid-connected inverter has several drawbacks, such as low efficiency, large size, high cost, safety and EMI problem, and lack of reliability and flexibility of capacity expansion. The authors first propose a DC-link inverter which solves the ground leakage current and EMI problem, bring the concept of modular inverter for photovoltaic PCU which improves the reliability and flexibility, and present a novel soft-switching scheme for the modular inverter. The inverter is formed by a ZVT-PWM boost converter, a ZVS-ZCS-PWM buck converter and a low frequency full-bridge inverter. The ZVT-PWM boost converter maintains the dc bus voltage and provides an auxiliary voltage source for the ZVS-ZCS scheme of the buck converter. The buck converter consists of a main switch and a novel ZVS-ZCS resonant cell, which generates a semi-sinusoidal dc current. The semi-sinusoidal dc current is inverted to sinusoidal and fed to the utility grid by the low frequency full-bridge inverter. No additional stresses are added to the main circuits by the soft-switching scheme. The size of the inverter is reduced because of the high frequency operation while the overall efficiency is kept high. The operating principles are introduced and the circuit is implemented to a 1 kW prototype module. A power density of 0.5 W/cm³ is achieved and the experimental results verify the soft-switching scheme with a maximum overall efficiency of 97% at the switching frequency of 100 kHz.