Variable Duty Cycle Approach to Improve CCM Boundary Range of Current-Fed Switched Inverter with the Modified PWM Scheme

Abstract

Active front-end based Impedance Source Inverters (ISIs), such as Current-Fed Switched Inverter (CFSI), and Switched boost inverter (SBI) are more popular due to less number of component count, inherent shoot-through protection, and higher AC gain. But, in these converters, intermediate DC voltage is kept higher due to constraint on modulation index. This intermediate voltage, in case of CFSI, is reduced by modifying the PWM scheme. But, at the lower intermediate voltage, the input current of the inverter increases. As a result, double line frequency current, and hence peak to peak ripple current, in the inductor of the boost stage increases. This increased ripple forces the CFSI to move into pseudo-DCM. This situation becomes more severe at the light load condition. In pseudo-DCM, AC output voltage of the ISIs distorts and gives rise in THD. In this paper, AC equivalent model of CFSI with the modified PWM scheme is derived. This helps in calculating the double line frequency component in the inductor current, and hence CCM/DCM boundary range. Additionally, a variable duty cycle scheme is used, which suppress the double line frequency component of the inductor current, and widen the CCM boundary range of CFSI. By implementing this technique, the converter can operate in CCM during the light load condition even at the lower DC-link voltage. A 500 W prototype of CFSI is designed to verify the control technique. By using the control technique, peak-to-peak ripple in the inductor current is reduced from 6.5 A to 1.4 A.