Multilevel converter with variable flying capacitor voltage used for virtual infinite capacitor

Abstract

We evaluate the flying capacitor multilevel converter (FCMC) with one switched-capacitor cell under asymmetric working condition, as a highly efficient hardware realization for the virtual infinite capacitor (VIC). The VIC is a type of electronic circuit, recently introduced to replace large, expensive and unreliable electrolytic capacitors, mainly intended for smoothing random fluctuations of a DC bus voltage V. By the proposed novel control algorithm, the pulsating power that the VIC takes away from the DC bus is shared among the main buffering and the flying capacitors, with the proportion set by a prescribed value. This facilitates to reduce the physical size of the converter, and hence achieve higher power density. This flying capacitor converter operates in an unconventional way, as the flying capacitor voltage is highly variable and hence the converter works under an asymmetric scenario. An average model of this specific working mode is presented, with the design of a linearized current controller. To improve the capacitance reduction ratio, the average value the of flying capacitor voltage is regulated to a relatively high value, instead of V/2. This is achieved by adjusting the phase shift of the PWM carriers. Both continuous and discontinuous conduction modes are investigated, and verified by simulation.