Investigation of Self-Output Voltage Balancing in Input-Parallel Output-Series DC–DC Converter

Voltage balancing control (VBC) between the upper and lower voltages is critical in neutral-point-clamped (NPC) inverter and bipolar dc grid systems due to the structure of cascaded dc capacitors. To address this issue, a high-voltage-gain boost converter with inherent output voltage balancing capability has been recently developed. The main benefit is that it can automatically balance the output voltages without additional control schemes or auxiliary circuits. Nevertheless, the output voltages are slightly unbalanced under certain load conditions, and the poor reverse recovery of body diodes leads to large switching losses due to hard-switching operation. This paper presents the principle of self-output voltage balancing in an input-parallel output-series (IPOS) dc-dc converter and proposes an enhanced self-balancing strategy to eliminate voltage imbalance at all load conditions. The process of implementing the proposed strategy enables zero-voltage switching (ZVS) of all the switches; hence, switching and reverse recovery losses can be significantly reduced. A 2-kW hardware prototype of the IPOS converter was built and tested to verify the feasibility of the proposed scheme.