Micro-inverters operating into the single-phase grid from new energy source with low-voltage output face the challenges of efficiency bottleneck and twice-line-frequency variation. This paper proposed a multilevel inverter based on bridge modular switched-capacitor (BMSC) circuits with its superiority in conversion efficiency and power density. The topology is composed of DC-DC and DC-AC stages with independent control for each stage, aiming to improve system stability and simplify the control method. The BMSC DC-DC stage, which can be expanded to synthesize more levels, not only features multilevel voltage gain but also partially replaces the original bulk input capacitor and functions as an active energy buffer to enhance power decoupling ability between DC and AC sides. In DC-AC stage, the control strategy of optimized unipolar frequency doubling sine-wave pulse-width modulation (UFD-SPWM) is proposed to improve the quality of output waveform. Meanwhile, the multilevel voltage phase has been optimized to reduce the power loss further. Finally, a prototype has been built and tested. Associated with the simulation, the experimental results validate the practicability of these analyses.
- Switched-capacitor circuit
- Multilevel inverter
- Power decoupling
- Optimized unipolar frequency doubling SPWM.
Fig.1 Topology of the proposed converter.(a) General topology of bridge modular switched-capacitor-based multilevel inverter (b) Seven-level inverter.
EXPECTED SIMULATION RESULTS
Fig.2 Simulation waveforms of seven-level inverter.(a) Us1_DS, Us3_DS, Us1a_DS and Us2a_DS. (b) UC2a, Ud, UX, Uo and io. (c) Spectrum of Uo.
Fig.3 Simulation comparison of power decoupling ability at different Cin. Under proposed control strategy:(a)Ui and Po. (b)Ud and Po. Under conventional control strategy:(c) Ui and Po. (d) Ud and Po.
A bridge modular switched-capacitor-based multilevel inverter with optimized UFD-SPWM control method is proposed in the paper. The switched-capacitor-based stage can obtain high conversion efficiency and multiple voltage levels. Meanwhile, it functions as an active energy buffer, enhancing the power decoupling ability and conducing to cut the total size of the twice-line energy buffering capacitance. Furthermore, voltage multi-level in DC-link reduces the switching loss of inversion stage because turn-off voltage stress of switches changes with phase of output voltage rather than always suffers from one relatively high DC voltage. Most importantly, the control method of UFD-SPWM, doubling equivalent witching frequency, is employed in the inversion stage for a high quality output waveform with reduced harmonic. In addition, the optimized voltage level phase maximizes the fundamental component in output voltage pulses to reduce harmonic backflow as possible. Hence, the comprehensive system efficiency has been promoted and up to peak value of 97.6%. Finally, two conversion stages are controlled independently for promoting reliability and decreasing complexity. In future work, detailed loss discussion, including theoretic calculation and validation of loss breakdown, will be presented.
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