Multilevel Hybrid Frequency Modulation With Adaptive DC Link Voltage Fluctuation and Dead-Time Elimination for Cascaded H-Bridge Inverter

With the same number of cascaded cells, the asymmetric H-bridge cascaded multilevel inverter (CMLI) achieves more output voltage levels. This paper proposes a multilevel hybrid frequency (HF) modulation composed of region-segmented low-frequency modulation and dead-time-eliminated high-frequency modulation for the asymmetric two-cell H-bridge CMLI. To reduce switching losses, high DC link voltage cascaded cell adopts low-frequency modulation, while low DC link voltage cell employs high-frequency modulation. After applying the proposed modulation, only one bridge arm operates at high frequency in the two-cell CMLI. Based on the switching patterns of cascaded units, the natural dead time in the driving signals of upper and lower switches within each bridge arm is analytically derived. The current zero-crossing detection debouncing scheme proposed in this paper can effectively prevent output current distortion. Based on the output power of each cascaded cell, the two-dimensional plane formed by the DC link voltage ratio and modulation index is divided into three operating regions. Simulation results verify that the multi-level hybrid frequency modulation can adaptively respond to DC-link voltage fluctuations and load characteristics. This paper has developed an experimental prototype with a combined MCU and FPGA control system as the core. The results verify the effectiveness of the multi-level hybrid frequency modulation.