Delta-Sigma Modulation of Two Stage Matrix Converter with Reference Voltage Correction

A two-stage matrix converter (TSMC) has the advantages of a conventional matrix converter, such as no DC-bus capacitance, flexible energy control, and a compact structure. Moreover, it simplifies the modulation process while avoiding the complex four-step commutation mechanism of direct matrix converters, becoming a potential power electronics topology. Currently, the voltage transfer ratio of TSMC using the classical method cannot exceed 0.866, and the electromagnetic noise caused by voltage harmonic near the switching frequency is relatively concentrated. According to the specific mathematical laws, the spread spectrum modulation technology transforms PWM waveforms into irregular pulse sequences. It improves the voltage transfer ratio and expands the output voltage spectrum to a wider range, thereby reducing output harmonics and electromagnetic noise. However, TSMC has a large number of power switches, and its switching coordination between the rectifier and inverter stage is complex. There are still many key issues to be addressed in the spread spectrum modulation technology of TSMC. Considering the simple structure and without complex random statistical characteristics, Delta-Sigma modulation (DSM) can significantly optimize the output spectrum, reduce the EMI level, and optimize the operational efficiency of the power converter. However, the modulation strategy of TSMC can be divided into rectifier-stage algorithms and inverter-stage algorithms. The absence of a DC-link capacitor of TSMC leads to fluctuations in DC-link voltage. The DSM of the inverter stage cannot accurately quantify and track the fluctuation patterns of DC-link voltage, making it difficult for TSMC to directly implement DSM in the inverter stage. This paper proposes a DSM scheme with reference voltage correction (RVC-DSM). Firstly, the operation principle of DSM indicates that as the order L of DSM increases, the suppression effect on low-frequency harmonics of voltage is better, and the modulator resolution is significantly improved. Specifically, the DSM resolution is increased by L+0.5 bits when its operating frequency is doubled. Then, to ensure the voltage utilization of TSMC, the rectifier stage still adopts SVM without zero-vector, and DSM is only employed on the inverter stage to achieve spread spectrum modulation. Meanwhile, limiting the switching frequency for switching loss reduction and solving the problem of voltage tracking are necessary. This paper utilizes the “Voronoi quantizer”, shaped like a Voronoi diagram as the vector partitioning rule, to implement the DSM strategy of TSMC. Finally, combined with the modulation target, the compensation coefficients of the DSM under different DC-link voltages are obtained by analyzing the DC-link voltage fluctuation in different sectors. Furthermore, the compensation coefficients are allocated to the reference voltage in real-time to eliminate the impact of DC-link voltage fluctuations on DSM, achieving efficient spread spectrum modulation of TSMC. Experiments were carried out on the platform with a dual-core controller of “FPGA+DSP” to verify the proposed RVC-DSM. The experimental results show that the harmonics originally related to the switching frequency (5 kHz, 10 kHz, …) under SVM modulation have been suppressed, and this part of harmonic energy has been extrapolated to the high-frequency band. The low-pass characteristics of the inductance load effectively reduce the conducted interference of TSMC in the 0～30 MHz band. At the same time, RVC-DSM directly employs the basic voltage vector of the inverter stage, ensuring good operating performance in the overmodulation region. Finally, benefiting from its spectrum-shaping effect, the operational efficiency of the RVC-DSM strategy is much higher than that of the SVM strategy. The following conclusions can be drawn through theoretical analysis and experimental verification. (1) By utilizing the spectrum shaping ability of the RVC-DSM modulator, the high-frequency harmonics near switching frequencies and its octave components are significantly reduced. (2) RVC-DSM can directly use the basic vector of the inverter stage. Its theoretical limit voltage transmission ratio fluctuates between 1.0 and 1.155, improving the TSMC voltage transmission ratio. (3) The switching frequency of RVC-DSM is directly related to its control period. By adopting a shorter control period, the tracking performance of the algorithm is better, and EMI can be improved significantly. © 2024 China Machine Press. All rights reserved.