A conceit of unipolar N-multiple frequency SPWM and the main circuit topology

The sinusoidal pulse width modulation (SPWM) has been widely used because of its simple implementation. The number of output voltage pulses and the frequency of the lowest harmonic voltage in unipolar SPWM are twice that of bipolar SPWM with the same switching frequency, so the unipolar SPWM facilitates the choice of filter and has better output waveforms. For the medium or low voltage high-current applications, high capacity switching devices such as IGBT (insulated gate bipolar transistor) are expected, but the capacitance of the device IGBT decreases with its switching frequency increasing. There is a contradiction in choices between high-capacity switching device and the filter. One of the solutions is to achieve higher equivalent output frequency by lower switching frequency. The idea of unipolar N-multiple-frequency SPWM is put forward in this paper and its principle is given in two-level voltage source inverter (VSI). The comparison between unipolar N-multiple frequency SPWM and the carrier phase-shifting SPWM (CPS-SPWM) is analyzed. Their essential differences are pointed out. Finally, a main circuit topology is established.