A Spread Spectrum Method for Four-level ANPC Inverters Based on Periodic Frequency Modulation

Four-level inverters can output multiple voltage levels, which can mitigate the negative impacts of dv/dt. However, high-frequency switching of the power device is a primary cause of dv/dt. Periodic switching frequency pulse width modulation (PSF-PWM) is widely used in power converters to reduce electromagnetic interference. In this study, a PSF-PWM method is introduced for four-level inverters. A triangular carrier wave generates a switching frequency sequence based on three periodic functions, while the central switching frequency remains consistent with that of conventional PWM methods. The fundamental principles behind the harmonic dispersion effect of the PSF-PWM method are analyzed by deriving an expression for the power spectral density. An experimental evaluation of the proposed method is conducted with a simulation and real measurements of the operation of a four-level active neutral point clamped (4L-ANPC) inverter as an example. The results show that the PWM harmonics near the switching frequency and their integer multiples are dispersed to the adjacent frequency bands. Thus, the device exhibits reduced electromagnetic interference compared to conventional PWM techniques in four-level ANPC inverters.