Switching period randomisation for multilevel converter modulation

The spectral energy associated with the carrier and sidebands of naturally sampled carrier based PWM can be spread by randomising the carrier (switch) half-period T_c = 1/2f_c. So long as the switch duty cycle each period still correctly reflects the value of the modulating fundamental waveform as sampled during that switch period, then the fundamental component will remain undistorted. Natural sampling will ensure this occurs. Carrier based PWM can be extended to (m+1) level multilevel converter waveform generation by creating m triangular carriers, each with an equal 2π/m phase displacement. Alternatively the carrier disposition strategy calls for m amplitude displaced triangular carriers, each of amplitude 1/m and frequency mf_c. Randomising these carrier sub-periods T₀ = 1/2mf_c is shown to generate (m+ 1) level PWM waveforms where the first (m-1) carrier groups are cancelled, while the remaining carrier and sidebands at multiples of mf_c are spectrally spread. Numerous five level simulation and experimentally gathered randomised PWM waveforms are presented, showing the effects of the variation of the degree of randomisation, modulation depth and pulse number.