Impact of a non-conventional PWM strategy on the DC link film capacitor sizing

In this paper, an analysis of the DC-link voltage ripple for classic and non-conventional Pulse Width Modulation (PWM) strategies such as the Unified-Double Carrier PWM (Uni-DC PWM) is derived. A focus will be held on the case where Film capacitors are applied. The latter have a very low Equivalent Series Resistance (ESR) which is responsible of losses in real capacitors (especially Electrolytic ones) hence, in the frequency range where the PWM harmonics do exist, a Film capacitor can be considered as having a pure capacitive impedance where the DC-link voltage ripple becomes highly dependent on the harmonic frequency distribution. Thus, it is important to determine the exact harmonic components needed for an analysis to be effective and this is where the double Fourier integral provides a mathematically rigorous application for studying PWM characteristics in the frequency domain. Despite the fact that the main objective of the Uni-DC PWM is to reduce the capacitor's RMS current, results show a reverse impact on the DC-link voltage ripple when a Film capacitor is used.