Spectrum analysis of switched-capacitor mode DPWM generator with Spread-Spectrum Clocking circuit

In this paper, we analyzed a spectrum of the SMPS (Switching Mode Power Supply) which uses DPWM (Digital Pulse Width Modulation) and adopts SSC (Spread Spectrum Clocking) circuit to reduce EMI (Electro-Magnetic Interference) noise. Generally, SSC circuits are used in order to reduce noise peaks by spreading switching frequency spectrum. Such noise peak reduction is verified with a general FFT (Fast Fourier Transform) method through sufficiently long time which includes many switching cycles. However, effects of drastic changes of power supply voltage such as voltage dropping, onetime noise or bounce in power distribution network are apparently not revealed in the spectrum because their effects are averaged down in the general FFT method. These ripples in power distribution become sources of EMI failures. For accurate spectrum analysis in the transition period, FFT should be performed separately with respect to each transient section. Therefore, we carried out short-time Fourier transform (STFT) with respect to several local window sections as well as general Fourier transform with respect to the overall interval. The separated local windows are the overshoot section and the undershoot section.