Impact of Lost Periodicity on Efficiency and Current Waveform Distortion of SMPS'

This paper investigates significant changes in the performance of switch-mode power supplies used in standard desktop PCs (PC-SMPS'), which might occur when a PC-SMPS transfers from high to low operating powers. Using the experimental results, it is shown that due to a partial discharging of dc link capacitor at low powers, the input ac current of tested PC-SMPS' loses its periodicity with supply voltage frequency (i.e. line frequency). In such cases, there is significant increase of PC-SMPS' current waveform distortion and substantial decrease of efficiency and operational power factors. In existing literature, this phenomena is analysed in terms of electrical and control circuit “instability”, denoted as a slow-scale bifurcation and chaotic operation, where the main concern was circuit design (or re-design) for achieving full, or at least improved operational stability. In this paper, however, lost periodicity is analysed from both metrological and analytical points of view, focusing on appropriate measurement and calculation procedures for evaluating changes in PC-SMPS' performance (efficiencies and power factors) and power quality characteristics (waveform distortion). The presented analysis can be applied for both testing and evaluating performance of larger SMPS' and other power electronic devices with a similar behaviour (e.g. PV inverters).