On Problems of Balance in Switched Mode Converters in Submarmonic Area

Common methods of balance criterions according to the linear theories are widely spread in SMPS development - despite the fact, that we have to deal with extremely nonlinear circuits, as for instance power switches and similar. The reason for the relatively successful applications of linear theories to nonlinear systems lies in fairly large distance between the high switching frequency on one hand and the low resonant frequency of smoothing output filter on the other hand. However, almost everyone involved in development of SMPS has been confronted with the unpleasant appearing of unsymmetrical pulses within the successive cycles. This paper shows some of the physical aspects of building such subharmonics. A very simple criterion for studying and avoiding described instability will be presented. It is based on comparison of propagation of duty cycle distortions in successive switching periods. Two practical examples which appeared during the recent development of SMPS, show the capability of the presented method. The first one deals with the pulse current control and the second one with the voltage control of forward converters. Excellent conformity between the experimental results and calculations reflects the full ability of the theoretical statement, thus giving the tools for additional study of balance in this problematic frequency area.