Analysis and design of a robust model-following control circuit for multiple-output isolated DC-DC converters with current injection control

A robust model-following control scheme for multiple-output isolated DC-DC converters with peak current mode control is presented. The aim of the proposed circuit is to reduce the closed loop audio-susceptibility and the output impedance of the converter without the need of a high bandwidth in any of the regulation loops. In this way the problems of using high crossover frequencies, such as modeling errors, sensitivity to components tolerances and noise, are avoided. This is an attractive point for designers because in multiple-output converters the regulation bandwidth is limited in practice by the effects derived from coupling the filter inductors of the outputs in the same core. The practical implementation of the proposed control circuit consists of adding an inner loop based on a low-pass first-order reference model and a conventional PI regulator, besides the outer voltage loop. An advantage, when compared with other methods based on reference models, is the low-pass nature of the reference model, which doesn't add noise limitations to the practical implementation. The design of the voltage regulator is straightforward if the crossover frequency of the voltage loop is chosen smaller than that of the inner loop. A systematic design procedure is explained.