Applying the theory of sliding mode to a cascade of boost-forward converters used as one-phase power supply unit factor pre-regulator

Two power converters connected in cascade provides the ideal solution for assuring a unity power factor and a good regulation of output voltage. The use of sliding modes allows power converter large signal modelling and leads to output robustness principally vis-a-vis fluctuation of load and unknown parameters. Two controls were studied. The first is of variable frequency and derives directly from the theory of variable structure systems (VSS). The resulting control is stable and robust. Stability and surface convergence criteria have been established. The second control is of fixed frequency and uses the close relationship between the sliding mode equivalent control and the duty cycle to calculate the duty cycle of the two switches directly via a DSP. Different laws of reaching surfaces are considered. The aim is to minimize and even suppress the static error introduced on account of switching to a fixed frequency. The results are almost similar to those of the first method. The control is in fact midway between sliding mode control and conventional voltage control.