Single stage AC/DC converter with input power factor correction

The full-bridge converter with isolation transformer is the preferred topology for medium and high power SMPS. A simple topology modification, i.e. the addition of two input inductors, and appropriate control methods permit to control simultaneously and accurately the output voltage and the input current, performing a new single stage SMPS. The circuit behaviour is strongly dependent on the output power level, as occurs with almost all single stage converters with power factor correction. In light load conditions, the voltage in the storage capacitor remains uncontrollable, even if an highly distorted input current is acceptable. This fact limits the use of those converters to some few applications where the ratio "nominal output power / minimum output power" is small, or when the converter can be switched off when the capacitor voltage is excessive. A different solution is presented in this paper, that consists in the addition of a static input switch, obtained by the replacement of two diodes in the input bridge rectifier by a pair of thyristors. Switching times are an integer number of periods and flickering is avoided by the use of a random control of the input switch.