Primary-controlled SMPS with adjustable switching frequency for output voltage and current control

摘要

The present invention relates to a method for controlling the output voltage of a primary-controlled switched-mode power supply unit having a primary-side switch and a transformer with an auxiliary winding in which an auxiliary voltage which images the output voltage is induced after the primary-side switch is opened. The voltage induced in the auxiliary winding is fed to a control circuit as the control variable. The present invention also relates to a control circuit for performing such a control method and to an associated switched-mode power supply unit. To adjust the output voltage and the output current in the simplest and most economical way while minimizing the cost of the components needed, the switching frequency of the primary-side switch is so adjusted in dependence on the auxiliary voltage in the present invention that the output voltage and the output current of the switched-mode power supply unit adopt values in accordance with a predetermined output characteristic. This may be a linear output characteristic.