Fixed-frequency boundary control of buck converters with second-order switching surface

A fixed-frequency boundary control of buck converter will be presented in this paper. The methodology is based on integrating the concept of variable hysteresis into the previously proposed boundary control technique with second-order switching surface. The switching frequency is maintained constant by using a frequency-to-voltage converter and comparing its output with a reference voltage to control the width of the hysteresis in the boundary controller. Compared with the methods using phase-locked loops, the proposed method has a wider lock-in range. Moreover, it integrates the advantages of the boundary control that the converter can settle into the steady state in two switching actions after large-signal input or output disturbances. The basic operating principles, stability analysis and design procedures will be given. The proposed control method has been successfully applied to control a 140 W, 24 V/12 V buck converter. The large-signal dynamic responses under supply voltage and load disturbances will be given.