Implementation of Voltage Regulation in a Spread-Spectrum-Clocked Buck Converter

Buck converters use spread-spectrum technique to reduce the electromagnetic emissions by distributing the electromagnetic interference in a frequency band around the switching frequency which consequently reduces the amplitude of the interference. Changing the switching frequency also changes the output voltage and causes an unwanted output voltage ripple. The output voltage ripple is then reduced by properly controlling the time delay used in the duty cycle controller of the buck converter. This paper presents a digital implementation of the duty cycle controller used in the buck converter which uses a phase-locked loop to generate the switching frequency. Changing the divider value of the phase-locked loop changes its output frequency, but this frequency change is not instantaneous and it is modelled as a second-order system response. The implementation of the duty cycle controller presented and analysed in this paper aims to compensate for this transient change of the output frequency, i.e. the switching frequency and keep the duty cycle of the buck converter constant. The paper focuses on the compensation algorithm used to generate a constant duty cycle, as well as on the challenges of its digital implementation.