EMI and Voltage Ripple Co-Optimization of a Spread-Spectrum Controller in Buck Converters

The buck converters can use spread-spectrum technique to ensure the reduction of electromagnetic emissions by distributing the energy around the switching frequency and reducing its amplitude. The switching frequency change causes the output voltage to change which manifests in an unwanted voltage ripple. This paper considers a spread-spectrum controller which uses a Phase Locked Loop (PLL) with a programmable divider to generate the changing switching frequency. The programmable divider uses a divider generator to generate the random sequence of dividers and the paper analyses generators based on Linear Feedback Shift Registers (LFRS) and memories. The Electromagnetic Interference (EMI) caused by the buck converter and the output voltage ripple are numerically modelled and then used to optimize divider generators. The results show that the memory-based generator designs have larger influence on EMI levels and duty cycle ripple which makes it more suitable for the co-optimization procedure. The co-optimization procedure results in memory-based designs which offer better performance in terms of EMI and duty cycle ripple compared to the LFSR-based designs. Implementation of the divider generators in field-programmable gate array (FPGA) shows that the memory-based generators have a larger programmable logic footprint. The measurements of the generator designs in a buck converter confirm the estimated relations of the modelled Figures-of-Merit.