Limited-Pool Random Carrier-Frequency PWM for Digitally Controlled dc-dc Converters

This paper investigates the application of a limited-pool random pulse-width modulation (RPWM) to high frequency digitally controlled DC-DC power converters. The use of a suitably selected small number of switching frequencies ensures the generation of a flat and widespread spectrum, while limiting the system complexity, especially from the integrated circuit (IC) point of view; moreover the spreading action achieved is almost equivalent to the one obtained with a larger ensemble of frequencies. In the presented solution, the randomization action affects both the switching period and the turn-on interval, realizing the random carrier-frequency modulation with fixed duty cycle (RCFMFD). Moreover, the generation of the random number stream is based on a simple linear feedback shift register. The design of the switching frequencies pool and the evaluation of system performances have been realized using an accurate time-based model of the spectrum analyser. Experimental results on a DC-DC synchronous buck converter, where the digital control has been implemented in field programmable gate array (FPGA), confirm the properties of the proposed solution