Spectral shaping of a randomized PWM DC-DC converter using maximum entropy probability distributions

Abstract

The use of random pulse width modulation has been known to help reduce Electromagnetic Interference (EMI) in power converters. The procedure for reducing EMI using Random Pulse Width Modulation (RPWM) techniques has always been to spread the harmonic power in the power density spectrum -otherwise caused by periodic PWM- by randomizing the PWM. This in turn reduces the amplitude of the EMI noise. Probability distribution functions (PDF) determine the extent to which EMI amplitudes could be reduced. This paper presents probability distribtions of the PWM which will result in the spreading out of harmonic power in the power density spectrum. The relationship between probability distributions and spreading out of harmonic power whilst maintaining constraints in a DC-DC converter is investigated. A probability distribution whose aim is to ensure maximal harmonic spreading and yet mainaint constraints is presented. The PDFs are determined from a direct application of the method of Maximum Entropy. It is shown that given a pool of randomized parameters, there exists a region through which maximum spreading happens and how this spreading out is compromized by having constraints.