Random carrier frequency modulation of PWM waveforms to ease EMC problems in switched mode power supplies

Electromagnetic compatibility (EMC) remains an issue which can present many problems to designers of high frequency power converter circuits. Usual techniques for alleviating the problems of interference generation by power converters involve the use of screening materials and filters. However, the effectiveness of such measures depends on the frequency of the interference and the power density at each frequency of interest. As pulse width modulation (PWM) generates high frequency harmonics at multiples of the switching frequency, modulation of the switching frequency may be used to spread the spectral power density present at these harmonic frequencies. This paper presents a technique of frequency-hopping spread-spectrum modulation which may be applied to switched-mode power converters to reduce the spectral power density at harmonics of the switching frequency. The influence of the peak frequency deviation and the modulation sequence length on the resultant spectrum are discussed. These predictions have been broadly confirmed in practice