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Spread spectrum clock generation techniques were originally developed to reduce electromagnetic interference (EMI) in communications and microprocessor systems working in the range of hundreds of megahertz. Nowadays, the switching frequency of power converters has been increasing up to values that make worthy the application of such switching frequency modulation techniques to reduce EMI emissions in power converters. Although random modulations have been applied before to power converters, periodic patterns can provide some advantages. First, theoretical principles of frequency modulation using three periodic patterns for the modulating function are presented. The influence of some important modulation parameters on the EMI reduction is analyzed and some considerations about the EMI filters design are also presented. The effectiveness of such methods in terms of EMI reduction is demonstrated theoretically and confirmed with experimental results obtained from tests carried out on two converters. The first one is a 2.5 W buck converter that can be switched up to 1 MHz and the second one is a 600 W boost converter switching at 40 kHz. In both cases, attenuations obtained in conducted EMI are evaluated. Finally, special attention has been paid to input current and output voltage ripple in order to evaluate possible undesired side-effects produced by this technique.
Date of Publication: Nov. 2007