A power-efficient method to mitigate the EMI of Switched-Mode Power Supplies

Modern microprocessors are predominately powered by the Switched-Mode Power Supplies. Fast switching of the power MOSFETs causes high-frequency voltage and current oscillations in the loop formed by the input decoupling and the MOSFETs. Such oscillations generate significant and broadband EMI noise. Conventional techniques to suppress this noise such as using snubbers, increasing MOSFET gate resistance or employing smaller MOSFETs result in higher power loss, shorter batter life and expensive cooling. This paper proposes an alternative approach to mitigate the EMI based on careful design of input decoupling. The paper shows how input decoupling can be optimized to provide low resistance path for low frequency ripple of the input current, while forcing high-frequency current to flow through a high resistance path. This method effectively increases the decay rate of the oscillations, which in turn decreases the EMI. This method was verified on two laptop designs and the experimental results showed 6dB of EMI reduction with no measurable degradation in the power efficiency.