An EMI Reduction Technique for Digitally Controlled SMPS

A spread spectrum technique and system for reducing average electromagnetic interference (EMI) in low-power digitally controlled dc-dc switch-mode power supplies (SMPS) are introduced. The technique utilizes very simple hardware, where the switching frequency of a SMPS is dynamically varied over a controlled range. This is achieved by changing the supply voltage of a ring-oscillator based digital pulse-width modulator in a pseudo-random fashion, through 128 discrete steps. The change is performed with a 1-b DeltaSigma digital-to-analog converter. Compensator design guidelines for this variable frequency system are provided for obtaining good dynamic response. The technique was tested with a 500-mW, 1.8-V buck converter prototype, whose switching frequency was varied from 1.74 to 2.84 MHz. A reduction of 23 dB in the conducted EMI with an efficiency degradation of less than 0.1 % was obtained, compared to fixed frequency operation.