EMI Reduction by Spread-Spectrum Clocking in Digitally-Controlled DC-DC Converters

SUMMARY This paper proposes spread-spectrum clock modulationalgorithms for EMI reduction in digitally-controlled DC-DC converters.In switching regulators using PWM, switching noise and harmonic noiseconcentrated in a narrow spectrum around the switching frequency cancause severe EMI. Spread-spectrum clock modulation can be used to min-imize EMI. In conventional switching regulators using analog control it isvery difficult to realize complex spread-spectrum clocking, however thispaper shows that it is relatively easy to implement spread-spectrum EMI-reduction using digital control. The proposed algorithm was verified usinga power converter simulator (SCAT). key words: PWM control, spread-spectrum clock modulation, EMI,digitally-controlled DC-DC converter 1. Introduction Most portable electronic devices have low-voltage high-current power supplies using high-efficiency switching reg-ulators. However, the considerable switching noise (andEMI) that switching regulators produce is a big demerit.EMI-related regulations are becoming more severe [1]. Reg-ulations prescribe limits on the generation of Electromag-netic Interference (EMI) and also noise-tolerance or Elec-tromagnetic Susceptibility (EMS) requirements; the combi-nation of both EMI and EMS is called EMC (Electromag-netic Compatibility). In many countries it is illegal to sellelectrical and electronic devices that do not comply with lo-cal EMC standards (in many cases devices must be type-approved, or tested and certified as complying). In mostconventional switching regulators, shielding and filteringare used to reduce EMI. While shielding and filtering areeffective ways of reducing noise, penalties associated withthis approach include the cost of transformers and coils andthe size/weight of shielding.We considered, as an alternative to extensive shield-ing and filtering, how to reduce EMI by switching regu-lator control-module design changes. After examining thepossible application of digital control systems with spread-spectrum technology [2]–[5] to analog-controlled switching