Improved Delta-Sigma Modulator for Direct Switch Control of a DC-DC Converter

Abstract

Digital Delta-Sigma modulators can be an interesting alternative to Pulse Width Modulation for generating switching signals of a power converter. Their noise-shaping behavior results in low output distortion for output frequencies well below the modulation frequency. To directly generate the switch control signals of a traditional DC-DC converter, a first-order modulator with a two-level quantizer is preferred as they are inherently stable for the entire output range. However, the noise attenuation of a first-order modulator is limited to 20dB per decade for frequencies below the modulation frequency. Additionally, they suffer from limit cycles due to the low modulator order and 2-level quantizer resolution. To overcome this problem, a fast feedback loop is proposed that acts on disturbances introduced by the quantizer and compensates low frequency noise. As a result, an improved Delta-Sigma modulator is obtained that remains stable for almost the entire output range but also provides improved noise attenuation like that of a second-order modulator. Moreover, the output noise signature is whitened resulting in less power being present at the limit cycle frequencies.