Quantization noise shaping in digital PWM converters

This paper addresses quantization noise shaping using sigma-delta (SigmaDelta) modulators in switched-mode power converters with digital pulse-width modulation (DPWM). It is shown that the SigmaDelta modulator design must take into account the effects of idle tones and DPWM nonlinearities, as well as the converter switching frequency and output filter constraints. Best results are obtained using 2^nd order or higher order SigmaDelta modulators in combination with center (dual-edge) DPWM. As an example, in an experimental synchronous buck power converter operating at 781 kHz and having a 4-bit center DPWM, a 44 dB reduction of the maximum noise spectral component over the 1^st order modulator is obtained using a modified 3^rd-order SigmaDelta modulator. Various design trade-offs are examined and limits of achievable DPWM resolution improvements are found as a guide to practical realizations of high-frequency digital controllers for DC-DC converters.