A noise-shaped switching power supply using a delta-sigma modulator

A technique to reduce in-band tones in switch-mode power supplies is described. It takes advantage of the noise-shaping properties of the delta-sigma (ΔΣ) modulator to eliminate the spikes normally present in switching power supplies. A framework is introduced for comparing the conventional pulsewidth modulated (PWM) controller and this approach. A buck converter test circuit is constructed that is designed for a PWM controller clocked at 200 kHz and then substituted with a ΔΣ modulator controller clocked at 400 kHz. The RMS noise power of the PWM controller is 14.9 mW compared to the rms noise power for the ΔΣ modulator of 75.85 mW measured in a 2-MHz bandwidth. Although the ΔΣ modulator rms noise power is higher, the noise floor is below the tones seen at the output of the PWM controller. A multibit ΔΣ modulator controller, however, provides a significant reduction in the spectral output of the power supply. Values of 3.75 and 0.24 mW rms noise power are observed at the output of a 2-bit and 4-bit ΔΣ modulator controller, respectively.