Audible Noise Reduction in Switched-Mode Power Supplies Operating in Burst Mode

Abstract

To improve the conversion efficiency at light load, burst mode (BM) control is widely employed by power converters. One drawback of hysteresis burst mode control is that the burst frequency changes with different loads, which would introduce audible noise to the system if burst frequency falls into audible frequency range. 2kHz to 4kHz would be the most sensitive audible frequency. The audible noise can be further reduced by decreasing the peak value of current pulses and proportionally increasing the burst frequency. In this paper, the new BM for reducing audible noise requires three thresholds to control the BM period at certain levels of light load. As the BM threshold represents the drain to source current, it delivers more or less power to the load by changing the threshold. This will generate higher output voltage ripple when more power is delivered and less output voltage ripple with less power delivery. By counting the BM period, the audible noise can be diminished by selecting the BM threshold. The presented BM audible noise reduction analyses are verified on flyback simulation circuit.