Methods of reducing audible noise caused by magnetic components in variable-frequency-controlled switch-mode converters

Methods and a circuit implementation of audible noise reduction in switch-mode converters with variable switching frequency are presented. The audible noise caused by magnetic components is reduced by controlling the switching frequency so that it stays above the audible range as the load decreases. This is accomplished by decreasing the peak value of the main switch current pulses in discrete steps until the peak value of the main switch current pulses is decreased to a level which is sufficiently low not to produce unacceptable audible noise. At very light loads and at no load, the audible noise reduction circuit can be disabled in order to meet various worldwide standards that limit the maximum input power. The performance of the proposed circuit was experimentally verified in a 90-W single-stage PFC flyback adapter.