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 (ANR) 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 remains 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 that is sufficiently low not to produce unacceptable audible noise. At very light loads and at no load, the ANR 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 power-factor-correction flyback adapter.