When using a leading edge triggered PFC controller such as the UCC3818 the designer may find that the converter, under extremely light-load conditions, will encounter an audible low noise.The unit under these conditions can go into the Over Voltage Protection (OVP) state because of the extremely light load. When this happens, the output of the current error amplifier can fall to ground because of the input offset voltage variations and the residual current from MOUT. When the unit comes out of the OVP state, the pulse width of the converter is going to start at a maximum resulting in an initial current spike. This can cause the windings of the PFC inductor to physically oscillate resulting in a periodic burst of audible noise. A single resistor from VREF to the positive input of the current error amplifier added to the circuit is sufficient to remove this problem. This problem and a full solution is described in the application report slua309C available at http://www.ti.com/lit/an/slua309c/slua309c.pdf
Posted by Colin Gillmor
