Operating frequency range reduction in a series resonant converter

A problem of the series resonant converter is the generation of audible noise under a light load since the operating frequency must be controlled approximately in inverse proportion to the load if a constant output voltage is desired. To reduce this operating frequency dependency on the load, this paper proposes the addition of an auxiliary resonant circuit to the output circuit of the conventional series resonant converter. The characteristics of the proposed converter have been analyzed and confirmed experimentally. It has the following features: (1) The proposed circuit can maintain constant output voltage by keeping the operating frequency in the range above the auxiliary resonant frequency. Therefore, no audible noise is generated if the auxiliary frequency is selected above the audio frequency range. (2) In the constant voltage control of the proposed circuit, the level of current necessary to maintain output voltage can be lowered by increasing the auxiliary resonant frequency. (3) In the range below the auxiliary resonant frequency, the output voltage increases despite decreasing operating frequency. Therefore, it is necessary to control and not operate the circuit in this range.