Series-resonant converter with reduced-frequency-range control

In this paper, a control method that improves performance of series-resonant converters that operate with a wide input-voltage and/or output-voltage range by substantially reducing their switching-frequency range. The switching-frequency-range reduction is achieved by controlling the output voltage with a combination of variable-frequency and delay-time control. Variable-frequency control is employed to control the primary switches, while delay-time control is used to control secondary-side rectifier switches provided in place of diode rectifiers. The performance evaluation of the proposed control was done on a 690-W prototype operating in a 200-450-V input-voltage range and delivering a 13.8-V output voltage. The prototype circuit exhibits the maximum full-load efficiency of 95.8% with a switching frequency variation from 48 kHz to 72 kHz over the entire input-voltage range.