A method and circuit for controlling a resonant DC/DC converter which adjusts an output voltage by changing a turn-on frequency of input switch devices of a resonant circuit of the converter. The method extends the range of the output voltage of the resonant circuit by adjusting the duty ratio of the switch devices based on the feedback signal of the load circuit. The method and circuit uses two modes to control resonance of the DC/DC converter-frequency modulation and frequency modulation plus pulse width modulation. Frequency modulation is used when the operating frequency of the power supply is low. Frequency modulation plus pulse width modulation is used when the operating frequency of the power supply is too high.