Power conversion circuit capable of adjusting the resonance period to improve the efficiency of low output power and control method thereof

A power conversion circuit, comprising:

• a transformer, comprising a primary coil and a secondary coil, wherein the primary coil is coupled between a switch node and a resonant node;
• a resonant capacitor, coupled between the resonant node and a ground, wherein a primary current flows through the resonant capacitor;
• a high-side transistor, providing an input voltage to the switch node based on a high-side driving signal;
• a low-side transistor, coupling the switch node to the ground based on a low-side driving signal;
• an auxiliary capacitor, coupled between the resonant node and an auxiliary node;
• an auxiliary switch, coupled between the auxiliary node and the ground;
• and
• a control circuit, generating the high-side driving signal and the low-side driving signal;
• wherein when a parasitic diode of the auxiliary switch is turned on, the auxiliary switch is turned on so that the auxiliary capacitor is connected in parallel with the resonant capacitor.

A control method for controlling a power conversion circuit, wherein the power conversion circuit comprises a resonant capacitor coupled between a resonant node and a ground, a transformer comprising a primary coil and a secondary coil, a high-side transistor providing an input voltage to a switch node, and a low-side transistor coupling the switch node to the ground, wherein the primary coil is coupled between the switch node and the resonant node, wherein the control method comprises:

• turning on the high-side transistor to magnetize the transformer and to charge the resonant capacitor;
• turning off the high-side transistor and turning on the low-side transistor so that the secondary coil generates an output voltage;
• and
• connecting an auxiliary capacitor in parallel with the resonant capacitor based on a primary current flowing through the resonant capacitor.