For secondary-resonance ACF, C_O1 is used to adjust the resonant time with L_K to fulfill the ZCS condition, so a large C_CLAMP will not compromise ZCS. Besides, during the on-time of low-side switch (Q_L), the small C_O1 is partially discharged by the load current at the same time. After Q_L turns off and the resonance begins, the discharged C_O1 makes the initial resonance voltage lower than the reflected clamp capacitor voltage across C_CLAMP, which forces more magnetizing current to be delivered to the output, so the conduction loss is reduced with less RMS current flowing through Q_H and the primary winding.