Power conversion apparatus and over current protection method thereof

A power conversion apparatus, comprising:

• a transformer, having a primary side and a secondary side, wherein a first end of the primary side receives an input voltage, and the secondary side provides an output voltage to an electronic device;
• a power switch, having a first end coupled to a second end of the primary side, a second end coupled to a first node, and a control end receiving a pulse width modulation signal;
• a first resistor, coupled between the first node and a ground potential;
• a feedback unit, coupled to the secondary side, for receiving the output voltage, and accordingly outputting a feedback signal related to a loading status of the electronic device;
• and
• a pulse width modulation controller chip, coupled to the control end of the power switch, the first node and the feedback unit, for generating an over current protection reference signal with variable slope according to the feedback signal and a system operation voltage of the pulse width modulation controller chip, and accordingly comparing a voltage of the first node and the over current protection reference signal with variable slope to determine whether or not to generate the pulse width modulation signal to switch the power switch.