Pwm controller with output current limitation

A switch power supply, comprising:

• a transformer having a primary side and a secondary side for converting the power received by the primary side and outputting the power to the secondary side;
• a power switch having a first terminal, a second terminal, and a controlled terminal, wherein the first terminal is coupled with the primary side of the transformer, and the second terminal is coupled with a grounding;
• a first detecting circuit coupled between the power switch and the grounding for generating a first detecting signal;
• a second detecting circuit coupled with the secondary side of the transformer for generating a second detecting signal;
• a compensating device coupled between the first detecting circuit and the second detecting circuit for compensating the second detecting signal with the first detecting signal;
• and
• a controller having a control terminal, a first detecting terminal, and a second detecting terminal, wherein the control terminal is coupled with the controlled terminal of the power switch, the first detecting terminal is coupled with the first detecting circuit for receiving the first detecting signal, the second detecting terminal is coupled with the second detecting circuit for receiving the compensated second detecting signal, and the controller performs a protecting operation according to the signal received by the compensated second detecting terminal.