Overcurrent protection circuit for switching power supply

An overcurrent protection circuit for a switching power supply, comprising:

• a transformer having a primary side comprising a primary winding and a secondary side comprising multiple secondary windings, the primary winding being provided with a DC current;
• a power switching element to switch on and off the DC current in the primary winding for inducing multiple currents in the multiple secondary windings, respectively, the currents being rectified and smoothed to provide multiple DC voltages, respectively, at the secondary side, thereby forming multiple voltage supply sections to output the multiple DC voltages at the secondary side;
• current detection resistors for detecting currents flowing in the primary side and the secondary side, respectively;
• a first switching element to be turned on when a voltage between both ends of the current detection resistor in the secondary side exceeds a given voltage value;
• a second switching element to be turned on when the first switching element is turned on;
• a first photocoupler to be turned on and off depending on state of the second switching element;
• a voltage detection resistor to detect a decrease of the voltage at the secondary side below a given voltage value;
• a third switching element to be varied in its on and off state in response to a voltage detected by the voltage detection resistor;
• a second photocoupler to be turned on and off depending on state of the third switching element;
• and
• a switching control circuit to control on-duty of the power switching element in response to turning on and off of the first and the second photocouplers and in response also to a current value detected at the primary side,
• wherein the current detection resistor in the secondary side is provided only in a line of one of the multiple secondary windings which, among the multiple voltage supply sections, belongs to a lower output voltage supply section for individually limiting the current flowing through the secondary winding within a maximum allowable current value.