| 摘要 |
When a main switching transistor (Q1) of a zero voltage switching power supply is conductive, a voltage (VR7) is developed in a current sensing resistor (R7) coupled in series with the transistor. The voltage in the current sensing resistor is coupled to a first input (base) of a comparator (Q3) of the control circuit (Q2, Q3). A second input (emitter) of the comparator is coupled to a capacitor (C6) that develops a voltage (VC6) that varies in accordance with an output voltage (IDOUT2) of the power supply. During a given conduction interval of the transistor, the comparator (Q3) is triggered in accordance with the difference between the current sensing resistor voltage and the capacitor voltage. An output of the comparator (Q3) is coupled to the base of the transistor for controlling the turn off instant of the transistor on a current pulse-by- current pulse basis. Under overload condition, when the transistor is turned off, a reverse collector current flows through the base-collector junction of the transistor. The reverse collector current produces a voltage (negative) in the current sensing resistor that is at opposite polarity with respect to the voltage there, during forward conduction of the transistor. The voltage produced by the sensed reverse current turns on a diode (D10) switch and charges the capacitor that is coupled to the comparator of the control circuit in a manner to reduce the peak forward collector current. Thereby, the peak forward current in the transistor decreases relative to a value that would have occurred without the protection. |
