| 摘要 |
PURPOSE:To obtain a switching power supply having an overcurrent protecting function in addition to the stabilizing function of output voltage, by detecting the overcurrent caused at the load side of a switching power supply and setting the reference voltage of an error detecting circuit at virtually zero by the detection signal of the overcurrent. CONSTITUTION:A current detecting resistance RSC is inserted to the output side of a rectifying/smoothing circuit which is connected to a secondary winding N2 of a transformer 11. The voltage value caused by the current flowing to the resistance RSC is supplied to a comparator B to be compared with the reference voltage Vrf2. If the current increases by some reason and the voltage caused to the resistance RSC exceeds the voltage Vrf2, a circuit 13 produces an output signal to make a transistor TRQ3 conduct. With the conduction of the TRQ3, a resistance Rf of an error detecting circuit 12 is short-circuited to set the reference voltage Vrf at zero. Thus control TRQ1 and Q2 are cut off to set the control current of a control winding NC at zero. As a result, the output voltage of a switching power supply is substantially set at zero to ensure a protective function to the overcurrent. When the output current is reset at the value less than the rated level, the voltage Vrf1 rises gradually by the time constant of a capacitor CA and a resistance RA to give the limitation to the rush current supplied from a capacitor C1. |
