| 摘要 |
PURPOSE: A switching power source device is provided to accurately control a output voltage and stable manner by improving the detection accuracy for the output voltage generated from a body diode of a synchronous rectifier connected in serial to a secondary coil when the main switch element is in the OFF time period. CONSTITUTION: A switching power source device(100) comprises a transformer(1), a main switching element(Q1), a secondary rectifying smoothing circuit(20), a detection circuit(30), and a control circuit(10). The transformer includes a primary coil(N1), a secondary coil(N2) and a third coil(N3). The main switching element is connected to the primary coil of the transformer. The secondary rectifying smoothing circuit rectifies and smooths electric power output from the secondary coil of the transformer by the switch on/off operation of the main switching element, and outputs the rectified and smoothed electric power to an external unit. The detection circuit indirectly detects the voltage output from the secondary rectifying smoothing circuit by using the voltage induced in the third coil of the transformer. The control circuit controls the switch on/off operation of the main switching element so as to stabilize the voltage output from the secondary rectifying smoothing circuit based on the detection voltage for the output voltage detected by the detection circuit. The secondary rectifying smoothing circuit includes a first synchronous rectifier(Q2) and a second synchronous rectifier(Q3). The first synchronous rectifier is connected in serial to the secondary coil, and performs an operation same as the switch on/off operation of the main switching element. The second synchronous rectifier is connected in parallel to the secondary coil, and performs an operation opposite to the switch on/off operation of the main switching element. The first and second synchronous rectifiers are arranged to rectify the output power of the secondary coil.
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