| 摘要 |
A switching power supply capable of reducing the size as well as noise is provided. A switching element is connected in series with a primary winding of a transformer, a rectifier/smoothing circuit is connected with a secondary winding and a wave-shaping circuit is connected with an auxiliary winding. An operating voltage of a control circuit controlling ON/OFF of the switching element is supplied from a connection node between the primary winding of the transformer and the switching element. The control circuit includes: an output voltage detection circuit that obtains, from a voltage output from the wave-shaping circuit, a feedback voltage in proportion to a voltage output from the rectifier/smoothing circuit and generates a signal for PWM control of the switching element; and a bottom detection circuit that detects, from the voltage output from the wave-shaping circuit, a bottom level of a ringing voltage. The control circuit switches the switching element from OFF to ON in accordance with a signal output from an oscillator internal to the control circuit or a signal output from the bottom detection circuit.
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