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Semiconductor device for controlling switching power supply and switching power supply unit using the same

  • US 20060092671A1
  • Filed: 10/26/2005
  • Published: 05/04/2006
  • Est. Priority Date: 11/02/2004
  • Status: Active Grant
First Claim
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1. A semiconductor device (51) for controlling a switching power supply, the semiconductor device being used for a switching power supply unit in which a direct-current input voltage is applied to a switching element (1) via a primary winding (103a) of a transformer (103), a direct-current voltage obtained by rectifying and smoothing an alternating current generated in a secondary winding (103b) of the transformer is controlled by a switching operation of the switching element, and power is supplied to a load, the semiconductor device controlling the switching operation of the switching element, said semiconductor device, comprising:

  • a control circuit having a transformer reset detection circuit (13) for detecting a reset state of the transformer from an alternating voltage generated in a tertiary winding (103c) of the transformer and outputting a transformer reset detection signal indicating the reset state, the reset state being generated by the switching operation of the switching element, an IV converter (29) for converting into a voltage a current value of control current indicating a change of the direct-current voltage based on the alternating current generated in the secondary winding (103b) of the transformer, and a light load detection circuit (32) for outputting, when detecting a light load as a load condition indicating power supply to the load according to a change of an output voltage from the IV converter, a control signal for controlling an intermittent operation of switching performed by the switching element, wherein a control electrode of the switching element is driven and the intermittent operation is controlled based on the transformer reset detection signal from the transformer reset detection circuit and the control signal from the light load detection circuit, the light load detection circuit outputs the control signal for controlling the intermittent operation so as to stop the switching operation of the switching element when the output voltage from the IV converter is lower than a light load detection lower limit voltage for detecting the light load, and restart the switching operation of the switching element when the output voltage from the IV converter is higher than a light load detection upper limit voltage for detecting the light load, the control circuit comprises a transformer reset detection time setting circuit (52) for setting a time of an effective period of the transformer reset detection signal from the transformer reset detection circuit, starting from stop of the switching operation in response to the control signal from the light load detection circuit, and switching on control unit for controlling timing of switching on at restart of the switching operation based on a relationship between the transformer reset detection setting time of the transformer reset detection time setting circuit and timing of the restart of the switching operation in response to the control signal from the light load detection circuit, and the switching on control unit turns on, when the control signal indicating the restart of the switching operation is outputted from the light load detection circuit within the transformer reset detection setting time of the transformer reset detection time setting circuit, switching at the restart of the switching operation in response to the transformer reset detection signal from the transformer reset detection circuit after the control signal is outputted.

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