Patent ID: 7075802
Filing Date: 2006-07-11
Classification: H02M,Y02B

Abstract:
1. A semiconductor device for controlling a switching power supply, in which a direct-current input voltage is applied to a switching element via a primary winding of a transformer, a direct-current voltage obtained by rectifying and smoothing an alternating current having been generated on a secondary winding of the transformer is controlled by a switching operation of the switching element, and power is supplied to a load, the semiconductor device comprising a control circuit, the control circuit comprising: a transformer reset detection circuit for detecting a reset state of the transformer generated by the switching operation of the switching element from an alternating voltage generated on a tertiary winding of the transformer, and outputting a transformer reset detection signal indicating the reset state; an I-V converter for converting into a voltage a value of a control current indicating a change in the direct-current voltage, the change being made according to the alternating current generated on the secondary winding of the transformer; and a light load detection circuit for outputting a control signal for controlling an intermittent operation of switching performed by the switching element when a light load is detected as a load indicating a magnitude of power supply to the load, the light load being detected according to a change in output voltage from the I-V converter, the light load detection circuit outputting the control signal for controlling the intermittent operation such that the switching operation of the switching element is stopped when the output voltage from the I-V converter is lower than a light load detection lower limit voltage for detecting the light load and such that the switching operation of the switching element is restarted when the output voltage from the I-V converter is higher than a light load detection upper limit voltage for detecting the light load, the control circuit driving a control electrode of the switching element and controlling the intermittent operation in response to the transformer reset detection signal from the transformer reset detection circuit and the control signal from the light load detection circuit, wherein the control circuit comprises a switching-on controller which starts counting, by means of a counter circuit, the transformer reset detection signal from the transformer reset detection circuit at a time when the switching operation is stopped in response to the control signal from the light load detection circuit, and controls timing of switching on at restart of the switching operation according to an order of the counting and the switching operation restarted in response to the control signal from the light load detection circuit, and the switching-on controller turns on switching at the restart of the switching operation at a time when the transformer reset detection signal is outputted from the transformer reset detection circuit after the control signal is outputted, when the control signal indicating the restart of the switching operation is outputted from the light load detection circuit before the counting of the counter circuit.