Semiconductor device for controlling power supply

A semiconductor device for controlling a power supply, wherein the semiconductor device generates and outputs a driving pulse to control on and off of a switching element so that an electric current flows intermittently in a primary side winding of a voltage converting transformer, and wherein the semiconductor device generates the driving pulse according to a voltage in proportion with the electric current flowing in the primary side winding of the transformer and an output detecting signal from a secondary side of the transformer, the semiconductor device comprising:

• a clock generating circuit which is provided with an oscillating circuit in which a frequency can be changed and which generates a clock signal to provide a timing to periodically turn on the switching element;
• a control circuit which provides a timing to turn off the switching element based on the voltage in proportion with the electric current flowing in the primary side winding of the transformer and the output detecting signal from the secondary side of the transformer;
• a setting terminal having a voltage set from outside the semiconductor device;
• a switch provided between a first power supply terminal to which an AC input voltage is supplied and a second power supply terminal to which a voltage induced to an auxiliary winding of the transformer is supplied;
• and
• an internal power supply voltage control circuit which controls on and off of the switch,
• wherein, when the voltage of the setting terminal is lower than a first voltage value set in advance, the device advances to a first stop mode in which the output of the driving pulse is stopped and the internal power supply voltage control circuit controls the on and off of the switch so that the voltage of the second power supply terminal is within a predetermined voltage range, and when the voltage of the setting terminal is higher than the first voltage value, under a condition that the output detecting signal from the secondary side is lower than a threshold which is the voltage of the setting terminal, the device advances to a second stop mode in which the output of the driving pulse is stopped.