Switching power supply device

1. A switching power supply device, comprising:

a transformer including:

a primary winding to one end of which an input voltage is applied;

a secondary winding; and

an auxiliary winding provided on a primary-winding side;

a switching element provided in a path of the primary winding;

an output capacitor having a potential at one end thereof fixed;

a rectifying element provided between another end of the output capacitor and one end of the auxiliary winding, with a cathode of the rectifying element on an output-capacitor side;

a control circuit controlling the switching element between ON and OFF, the control circuit including:

a power terminal connected to the other end of the output capacitor;

a HIGH-voltage terminal connected to an input side of the input voltage; and

a start-up circuit provided between the power terminal and the HIGH-voltage terminal, the start-up circuit passing a first current for charging the output capacitor via the power terminal; and

a current supplementing circuit receiving the input voltage and connected to the HIGH-voltage terminal, the current supplementing circuit passing a second current to the output-capacitor side based on the first current,

wherein a load is connectable to the other end of the output capacitor, and

wherein the current supplementing circuit includes:

a resistor element to one end of which the input voltage is applied and to another end of which the HIGH-voltage terminal is connected; and

a transistor of which a control end is connected to the another end of the resistor element, the transistor having the input voltage applied to a current-in end thereof, the transistor having a current-out end thereof connected to the other end of the output capacitor, and

wherein when, as a result of the output capacitor being charged, a voltage at the power terminal exceeds a first threshold voltage, the start-up circuit shuts off the first current, and

in response to the first current being shut off, the current supplementing circuit makes the second current substantially zero.

2. The switching power supply device of claim 1 , wherein

the transistor is a PNP transistor of which

a base is the control end,

an emitter is the current-in end, and

a collector is the current-out end.

3. The switching power supply device of claim 1 , wherein the start-up circuit includes:

a charge transistor provided between the HIGH-voltage terminal and the power terminal;

a first comparison circuit comparing the voltage at the power terminal with the first threshold voltage; and

a switch provided in a path of the charge transistor, the switch being switched according to a result of comparison by the first comparison circuit.

4. The switching power supply device of claim 3 , wherein the start-up circuit further includes:

a second comparison circuit comparing the voltage at the power terminal with a second threshold voltage lower than the first threshold voltage; and

a current source connected in parallel with the switch, the current source limiting a current, and

the switch and the current source are switched between ON and OFF according to results of comparison by the first and second comparison circuits.

5. The switching power supply device of claim 3 , wherein the charge transistor is an N-channel MOSFET biased so as to be normally ON.

6. An electronic appliance, comprising:

the switching power supply device of claim 1 ; and

wherein the load is connected to the other end of the output capacitor in the switching power supply device.

7. The electronic appliance of claim 6 , wherein

the switching power supply device is an AC/DC converter,

the electronic appliance is an air conditioner further including:

a motor; and

a motor driving circuit receiving an alternating-current voltage to drive and control the motor, and

the load is a control circuit that controls the motor driving circuit.

8. The electronic appliance of claim 6 , wherein the electronic appliance is an LED lighting device further including:

an LED load connected on a secondary side of the transformer in the switching power supply device,

the control circuit of the switching power supply device has a function of dimming the LED load, and

the load is a control circuit that feeds the control circuit with a dimming signal.