Switching power supply device with a switching control circuit

1. A switching power supply device comprising:

a power transformer including a primary winding, a secondary winding, and an auxiliary winding;

a switching element which is connected to said primary winding and performs a switching operation to supply and stop supplying a first direct-current voltage to said primary winding;

an output voltage generation circuit which converts, into a second direct-current voltage, an alternating-current voltage induced in said secondary winding through the switching operation of said switching element, and supplies the second direct-current voltage to a load;

a transformer reset detection circuit which monitors a voltage signal of said auxiliary winding and generates a transformer reset signal according to a decrease in the voltage signal;

a secondary-side on-time signal generation circuit which generates a secondary-side on-time signal indicating a secondary-side on-time that is a time period from when said switching element is turned off to when the transformer reset signal is generated;

a feedback control circuit which generates a feedback signal corresponding to a voltage level of the second direct-current voltage;

a switching control circuit which controls the switching operation of said switching element according to the feedback signal so that the second direct-current voltage falls within a normal operation specified range; and

an overvoltage detection circuit which generates an overvoltage detection signal when the secondary-side on-time indicated by the secondary-side on-time signal becomes smaller than a set value,

wherein the set value is smaller than the secondary-side on-time resulting when the second direct-current voltage is controlled to be within the normal operation specified range, and

said switching control circuit controls the switching operation of said switching element so that the second direct-current voltage becomes equal to or below an overvoltage specified value when said overvoltage detection circuit generates the overvoltage detection signal.

2. The switching power supply device according to claim 1 ,

wherein the set value is smaller than a minimum secondary-side on-time resulting when the second direct-current voltage is controlled to be within the normal operation specified range.

3. The switching power supply device according to claim 1 ,

wherein the set value is a constant value.

4. The switching power supply device according to claim 1 ,

wherein said feedback control circuit is connected to said auxiliary winding and generates the feedback signal according to a voltage level of said auxiliary winding,

said feedback control circuit includes a heavy load detection circuit which generates a heavy load detection signal when the voltage level of said auxiliary winding becomes lower than a heavy load detection voltage which is preset,

said overvoltage detection circuit generates the overvoltage detection signal when said heavy load detection circuit generates the heavy load detection signal and the secondary-side on-time indicated by the secondary-side on-time signal becomes smaller than the set value, and

the set value is smaller than a minimum secondary-side on-time resulting when the voltage level of said auxiliary winding is lower than the heavy load detection voltage.

5. The switching power supply device according to claim 1 ,

wherein said feedback control circuit is connected to said auxiliary winding and generates the feedback signal according to a voltage level of said auxiliary winding,

said switching control circuit controls the switching operation of said switching element according to an amount of the load using two or more control methods so as to adjust power supplied to the load, said switching control circuit switching among the two or more control methods according to the feedback signal, and

said overvoltage detection circuit changes the set value for each of the two or more control methods.

6. The switching power supply device according to claim 5 ,

wherein the two or more control methods include:

a pulse width modulation (PWM) control method of controlling, according to the feedback signal, a peak value of a current flowing through said switching element so that the second direct-current voltage falls within the normal operation specified range; and

a pulse frequency modulation (PFM) control method of controlling a switching frequency of said switching element according to the feedback signal so that the second direct-current voltage falls within the normal operation specified range,

said switching control circuit uses the PWM control method when the second direct-current voltage is higher than a first voltage level which is preset, and uses the PFM control method when the second direct-current voltage is lower than the first voltage level, and

said overvoltage detection circuit uses a first value as the set value when the second direct-current voltage is higher than the first voltage level, and uses a second value as the set value when the second direct-current voltage is lower than the first voltage level, the second value being higher than the first value.

7. The switching power supply device according to claim 1 ,

wherein said feedback control circuit is connected to said auxiliary winding and generates the feedback signal according to a voltage level of said auxiliary winding,

said switching control circuit controls, according to the feedback signal, a peak value of a current flowing through said switching element so that the second direct-current voltage falls within the normal operation specified range, and

said overvoltage detection circuit increases the set value when the peak value increases.

8. The switching power supply device according to claim 1 ,

wherein said feedback control circuit decreases a switching frequency of said switching element when said overvoltage detection circuit generates the overvoltage detection signal.

9. The switching power supply device according to claim 1 ,

wherein, when said overvoltage detection circuit generates the overvoltage detection signal, said feedback control circuit decreases a peak value of a current flowing through said switching element.

10. The switching power supply device according to claim 1 ,

wherein, when said overvoltage detection circuit generates the overvoltage detection signal, said feedback control circuit varies the feedback signal in value in a direction in which the feedback signal varies in value when the voltage level of the second direct-current voltage increases, and

when said overvoltage detection circuit generates the overvoltage detection signal, said switching control circuit controls the switching operation of said switching element according to the varied feedback signal so that the second direct-current voltage becomes equal to or below the overvoltage specified value.