Power supply device and driving method thereof

1. A power supply device generating an output power by using an AC line voltage generated through a rectifying of an AC input, comprising:

a transformer including a first coil input with the AC line voltage;

a power switch connected to the first coil;

a feedback circuit generating a feedback voltage corresponding to an output voltage of the output power; and

a switch control circuit receiving a sensing voltage corresponding to a drain current flowing to the power switch and the feedback voltage, generating a threshold voltage by controlling a feedback current every switching cycle of the power switch, and comparing the sensing voltage and the threshold voltage to control the switching operation of the power switch,

wherein the feedback current includes a first current to generate the feedback voltage, and the threshold voltage follows a curved line waveform of which an increasing slope thereof is decreased during the switching cycle.

2. The power supply device of claim 1 , wherein

the switch control circuit includes

a slope compensator maintaining constant impedance during a predetermined initial period from the switching cycle time, and having impedance that is increased after the passage of the initial period, and

the feedback current includes a second current supplied to the slope compensator.

3. The power supply device of claim 2 , wherein

the slope compensator includes:

a first resistor;

a second resistor connected to the first resistor in series;

a capacitor connected to the second resistor; and

a switch connected in parallel to the capacitor,

wherein the second current flows through the first resistor, the second resistor, and the switch during the initial period, and the second current flows through the first resistor, the second resistor, and the capacitor after the initial period.

4. The power supply device of claim 3 , wherein

the switch control circuit further includes

a feedback controller generating the feedback current and connected to the slope compensator and the feedback circuit, and

the feedback controller generates the threshold voltage by using a third current among a remaining current of which the first current and the second current are subtracted from the feedback current.

5. The power supply device of claim 4 , wherein

the feedback controller includes

a first diode including an anode connected to the current copy circuit,

a third resistor connected to a cathode of the first diode, and

a fourth resistor connected to the third resistor in series,

wherein the third current flows to the third resistor and the fourth resistor through the first diode, and the threshold voltage is determined by the third current and the fourth resistor.

6. The power supply device of claim 5 , wherein

the feedback controller includes

a second diode including an anode connected to the current copy circuit and a cathode connected to the feedback circuit, and

a third diode including an anode connected to the current copy circuit and a cathode connected to the slope compensator.

7. The power supply device of claim 6 , wherein

the feedback controller further includes

a fourth diode including an anode connected to the current copy circuit and a cathode connected to the peak resistor, and

the fourth current of the feedback current except for the first to third currents is supplied to the peak resistor.

8. The power supply device of claim 3 , wherein

the switch control circuit controls a switching operation of the power switch by using a clock signal having a predetermined frequency,

the switch is switching-operation according to the clock signal, and the initial period is a first level (a high level) directly before the power switch is turned on by the clock signal.

9. The power supply device of claim 8 , wherein

the switch control circuit controls a turn-off time of the power switch according to a result of comparing the threshold voltage and the sensing voltage, and includes a PWM controller turning on the power switch when the clock signal is changed from the first level to the second level.

10. The power supply device of claim 9 , wherein

the PWM controller includes

a comparator comparing the threshold voltage and the sensing voltage,

an LEB generating the output of the third level during a predetermined blanking period from the turn-on time of the power switch, and

a first logical calculation unit logical-calculating the output of the LEB and the comparator to generate an off control signal controlling a turn-off time of the power switch.

11. The power supply device of claim 10 , wherein

the first logical calculation unit generates the off control signal of a level that does not turn off the power switch by the third level output of the LEB during the blanking period.

12. The power supply device of claim 11 , wherein

the PWM controller includes

an oscillator generating a clock signal,

an SR flip-flop generating a duty control signal according to the clock signal and the off control signal, and

a second logical calculation unit generating a gate control signal according to the clock signal and the output of the SR flip-flop,

wherein the SR flip-flop changes the level of the duty control signal into a fourth level when the level of the clock signal is changed from the second level to the first level after the turn-off time of the power switch,

the first logical calculation unit changes the level of the off control signal into a fifth level in synchronization with a time at which the sensing voltage reaches the threshold voltage, and

the second logical calculation unit generates a gate control signal turning off the power switch according to an off control signal of the fifth level, and generates the gate control signal turning on the power switch when the level of the clock signal is changed from the first level to the second level after the generation of the duty control signal of the fourth level.

13. A driving method of a power supply device including a feedback circuit generating a feedback voltage corresponding to an output voltage and a power switch controlling the output voltage, comprising:

generating a feedback current;

supplying a first current among the feedback current to a feedback circuit to generate a feedback voltage corresponding to the output voltage;

forming a path through which a second current flows among the feedback current and increasing impedance of the path among a switching cycle of the power switch; and

generating a threshold voltage to control a turn-off time of the power switch by using a third current among the feedback current.

14. The driving method of claim 13 , further comprising

constantly maintaining the impedance of the path during a predetermined initial period from the switching cycle time of the power switch.

15. The driving method of claim 13 , wherein

the path includes a resistor and a capacitor, and

the increasing of the impedance of the path includes

charging the capacitor by the second current and

decreasing the second current by the charging of the capacitor.

16. The driving method of claim 15 , wherein

the second current is decreased with an exponential function according to a time constant determined by the resistor and the capacitor.

17. The driving method of claim 13 , further comprising

comparing the threshold voltage and a sensing voltage corresponding to a current flowing to the power switch to determine a turn-off time of the power switch.

18. The driving method of claim 17 , wherein

the second current is decreased with a shape in which a decreasing slope is decreased, and the third current is decreased with a shape in which an increasing slope is decreased.