Apparatus and method for controlling pulse width modulation switching frequency

1. An apparatus for controlling Pulse Width Modulation (PWM) switching frequency, comprising:

a power factor correction (PFC) converter configured to convert an AC voltage into a DC voltage, the converter comprising a transistor controlling an operation of the PFC converter and a current sensor connected in series with the transistor for measuring an electric current flowing through the transistor; and

a controller configured to receive an output at the center of an on-duty period of a switching signal of a previous cycle from the current sensor and calculate a duty ratio of a switching signal of a next cycle,

wherein the controller applies the calculated duty ratio to the switching signal of the next cycle at the center of in an on-duty period of the switching signal of the next cycle,

wherein the controller:

receives an analog current value measured by the current sensor at the center of the on-duty period of the switching signal of the previous cycle,

converts the analog current value into a digital signal to then calculate a variation of the duty ratio between the switching signal of the previous cycle and the switching signal of the next cycle based on the converted digital signal, and

retards or advances timing of measuring the current by the current sensor based on the variation of the duty ratio such that the retarded or advanced timing of measuring the current becomes the center of an on-duty period of the switching signal of the next cycle.

2. The apparatus of claim 1 ,

wherein the PFC converter comprises a rectifier stage to rectify an AC voltage by using a plurality of diodes; and

a control circuit comprising a first capacitor connected in parallel to the rectifier stage, and an inductor connected in series to one side of the first capacitor, a diode and a second capacitor,

wherein one end of the transistor is connected between the inductor and the diode, and another end of the transistor is connected to the second capacitor.

3. The apparatus of claim 2 , further comprising:

a DC-DC converter controlled by the controller, the DC-DC converter comprising a transformer for stepping up/down an output voltage applied to the second capacitor to output it to a secondary side.

4. The apparatus of claim 1 , wherein a magnitude of a current measured by the current sensor increases in the on-duty ratio period of the switching signal.

5. The apparatus of claim 1 , wherein the controller compares a duty ratio of a first cycle with a duty ratio of a second cycle subsequent to the first cycle of the switching signal, and retards the timing of measuring the current by the current sensor if the duty ratio of the first cycle is smaller than the duty ratio of the second cycle.

6. The apparatus of claim 5 , wherein the controller advances the timing of measuring the current by the current sensor if the duty ratio of the first cycle is greater than the duty ratio of the second cycle.

7. A method for controlling Pulse Width Modulation (PWM) switching frequency, the method comprising:

measuring an electric current flowing through a transistor through a current sensor connected in series with the transistor for controlling an operation of a PFC converter;

receiving data on the measured current at the center of an on-duty period of a switching signal of a previous cycle to calculate a duty ratio of a switching signal of a next cycle based on the data; and

applying the calculated duty ratio to the switching signal of the next cycle at the center of in an on-duty period of the switching signal of the next cycle,

wherein the receiving data on the measured current at the center of an on-duty period of a switching signal of the previous cycle to calculate a duty ratio of the switching signal of the next cycle based on the data comprises:

receiving an analog current value measured by the current sensor in at the center of the on-duty period of the switching signal of the previous cycle;

converting the analog current value into a digital signal to then calculate a variation of the duty ratio between the switching signal of the previous cycle and the switching signal of the next cycle based on the converted digital signal; and \

retarding or advancing timing of measuring the current by the current sensor based on the variation of the duty ratio such that the retarded or advanced timing of measuring the current becomes center of an on-duty period of the switching signal of the next cycle.

8. The method of claim 7 ,

wherein the PFC converter comprises a rectifier stage to rectify an AC voltage by using a plurality of diodes; and

a control circuit comprising a first capacitor connected in parallel to the rectifier stage, and an inductor connected in series to one side of the capacitor, a diode and a second capacitor, and

wherein one end of the transistor is connected between the inductor and the diode, and another end of the transistor is connected to the second capacitor.

9. The method of claim 7 , wherein a magnitude of a current measured by the current sensor increases in the on-duty ratio period of the switching signal.

10. The method of claim 7 , further comprising: comparing a duty ratio of a first cycle with a duty ratio of a second cycle subsequent to the first cycle of the switching signal; and retarding the timing of measuring the current by the current sensor if the duty ratio of the first cycle is smaller than the duty ratio of the second cycle.

11. The method of claim 10 , further comprising: advancing the timing of measuring the current by the current sensor if the duty ratio of the first cycle is greater than the duty ratio of the second cycle.