Method of power factor correction

Abstract

A method of power factor correction is disclosed. The method uses the slope of the voltage waveform to determine the phase angle of the voltage. Based on the phase angle, a current waveform is generated that is in phase with the voltage. The slope of the voltage signal is calculated as the derivative of voltage with respect to time. The resultant current signal is in ratio with the voltage signal. Additionally, the current signal has zero or near zero phase displacement with respect to the voltage signal. Repeatedly performing the steps of the method allows a continuous current signal to be provided. As load characteristics change, the method quickly adapts the power to compensate. As a result, reliable and effective power factor correction is achieved.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. In the drawings,

FIG. 1A is a graph illustrating in-phase voltage and current signals in a resistive circuit;

FIG. 1B is a graph illustrating out-of-phase voltage and current signals in a varying load circuit;

FIG. 1C is a diagram illustrating a current waveform of a conventional average current control method;

FIG. 1D is a diagram illustrating a current waveform of a conventional discontinuous current PWM control method;

FIG. 2 is a flowchart illustrating a method of power factor correction according to an embodiment of the present invention;

FIGS. 3A-3C are graphs illustrating determining the slope of a non-linear signal;

FIG. 4A is a graph illustrating a voltage signal over time; and

FIG. 4B is a graph illustrating a resultant current signal obtained by the method of power factor correction according to an embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts.

Refer to FIG. 2, which is a flowchart illustrating a method of power factor correction according to an embodiment of the present invention.

As shown in FIG. 2, the method 200 begins by using the slope of the voltage waveform to determine the phase angle of the voltage in Step 210. From this phase angle of the voltage, a current waveform is generated that is in phase with the voltage in Step 220. The in-phase current waveform is then provided to the rest of the system in Step 230.

By repeating the steps above, any change in load characteristics will immediately be compensated for by the present invention in order to provide a sinusoidal current signal with zero phase displacement.

Refer to FIGS. 3A-3C, which are graphs illustrating determining the slope of a non-linear signal.

In an embodiment of the present invention the slope is determined by the following method. The derivative of a function f at x is the slope of the tangent line to the graph off at x. Since only one point is know from the sample, the tangent line is approximated with multiple secant lines. The secant lines have a progressively shorter distance between the intersecting points. The slope of the tangent line is obtained by taking the limit of the slopes of the nearby secant lines. The derivative is determined by taking the limit of the slope of secant lines as they approach the tangent line. The derivative of f at x is the limit of the value of the difference quotient as the secant lines get closer and closer to be a tangent line.

The slope of the line through the points (x, f(x)) and (x+h, f(x+h)) shown in FIG. 3C is (f/(x+h)−f(x))/h.

Therefore, the slope of the voltage sample can be determined by the equation dvldt or the derivative of voltage over the derivative of time or in other words, the derivative of voltage with respect to time.

Refer to FIG. 4A which is a graph illustrating a voltage signal over time and to FIG. 4B, which is a graph illustrating a resultant current signal obtained by the method of power factor correction according to an embodiment of the present invention.

As shown in FIGS. 4A and 4B, the current signal is in-phase and in ratio to the voltage signal. The number of voltage samples can be predetermined according to requirements. In situations where high accuracy is required, more samples can be taken.

In another embodiment of the present invention the slope of the voltage signal is obtained by taking two voltage samples. Next the slope of the voltage signal is calculated as voltage at second sample minus voltage at first sample divided by time at second sample minus time at first sample.

The accuracy of the method in this embodiment is mainly dependant on the frequency of samples taken. The more frequent the samples, the more accurate the resultant current signal is.

It will be apparent to those skilled in the art that various modifications and variations can be made to the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the invention and its equivalent.

Claims

1. A method of power factor correction comprising the steps of: using a slope of a voltage waveform to determine a phase angle of the voltage; andgenerating a current waveform that is in phase with the voltage based on the phase angle of the voltage.

2. The method of power factor correction of claim 1, further comprising providing the current waveform to a load.

3. The method of power factor correction of claim 1, where the slope of the voltage waveform is calculated as the derivative of voltage with respect to time.

4. The method of power factor correction of claim 1, where the slope of the voltage waveform is calculated after taking two samples as voltage at second sample minus voltage at first sample divided by time at second sample minus time at first sample.

5. The method of power factor correction of claim 1, where the method is incorporated into an integrated circuit.

6. The method of power factor correction of claim 1, further comprising repeating the steps in order to create a continuous current signal.

7. The method of power factor correction of claim 1, where phase difference between the voltage waveform and the current waveform is zero.

8. A method of power factor correction comprising the steps of: determining a slope of a voltage signal;using the slope of the voltage signal to determine a phase angle of the voltage signal; andgenerating a current signal that is in phase with the voltage signal based on the phase angle of the voltage signal.

9. The method of power factor correction of claim 8, where the slope of the voltage signal is calculated as the derivative of voltage with respect to time.

10. The method of power factor correction of claim 8, where the slope of the voltage signal is calculated after taking two samples as voltage at second sample minus voltage at first sample divided by time at second sample minus time at first sample.

11. The method of power factor correction of claim 8, further comprising repeating the steps in order to create a continuous current signal.

12. The method of power factor correction of claim 8, where the method is incorporated into an integrated circuit.

13. The method of power factor correction of claim 8, where phase difference between the voltage signal and the current signal is zero.

14. A method of power factor correction comprising the steps of: determining a slope of a voltage waveform;determining a phase angle of the voltage waveform based on the slope;generating a current waveform in phase with the voltage waveform based on the phase angle; andrepeating the steps in order to provide a continuous current signal.

15. The method of power factor correction of claim 14, where the slope of the voltage waveform is calculated as the derivative of voltage with respect to time.

16. The method of power factor correction of claim 14, where the slope of the voltage waveform is calculated after taking two samples as voltage at second sample minus voltage at first sample divided by time at second sample minus time at first sample.

17. The method of power factor correction of claim 14, further comprising providing the current waveform to a load.

18. The method of power factor correction of claim 14, where the method is incorporated into an integrated circuit.

19. The method of power factor correction of claim 14, where phase angle between the voltage signal and the current signal is zero.