# Patent

## Title
Adaptive current limit for power factor correction

## Abstract

In one embodiment, an apparatus for performing power factor correction is provided. A power factor corrector includes an input configured to sense a current from an input circuit. A reference generator generates a current limit based on an input voltage. The current limit reference is dynamically changed based on the input voltage. A control signal generator controls the current in the input circuit based on a comparison of the current and the generated current limit.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
The present application claims priority to U.S. Provisional App. No. 61/169,922 for “Constant Power Limit for Power Factor Correction” filed Apr. 16, 2009, the contents of which is incorporated herein by reference in their entirety.


BACKGROUND
The present disclosure generally relates to power factor correction.
Unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
Power factor is a ratio of real power flowing to a load to apparent power. Power factor may be described as a number between 0 and 1 or expressed as a percentage. It is desirable to have the power factor be closer to 1 or 100%.
Two factors may affect the power factor. A displacement factor is when a current waveform is not in-phase with a voltage waveform. A distortion factor is when the current waveform is not sinusoidal; that is, distortion may be present in the current waveform. Power factor correction may be used to correct these two factors.
FIG. 1 shows two waveforms illustrating effects of the displacement and distortion factors. A voltage waveform 102 and two current waveforms 104a and 104b are shown. The two current waveforms 104a and 104b illustrate the displacement factor and the distortion factor separately.
Current waveform 104a shows the displacement factor. A phase difference 1 exists between voltage waveform 102 and current waveform 104a. Current waveform 104a is thus delayed with respect to voltage waveform 102.
Current waveform 104b shows the distortion factor. Current waveform 104b is in phase with voltage waveform 102; however, current waveform 104b is distorted. For example, total harmonic distortion (THD) is present.
A combination of the displacement factor and distortion factor causes the power factor to be lower. Power factor correction is used to shape the current waveform to make it sinusoidal and in-phase with voltage waveform 102, which raises the power factor.
During power factor correction, it may be desirable to limit the maximum current in an input circuit. If the current is not limited, a system may be damaged. For a given system, input power is given by the root mean square (rms) of the input voltage Vin rms multiplied by the root mean square of the input current Iin rms. The range of the input voltage Vin rms is typically 85V-277V. The maximum current occurs at the minimum input voltage Vin rms of the range for constant input power. FIG. 2 shows a graph 200 of input power vs. input voltage Vin rms for constant input current. As shown, as the input voltage Vin rms increases, the input power increases. At a point 202, the input voltage Vin rms is at its lowest and the current is expected to be at its highest. The maximum current limit is thus set at a percentage above the current that occurs at point 202 because this is expected to be the maximum current over the range of voltages for the input voltage Vin rms. The maximum current limit is constant and does not change. Because the limit is constant, this may lead to high input power over the voltage range when the current limit is reached at every switching cycle of a switched mode power supply.
SUMMARY
In one embodiment, an apparatus for performing power factor correction is provided. A power factor corrector includes an input configured to sense a current from an input circuit. A reference generator generates a current limit based on an input voltage. The current limit reference is dynamically changed based on the input voltage. A control signal generator controls the current in the input circuit based on a comparison of the current and the generated current limit.
In one embodiment, an apparatus is provided comprising: an input configured to sense a current and input voltage from an input circuit; a reference generator configured to generate a current limit based on the input voltage, wherein the current limit is adaptively changed over at least a portion of a cycle of the input voltage; and a control signal generator configured to control the current in the input circuit, the control of the current based on a comparison of the sensed current and the generated current limit.
In one embodiment, the current limit is determined from a sinusoidal current limit profile over the at least the portion of the cycle of the input voltage.
In one embodiment, the sinusoidal current limit profile is based on a peak value of the current limit, the peak value of the current limit based on a peak value of the input voltage.
In one embodiment, the control signal generator comprises a comparator configured to compare the current limit and the sensed current. The control signal generator is configured to output a control signal based on the comparison.
In another embodiment, a method is provided comprising: sensing a current and input voltage from an input circuit; generating a current limit based on the input voltage of the input circuit, wherein the current limit is adaptively changed over at least a portion of a cycle of the input voltage; and controlling the current in the input circuit, the control of the current based on a comparison of the sensed current and the generated current limit.
In one embodiment, the current limit is determined from a sinusoidal current limit profile over the at least the portion of the cycle of the input voltage.
In one embodiment, the sinusoidal current limit profile is based on a peak value of the current limit, the peak value of the current limit based on a peak value of the input voltage.
In one embodiment, the method further comprises: receiving the input voltage; determining a peak value of the input voltage and an instantaneous angle of the input voltage; determining a peak value of a sensed voltage, the sensed voltage being a margin above the peak value of the input voltage; generating the current limit by applying the peak value of the sensed voltage to a sine of the instantaneous angle.
The following detailed description and accompanying drawings provide a better understanding of the nature and advantages of the present invention.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows two waveforms illustrating effects of the displacement and distortion factors.
FIG. 2 shows a graph of input power vs. an input voltage Vin rms for constant input current.
FIG. 3 depicts a simplified system for power factor correction according to one embodiment.
FIG. 4 depicts a more detailed example of the system according to one embodiment.
FIG. 5A depicts a graph showing the sensed voltage Vrsns over the half cycle according to one embodiment.
FIG. 5B depicts a graph showing the current limit profile according to one embodiment.
FIG. 5C depicts a graph showing the instantaneous power obtained by the sinusoidal current limit and the constant limit according to one embodiment.
FIG. 6 depicts a graph of the change in constant and sinusoidal current limits implementations according to one embodiment.
FIG. 7 depicts a graph of the relationship between a peak current limit and the input voltage Vin rms according to one embodiment.
FIG. 8 depicts an example of an adaptive current limiting according to one embodiment.
FIG. 9 depicts a more detailed example of the adaptive current limiting according to one embodiment.
FIG. 10 depicts a graph of two piecewise linear curves that may be used to determine a peak of a sinusoidal current limit according to one embodiment.
FIG. 11 depicts the variation of input power vs. the input voltage Vin rms according to one embodiment.
FIG. 12 depicts a simplified flowchart of a method for controlling the current according to one embodiment.



DETAILED DESCRIPTION
Described herein are techniques for current limiting in power factor correction. In the following description, for purposes of explanation, numerous examples and specific details are set forth in order to provide a thorough understanding of embodiments of the present invention. Particular embodiments as defined by the claims may include some or all of the features in these examples alone or in combination with other features described below, and may further include modifications and equivalents of the features and concepts described herein.
FIG. 3 depicts a simplified system 300 for power factor correction according to one embodiment. System 300 includes an input circuit 302, a load 304, and a power factor corrector 306. A power supply, such as a switch mode power supply, is also coupled to input circuit 302.
Power factor corrector 306 shapes an input current waveform of input circuit 302 such that it is sinusoidal and in phase with a voltage waveform of input circuit 302. In one embodiment, power factor corrector 306 receives a current Isns that is sensed from input circuit 302. For example, the current Isns may be sensed across a resistor of input circuit 302. Also, power factor corrector 306 receives an output voltage Vfp sensed across load 304. The sensed current Isns and the output voltage Vfp are used to shape the input current to be in-phase with an input voltage waveform and sinusoidal. For example, a distorted input current waveform is shaped to be sinusoidal like the voltage waveform. Also, an input current that is out of phase with the voltage waveform is shifted to be in phase with the voltage waveform. A person skilled in the art will appreciate how power factor correction is performed based on the teachings and disclosure herein.
Particular embodiments are directed to limiting a maximum current in input circuit 302 during the power factor correction. In one embodiment, power factor corrector 306 provides over-current protection (OCP). Limiting the maximum current in input circuit 302 protects system 300 from being damaged. A current limit is used to limit the maximum current. Particular embodiments adaptively change the current limit. Adaptively changing the current limit also limits variations in maximum output power limit, which further protects system 300. For example, the variation in current is reduced, which reduces the variation in power consumed.
Power factor corrector 306 uses the sensed current Isns to determine whether to limit the current in input circuit 302. For example, power factor corrector 306 compares the sensed current Isns to a generated current limit. If the current limit is exceeded, power factor corrector 306 outputs a control signal that limits the current in input circuit 302. For example, power factor corrector 306 stops a transistor from turning on for a current pulse width modulation (PWM) cycle. The PWM cycle is the cycle of a signal that turns the transistor on and off. This ensures that the current through input circuit 302 does not exceed a maximum current defined by the current limit.
FIG. 4 depicts a more detailed example of system 300 according to one embodiment. In one embodiment, input circuit 302 includes a flyback converter. Although a flyback converter is described, power factor corrector 306 may be used in other converter topologies. The flyback converter may be used in notebook power supply adapters, such as switched mode power supplies.
Input circuit 302 includes a diode bridge 404 and a capacitor 406. Load 304 may be any load, such as a notebook computer. Load 304 includes inductors 408a and 408b, a capacitor 410, and resistors 412a, 412b, and 412c. A metal-oxide-semiconductor field-effect transistor (MOSFET) 402 and resistor 414 are also included to provide current limiting. A person of skill in the art will understand the operation of input circuit 302 and the flyback converter in accordance with the disclosure and teachings herein.
An input current I, flows through input circuit 302 and can be sensed at resistor 414. The sensed current is referred to as Isns. Also, the output voltage Vfb is sensed in between resistors 412b and 412c and received at power factor corrector 306 through an isolator 416. The output voltage Vfb is used to determine amplitude of a reference current that is compared with the input current. The comparison is used to shape the input current to be sinusoidal with the input voltage waveform in power factor correction.
MOSFET 402 is controlled by power factor corrector 306 to limit current and also output power variation. For example, MOSFET 402 may be turned off if the current limit is reached in input circuit 302. Turning off MOSFET 402 stops current flowing through input circuit 302. Although MOSFET 402 is described, it will be understood that any component may be used to stop current flow in input circuit 302.
Power factor corrector 306 determines the current limit that may be adaptively, for example, changed over a half cycle of the voltage waveform. Although a half cycle is described, other portions of the cycle may be used. The adaptive current limit is compared to the sensed current Isns over the half cycle of the voltage waveform. In one embodiment, if sensed current Isns is greater than the current limit during the half cycle, MOSFET 402 is turned off using a switching signal (SW). This limits the current in system 300. Although turning MOSFET 402 off is described, it will be understood that other devices may be used in limiting current.
In one embodiment, a current limit profile is used to dynamically change the current limit. The current limit profile may be the range of values of the current limit of the half cycle. In one embodiment, the current limit profile is sinusoidal over the half cycle of the voltage waveform. The current limit profile may be determined by calculating a peak value of the input current. A margin above the peak value of the input current is set as the peak value of a sinusoidal current limit profile. The current limit profile is generated in-phase with the input voltage
The current limit profile reduces the average power limit and also the variation in the power limit over the input voltage Vin rms range. An example of the current limit profile, power used, and power variation will be described for a 36 W adaptor. FIG. 5A depicts a graph 500 showing the voltage across resistor 414 over the half cycle according to one embodiment. A waveform 501 shows a half cycle of the voltage across resistor 414. The current limit is adaptively changed sinusoidally based on the peak voltage calculated for each pulse of the voltage.
FIG. 5B shows a graph 502 the current limit profile according to one embodiment. A first waveform 503 shows the peak current profile at 100% load. A second waveform 504 shows a sinusoidal current limit profile Ipk—limit with a 30% margin. The current limit profile Ipk—limit shows the current limit values in the half cycle. Although a 30% margin is described, other margins may be used.
A waveform 506 shows a constant current limit that is conventionally used. For example, the conventional constant current limit takes the peak current from waveform 502 and adds a 30% margin onto the peak current found at a point 508. The conventional current limit that is determined at point 508 is kept constant throughout the half cycle as shown in waveform 506.
The power obtained using the conventional constant current limit profile and the sinusoidal current limit profile is different. FIG. 5C depicts a graph 507 showing the instantaneous power obtained by the sinusoidal current limit and the constant limit according to one embodiment. A waveform 508 shows the instantaneous power (inst_pow_limit_sinu) obtained by the sinusoidal current limit profile shown in waveform 504 of FIG. 5B. A waveform 510 shows the instantaneous power (inst_pow_limit_const) obtained by using the constant limit profile shown by waveform 506. In one example, the average power from the instantaneous power profiles is 60 watts for the sinusoidal current limit profile and 90 watts for the constant profile limit. Average power may be the area under each curve and the sinusoidal current limit profile uses less average power.
Using the sinusoidal current limit profile also reduces the variation in power limit. FIG. 6 depicts a graph of the change in constant and sinusoidal current limits implementations according to one embodiment. A waveform 602 shows the variation in power using the sinusoidal current limit profile. As shown, a variation of 20 watts of power occurs over the range of the input voltage Vin rms. A waveform 604 shows the power used with the conventional constant current limit profile. As shown, the average power varies over 40 watts for the range of the input voltage Vin rms for the conventional constant current limit profile.
Particular embodiments use the peak current for a given rms input voltage Vin rms to determine the current limit profile. In one embodiment, the current limit profile is 130% of a calculated peak current. The peak current varies with respect to the input voltage Vin rms. For example, if the input voltage Vin rms is known, then peak current Ipk can be determined. Then, the current limit Ipk—limit is determined A relationship between Vin rms and the current limit profile can thus be determined from the peak current Ipk. FIG. 7 shows a graph 700 of the relationship between the current limit and the input voltage Vin rms according to one embodiment. A waveform 702 shows the values of a peak value of the current limit profile within the range of the input voltage Vin rms. If the value of Vin rms is known, then the peak value of the current limit profile can be determined.
The above relationship between the peak value of the current limit profile and the input voltage Vin rms may be used by power factor corrector 306 to limit the current in system 300. FIG. 8 shows more detailed example of adaptive current limiting in power factor corrector 306 according to one embodiment. A reference generator 802 receives a value of the input voltage Vin rms. This is the input voltage at input circuit 302. Reference generator 802 can then determine a peak value of the current limit based on the input voltage. A current limit profile is then used to determine a current limit to be used in a comparison with the sensed current Isns. This process will be described in more detail below.
A control signal generator 804 receives the current sensed Isns across resistor 404. Control signal generator 804 may compare the sensed current Isns and the current limit. Based on the comparison, the control signal may turn off MOSFET 402 to limit the current. For example, if the sensed current Isns exceeds the current limit, then the control signal may turn off MOSFET 402. This limits the input current. In one embodiment, the input current may be turned off for the remainder of the pulse width modulation (PWM) cycle. The PWM cycle is the signal that power factor corrector 306 outputs to MOSFET 402 to switch MOSFET 402 on and off. In this case, no more power transfer occurs across load 304. The comparison may be determined at every pulse of the sensed current.
The generation of the current limit sent to control signal generator 804 will now be described in more detail. FIG. 9 depicts a more detailed embodiment of adaptive current limiting in power factor corrector 306 according to one embodiment. Vin computational logic 902 receives the input voltage Vin rms. A one bit signal (N, M) may be sent to a predictive input sine block 904 indicating a digital representation of the input voltage Vin. This moves from processing in the analog domain to the digital domain. Predictive input sine block 904 determines a peak value of input voltage Vin and also an instantaneous angle θ of the input voltage Vin. The instantaneous angle θ and peak value Vpk of the pulse may be used to determine the current limit.
The peak input voltage Vpk is sent to a voltage Vrsns generator 906. Vrsns may be the voltage sensed across resistor 414. The peak input voltage Vpk is used to determine the peak value of voltage Vrsns (peak value). A chip may process voltage values. That is, it is the voltage value that corresponds to the peak value of the current limit profile discussed above.
The value of the voltage Vrsns (peak value) may be determined in different ways. In one embodiment, a look-up table may be used. For example, a look-up table includes the values of the peak input voltage Vpk and corresponding voltages for the voltage Vrsns (peak value). In another embodiment, an equation may be used. For example, FIG. 10 shows a graph 1000 of two piecewise linear curves that may be used to determine the peak of a sinusoidal current limit according to one embodiment. Two piecewise linear curves 1002a and 1002b are used instead of a continuous non-linear curve because the non-linear curve may yield values that are too general. Although two piecewise linear curves are shown, it will be understood that other equations may be used to determine the voltage Vrsns (peak value).
The two piecewise linear curves may be used to approximate the relationship of the voltage Vrsns to the input voltage Vin rms. For example, the value of the voltage Vrsns with respect to the input voltage Vin rms are approximated using piecewise linear curves 1002a and 1002b. Using the equations for piecewise linear curves 1002a and 1002b, the value of the peak input voltage Vpk may be used to determine the value of the voltage Vrsns (peak value).
Referring back to FIG. 9, a current limit generator 908 receives the instantaneous angle θ from predictive input sine logic 904 and the value of the voltage Vrsns (peak value) from Vrsns generator 906. Current limit generator 908 applies the value of the voltage Vrsns (peak value) to a sinusoidal profile to determine the current limit. For example, the angle is used to apply a sinusoidal profile to the value of the voltage Vrsns (peak value) to determine a current limit. The equation Vrsns (peak value)*sin(θ) may be used to determine the value of the current limit. For example, once the voltage profile from the equation is determined, it can be converted to a current profile based on a value of resistor 414, which is the current limit profile. As the voltage Vrsns (peak value) and the instantaneous angle θ vary, the value of the current limit varies.
Current limit generator 908 determines the current limit in the digital domain. A digital-to-analog converter (DAC) 910 receives the current limit and converts it to an analog signal for a comparison.
A comparator 912 receives the current limit and the sensed current Isns. A comparison is performed to determine if the sensed current Isns exceeds the current limit.
Comparator 912 outputs a control signal based on the comparison. For example, the control signal may turn off MOSFET 4-402 if the current limit is exceeded by the sensed current Isns. In one example, if the current limit is exceeded in the PWM cycle, MOSFET 4-402 is turned off immediately for the rest of the PWM cycle thereby limiting further power transfer and protecting system 3-300.
Using the example of adaptive current limiting in power factor corrector 3-306 shown in FIG. 9 and the piecewise linear model of FIG. 10, the variation of the input power is reduced from 20 watts to 6 watts. FIG. 11 shows the variation of input power vs. the input voltage Vin rms according to one embodiment. A first waveform 1102a shows the variation in power when equation 10-1002a is used to determine the voltage Vrsns (peak value). A waveform 1102b shows the input power when the equation corresponding to equation 10-1002b is used. As shown, a variation of 6 watts occurs in the input power using piecewise linear equations 10-1002a and 10-1002b. The power variation is different in FIG. 11 as compared to FIG. 6 because the peak of the sinusoidal current limit is changed based on equations 10-1002a and 10-1002b. In FIG. 6, the current limit is constant for all Vin rms.
A waveform 1104 shows a theoretical power limit. The theoretical power limit is 130% of the input power. This is the theoretical power limit occurs when the current limit is sinusoidal and the peak value of the sinusoidal current limit is changed according to the input voltage Vin rms accurately.
FIG. 12 depicts a simplified flowchart 1200 of a method for controlling the current according to one embodiment. At 1202, the input voltage Vin rms is received. At 1204, a peak input voltage Vpk is determined.
At 1206, the value of the voltage Vsns (peak value) is determined. At 1208, a current limit is generated by applying a sinusoidal profile to the value of the voltage Vsns (peak value). In one embodiment, the current limit may be converted from a digital to analog value.
At 1210, the sensed current Isns is received. At 1212, a comparison of the sensed current Isns and the current limit may then be performed. At 1214, the control signal is generated based on the comparison of sensed current Isns and the generated current limit. The above method is performed over the half cycle of the input voltage.
As used in the description herein and throughout the claims that follow, “a”, “an”, and “the” includes plural references unless the context clearly dictates otherwise. Also, as used in the description herein and throughout the claims that follow, the meaning of “in” includes “in” and “on” unless the context clearly dictates otherwise.
The above description illustrates various embodiments of the present invention along with examples of how aspects of the present invention may be implemented. The above examples and embodiments should not be deemed to be the only embodiments, and are presented to illustrate the flexibility and advantages of the present invention as defined by the following claims. Based on the above disclosure and the following claims, other arrangements, embodiments, implementations and equivalents may be employed without departing from the scope of the invention as defined by the claims.

## Claims

1. An apparatus comprising:
an input configured to sense a current and input voltage from an input circuit;
a reference generator configured to generate a current limit based on the input voltage, wherein the current limit is adaptively changed over at least a portion of a cycle of the input voltage; and
a control signal generator configured to control the current in the input circuit, the control of the current based on a comparison of the sensed current and the generated current limit.

2. The apparatus of claim 1, wherein the current limit is determined from a sinusoidal current limit profile over the at least the portion of the cycle of the input voltage.

3. The apparatus of claim 2, wherein the sinusoidal current limit profile is based on a peak value of the current limit, the peak value of the current limit based on a peak value of the input voltage.

4. The apparatus of claim 3, wherein the reference generator determines the current limit based on an angle of the input voltage applied to the sinusoidal current limit profile.

5. The apparatus of claim 3, wherein a peak value of the sinusoidal current limit profile is a margin above the peak value of the current.

6. The apparatus of claim 5, further comprising a look up table used by the reference generator to determine the peak value of the sinusoidal current limit profile.

7. The apparatus of claim 1, wherein the cycle comprises a half cycle.

8. The apparatus of claim 1, wherein the control signal generator comprises a comparator configured to compare the current limit and the sensed current, wherein the control signal generator is configured to output a control signal based on the comparison.

9. The apparatus of claim 1, wherein the control signal turns off a component in the input circuit to limit current through the input circuit.

10. The apparatus of claim 9, wherein the component comprises a MOSFET.

11. The apparatus of claim 9, wherein the component is turned off during a pulse width modulation cycle of a pulse width modulation signal being input to component.

12. A method comprising:
sensing a current and input voltage from an input circuit;
generating a current limit based on the input voltage of the input circuit, wherein the current limit is adaptively changed over at least a portion of a cycle of the input voltage; and
controlling the current in the input circuit, the control of the current based on a comparison of the sensed current and the generated current limit.

13. The method of claim 12, wherein the current limit is determined from a sinusoidal current limit profile over the at least the portion of the cycle of the input voltage.

14. The method of claim 13 wherein the sinusoidal current limit profile is based on a peak value of the current limit, the peak value of the current limit based on a peak value of the input voltage.

15. The method of claim 14, wherein a peak value of the sinusoidal current limit profile is a margin above the peak value of the current.

16. The method of claim 14, further comprising determining the peak value of the sinusoidal current limit profile based on a look-up table or an equation.

17. The method of claim 13, wherein the current limit is based on an angle of the input voltage applied to the sinusoidal current limit profile.

18. The method of claim 12, further comprising:
comparing the current limit and the sensed current; and
outputting a control signal based on the comparison.

19. The method of claim 12, further comprising:
receiving the input voltage;
determining a peak value of the input voltage and an instantaneous angle of the input voltage;
determining a peak value of a sensed voltage, the sensed voltage being a margin above the peak value of the input voltage;
generating the current limit by applying the peak value of the sensed voltage to a sine of the instantaneous angle.

20. The method of claim 12, wherein the cycle comprises a half cycle.