Abstract:
An AC/DC power conversion apparatus comprises an AC/DC converter for converting AC power to DC power for a load and a controller that maintains a power factor of the load as the load varies. The AC/DC converter includes an inductor and a plurality of switches that alternately connects and disconnects the inductor to and from an AC power source, to generate the DC power for the load. The plurality of switches is controlled by a plurality of switch drive signals generated by the controller, based on comparisons of an AC voltage from the AC power source to a DC output voltage produced by the AC/DC converter. To maintain the power factor of the load, the controller is configured to adjust the frequency of the plurality of switch drive signals in response to variations in the load while holding the duty cycles of the switch drive signals constant.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation-in-part of U.S. patent application Ser. No. 12/841,608, filed on Jul. 22, 2010, the disclosure of which is incorporated herein by reference in its entirety and for all purposes. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates in general to power conversion and in particular to methods and apparatus for converting alternating current (AC) to direct current (DC). 
     BACKGROUND OF THE INVENTION 
     Many household and industrial machines and devices are powered by a direct current (DC) power source that has been rectified from alternating current (AC) power provided by the AC mains. The AC-to-DC rectification is typically accomplished using a bridge rectifier  104  (or “diode bridge”) comprised of four diodes  102 - 1 ,  102 - 2 ,  102 - 3   102 - 4  configured as shown in  FIG. 1 . The bridge rectifier  104  converts the positive and negative half cycles of the AC input voltage Vin to a full-wave-rectified waveform of constant polarity. (See  FIGS. 2A and 2B ). To produce the desired steady DC output voltage Vout across a load  108 , the rectified waveform is filtered by a smoothing circuit, which in its simplest form comprises a smoothing capacitor  106  coupled to the output of the bridge rectifier  104 . The smoothing capacitor  106  functions to maintain the DC output voltage Vout near the peak voltage Vpeak during the low portions of the AC input voltage Vin, as shown in  FIG. 2C . Some amount of AC ripple is superimposed on the DC output Vout, even following filtering by the smoothing capacitor  106 . The ripple may or may not be tolerable, depending on the application. In applications where it is not tolerable, additional filtering can be employed to reduce it to an acceptable level. 
     The AC/DC converter  100  in  FIG. 1  generates a DC output voltage Vout near the peak voltage Vpeak of the AC input voltage Vin (see  FIG. 2C ). However, many applications require a much lower voltage. For example, many machines and devices require a DC voltage of 12 volts DC or less but the peak voltage Vpeak of the center-tapped 120 volts RMS (root mean square) residential mains is near 170 V. To lower the DC voltage to the required level, a step-down transformer or DC-DC converter  302  (i.e., “buck converter”) is used.  FIG. 3  illustrates use of a DC-DC converter  302 . The DC-DC converter  302  comprises a switch (typically a metal-oxide-semiconductor field effect transistor (MOSFET))  304 , a diode (or, alternatively, a second MOSFET)  306 , an inductor  308 , a filter capacitor  310 , and a pulse-width modulator (PWM) control  312 . The PWM control  312  controls the opening and closing of the switch  304  at a fixed frequency f that is much higher than the 60 Hz line frequency (typically greater than 1 kHz). When the switch  304  is turned on, current flows through it, the inductor  308 , and then into the filter capacitor  310  and the load  108 . The increasing current causes the magnetic field of the inductor  308  to build up and energy to be stored in the inductor&#39;s magnetic field. When the switch  304  is turned off, the voltage drop across the inductor  308  quickly reverses polarity and the energy stored by the inductor  308  is used as a current source for the load  108 . The DC output voltage Vout is determined by the proportion of time the switch  304  is on (t ON ) in each period T, where T=1/f. More specifically, Vout=DVin(dc), where D=t ON /T is known as the “duty cycle” and Vin(dc) is the source DC input voltage provided at the output of the bridge rectifier  104 . The PWM control  312  is configured in a feedback path, allowing it to regulate the DC output voltage Vout by modulating the duty cycle D. 
     Although the AC/DC converter  300  in  FIG. 3  addresses the inability of the AC/DC converter  100  in  FIG. 1  to step down the DC voltage to a lower DC voltage, it does not address another well-known problem of conventional AC/DC converters—low power factor. The power factor of an AC/DC converter is a dimensionless number between 0 and 1 indicating how effectively real power from an AC power source is transferred to a load. An AC/DC converter with a low power factor draws more current from the mains than one having a high power factor for the same amount of useful power transferred. A low power factor can result due to the input voltage Vin being out of phase with the input current Iin or by action of a nonlinear load distorting the shape of the input current Iin. The latter situation arises in non-power-factor-corrected AC/DC converters, such as those described in  FIGS. 1 and 3 , which as described above use a diode bridge  104 . The filter capacitor  106  of the AC/DC converter  100  in  FIG. 1  (and, similarly, the filter capacitor  310  of the AC/DC converter  300  in  FIG. 3 ) remains charged near the peak voltage Vpeak for most of the time. This means that the instantaneous AC line voltage Vin is below the filter capacitor  106  voltage for most of the time. The diodes  102 - 1 ,  102 - 2 ,  102 - 3   102 - 4  of the bridge rectifier  104  therefore conduct only for a small portion of each AC half-cycle, resulting in the input current Iin drawn from the mains being a series of narrow pulses, as illustrated in  FIG. 4 . Note that although the input current Iin is in phase with the AC input voltage Vin, it is distorted and, therefore, rich in harmonics of the line frequency. The harmonics lower the power factor, resulting in reduced conversion efficiency and undesirable heating in the AC mains generator and distribution systems. The harmonics also create noise that can interfere with the performance of other electronic equipment. 
     To reduce harmonics and increase the power factor, conventional AC/DC converters are often equipped with a power factor correction (PFC) pre-regulator. The PFC pre-regulator can be formed in various ways. One approach employs a PFC boost converter  502  coupled between the bridge rectifier  104  and the DC-DC converter  302 , as shown in the power-factor-corrected AC/DC converter  500  in  FIG. 5 . The PFC boost converter  502  comprises an inductor  504 , switch  506 , diode  508 , output capacitor  510  and a PFC control  512 . The PFC control  512  controls the on and off state of the switch  506 . When the switch  506  is switched on, current from the mains flows through the inductor  504 , causing energy to build up and be stored in the inductor&#39;s magnetic field. During this time, current to the DC-DC converter  302  and load  108  is supplied by the charge in the capacitor  510 . When the switch  506  is turned off, the voltage across the inductor  504  quickly reverses polarity to oppose any drop in current, and current flows through the inductor  504 , the diode  508  and to the DC-DC converter  302 , recharging the capacitor  510  as well. With the polarity reversed, the voltage across the inductor  504  adds to the source input DC voltage, thereby boosting the input DC voltage. The PFC boost converter  502  output voltage is dependent on the duty cycle D of the on-off switch control signal provided by the PFC control circuit  512 . More specifically, the PFC boost converter  502  output voltage is proportional to 1/(1−D), where D is the duty cycle and (1−D) is the proportion of the switching cycle T (i.e., commutation period) that switch  506  is off. In addition to setting the duty cycle D, the PFC control  512  forces the DC-DC converter  302  and load  108  to draw current that on average follows the sinusoidal shape of the AC input voltage Vin, thereby reducing harmonics and increasing the power factor of the AC/DC converter  500 . 
     The power-factor-corrected AC/DC converter  500  is suitable for many applications. However, it has a number of drawbacks. First, the AC/DC converter is less efficient than desired, particularly since the AC-to-DC power conversion requires two stages—the PFC boost converter  502  front end and the DC-DC converter  302  final stage. Second, the converter  500  has a large parts count, including parts necessary to implement the two control circuits (PFC control  512  and PWM control  312 ), which increases design complexity and cost, and makes the converter  500  more susceptible to failure. Third, the PFC boost converter  502  generates very high voltages, which stress the converter&#39;s parts and raise safety concerns. 
     It would be desirable, therefore, to have AC/DC conversion methods and apparatus that are efficient at converting AC to DC, avoid power factor degradation attributable to using a bridge rectifier, do not require voltage boosters to counteract power factor degradation, and do not have a large parts count. 
     SUMMARY OF THE INVENTION 
     Methods and apparatus for converting alternating current (AC) to direct current (DC) are disclosed. An exemplary AC/DC converter that converts an AC input voltage Vin, such as may be provided by the AC mains, to a DC output voltage comprises an inductor, a capacitor, a plurality of switches, and a controller. The controller configures the plurality of switches, inductor, and capacitor to operate as a buck converter during times when Vin&gt;Vout and to operate as an inverting buck converter during times when Vin&lt;−Vout. 
     In one embodiment of the invention, the controller modulates the duty cycles of the plurality of switches to regulate the DC output voltage Vout to the desired, constant output level. In another embodiment of the invention, the duty cycles of the switches are held constant but their frequency is changed in response to variations in the load. On average, the input current to the AC/DC converter is inversely proportional to the frequency of the switch drive signals. Therefore, by holding the duty cycles of the switch drive signals constant and adjusting their frequency as the load varies, the input current is forced to adapt to changes in the load and the power factor is maintained, as a result. 
     The AC/DC converter of the present invention converts the AC input voltage Vin to the DC output voltage Vout directly, i.e., without the need for a bridge rectifier or transformer to complete the AC-to-DC conversion. Direct AC to DC conversion avoids power factor degradation problems attributable to use of bridge rectifiers, obviates the need for specialized power factor correction pre-regulator circuitry, and results in a low parts count and an energy-efficient design. 
     Further features and advantages of the invention, including descriptions of the structure and operation of the above-summarized and other exemplary embodiments of the invention, will now be described in detail with respect to accompanying drawings, in which like reference numbers are used to indicate identical or functionally similar elements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram of a conventional alternating current to direct current (AC/DC) converter; 
         FIG. 2A  is a signal diagram of the AC input voltage Vin applied to the AC input of the AC/DC converter in  FIG. 1 ; 
         FIG. 2B  is a signal diagram of the unfiltered, full-wave-rectified voltage waveform produced at the output of the bridge rectifier of the AC/DC converter in  FIG. 1 ; 
         FIG. 2C  is a signal diagram of the DC output voltage of the AC/DC converter in  FIG. 1  after having been filtered by a smoothing capacitor; 
         FIG. 3  is a circuit diagram of an AC/DC converter equipped with a step-down buck converter to step down the DC output voltage to a level lower than possible using just a bridge rectifier and smoothing capacitor; 
         FIG. 4  is a signal diagram illustrating how the bridge rectifier used by the AC/DC converters in  FIGS. 1 and 3  causes current to be drawn from the AC power source in narrow pulses that are rich in harmonics; 
         FIG. 5  is a circuit diagram of an AC/DC converter having a step-down buck converter and a power-factor-correcting boost converter that compensates for power factor degradation caused by the AC/DC converter&#39;s bridge rectifier; 
         FIG. 6  is a circuit diagram of an AC/DC converter, according to an embodiment of the present invention; 
         FIG. 7  is a signal diagram of the AC input voltage Vin supplied to the AC/DC converter in  FIG. 6  and its relationship to the DC output voltage Vout generated by the AC/DC converter and its inverse −Vout; 
         FIG. 8  is a table showing how the switches of the AC/DC converter in  FIG. 6  are switched and driven, depending on the instantaneous value of the AC input voltage Vin compared to the DC output voltage Vout generated by the AC/DC converter in  FIG. 6  and its inverse −Vout; 
         FIG. 9  is a circuit diagram illustrating how the AC/DC converter in  FIG. 6  reduces to and operates as a buck converter during times of positive half cycles of the AC input voltage when Vin&gt;Vout; 
         FIG. 10  is a circuit diagram illustrating how the AC/DC converter in  FIG. 6  reduces to and operates as an inverting buck converter during times of negative half cycles of the AC input voltage when Vin&lt;−Vout; 
         FIG. 11  is a circuit diagram of a comparison circuit that forms part of the controller of the AC/DC converter in  FIG. 6  and which compare the AC input voltage Vin to the DC output voltage Vout to determine times whether Vin&gt;Vout and Vin&lt;−Vout; 
         FIG. 12  is a circuit diagram of a switch control circuit that forms part of the controller of the AC/DC converter in  FIG. 6  and which operates to control the switching of the switches of the AC/DC converter in  FIG. 6 ; and 
         FIG. 13  is a simplified signal diagram depicting the current i L (t) that flows through the inductor of the AC/DC converter in  FIG. 6 . 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 6 , there is shown an alternating current to direct current (AC/DC) converter  600 , according to an embodiment of the present invention. The AC/DC converter  600  comprises first, second, third and fourth switches  602 ,  604 ,  606  and  608 , an inductor  610 , a smoothing capacitor  612 , and a controller  614 . The first switch  602  is coupled between one terminal of the AC input and a first terminal of the inductor  610 ; the second switch  604  is coupled between the first terminal of the inductor  610  and the opposing-polarity terminal of the AC input; the third switch  606  is coupled between the AC input and the second terminal of the inductor  610 ; and the fourth switch  608  is coupled between the second terminal of the inductor  610  and the positive DC output terminal. The controller  614  generates switch drive signals for controlling the switching of the first, second, third and fourth switches  602 ,  604 ,  606  and  608 , depending on the instantaneous AC input voltage Vin compared to the DC output voltage, and selectively modulates the duty cycles of the first, second, third and fourth switches  602 ,  604 ,  606  and  608  switches so that the DC output voltage Vout is maintained at the desired level, as is explained in more detail below. 
     The components of the AC/DC converter  600  comprise discrete devices, one or more integrated circuit (IC) chips, or a combination of discrete devices and IC chips. In one embodiment, the controller  614  and first, second, third, and fourth switches  602 ,  604 ,  606  and  608  are integrated in a single IC chip manufactured in accordance with a standard complementary metal-oxide-semiconductor (CMOS) fabrication process, with the first, second, third, and fourth switches  602 ,  604 ,  606  and  608  comprising metal-oxide-semiconductor field-effect transistors (MOSFETs). In another embodiment, the first, second, third, and fourth switches  602 ,  604 ,  606  and  608  are formed in a first IC chip and the controller is formed in a second IC chip. Whereas the first, second, third, and fourth switches  602 ,  604 ,  606  and  608  comprise silicon-based MOSFETs in the exemplary embodiment just described, other types of switching devices may be used, including conventional switches, diodes, relays, or other semiconductor-based or non-semiconductor-based switching devices. For example, in applications requiring fast switching speeds, compound-semiconductor-based transistor devices, such as high electron mobility transistors (HEMTs) or heterojunction bipolar transistors (HBTs), may be used to implement the first, second, third, and fourth switches  602 ,  604 ,  606  and  608  switches, instead of silicon-based MOSFETs. For the purpose of this disclosure, the term “switch” is used in its broadest sense to include all of these types of switches and any other suitable switching device. The inductor  610  and capacitor  612  may also be integrated in the one or more IC chips, or either or both of these devices may be discrete devices coupled to external pins of the one or more IC chips. 
     The AC/DC converter  600  is configured to directly convert an AC input voltage Vin, such as may be provided by the AC mains, to a DC output voltage Vout, without the need for a diode bridge or a step-down transformer. Direct conversion is accomplished by controlling and modulating the on/off states of the first, second, third, and fourth switches  602 ,  604 ,  606  and  608  using the controller  614 . More specifically, depending on the instantaneous AC input voltage Vin compared to the DC output voltage Vout, the switches are turned on (closed), turned off (opened), driven by a switch drive signal of duty cycle D, or driven by a complementary switch drive signal of duty cycle (1−D). The switch drive signal (labeled “D” in  FIG. 6 ) and the complementary switch drive signal (labeled “1−D” in  FIG. 6 ) are periodic (or semi-periodic) and have a common, fixed switching frequency f=1/T, where T is the switching period. As illustrated in the signal diagram in  FIG. 7  and shown in the switching table in  FIG. 8 , when Vin&gt;Vout, the first switch  602  is driven by the switch drive signal at a duty cycle t ON /T=D, the second switch  604  is driven by the complementary switch drive signal at a duty cycle (T−t ON )/T=(1−D), the third switch  606  is turned off, and the fourth switch  608  is turned on. When Vin&lt;−Vout, the first switch  602  is turned off, the second switch  604  is turned on, the third switch  606  is driven by the switch drive signal at a duty cycle D, and the fourth switch is driven by the complementary switch drive signal at a duty cycle (1−D). Finally, when Vin is greater than −Vout but less than Vout, i.e. when |Vin|&lt;Vout, the first, second, third, and fourth switches  602 ,  604 ,  606  and  608  are turned off. 
     The DC output voltage of the AC/DC converter  600  is equal to D|Vin|, where |Vin| is the absolute value of the instantaneous AC input voltage. According to one embodiment, the controller  614  modulates the duty cycle D, regulating the DC output voltage Vout so that it is maintained at a constant level. The duty cycle D may also be managed to improve the power factor of the AC/DC converter  600 . Whereas D is modulated to maintain the DC output voltage Vout at a constant level in the exemplary embodiment described here, in general Vout, D, and Vin are all variables. Accordingly, Vout need not necessarily be maintained at a constant level. 
     That Vout=|Vin| is more readily apparent by understanding that the AC/DC converter  600  comprises an integrated (i.e., conjoined) buck converter and an inverting buck converter. During the positive half cycles of the AC input waveform when Vin&gt;Vout, the third switch  606  is off, the fourth switch  608  is on, and the AC/DC converter  600  reduces to and operates as a buck converter  600 A, as illustrated in  FIG. 9 . The first and second switches  602  and  604  serve as the high-side and low-side switches of the buck converter and are driven by the switch drive signal at duty cycle D and complementary switch drive signal at a duty cycle (1−D), respectively. The first and second switches  602  and  604  therefore alternately configure the inductor  610  between storing energy and supplying current during positive half cycles of the AC input voltage when Vin&gt;Vout, and the DC output voltage Vout=DVin. 
     During the negative half cycles of the AC input waveform when Vin&lt;−Vout, the first switch  602  is off, the second switch  604  is on, and the AC/DC converter  600  reduces to and operates as what may be referred to as an “inverting” buck converter  600 B, as illustrated in  FIG. 10 . The third and fourth switches  606  and  608  are driven by the switch drive signal D and complementary switch drive signal (1−D), respectively. The inverting buck converter  600 B inverts the negative input voltage Vin, alternately configuring, by the switching action of the third and fourth switches  606  and  608 , the inductor  610  between storing energy and supplying current during the negative half cycles of the AC input voltage when Vin&lt;−Vout, to produce an output voltage Vout equal to D|Vin|. Hence, considering both positive and negative half cycles, the AC/DC converter  600  produces a DC output voltage Vout=D|Vin|. 
     The controller  614  of the AC/DC converter  600  includes a comparison circuit that continually compares the AC input voltage Vin to the DC output voltage Vout, to determine whether Vin&gt;Vout or Vin&lt;−Vout.  FIG. 11  is a drawing of an exemplary comparison circuit  1100  that performs this task. The comparison circuit  1100  comprises first and second comparators  1102  and  1104 , an inverting amplifier  1106 , a first voltage divider including resistors  1108  and  1110 , and a second voltage divider including resistors  1112  and  1114 . The first voltage divider scales the AC input voltage down to a scaled AC input voltage αVin so that the voltage is within the acceptable input voltage range limit of the first comparator  1102 . The second voltage divider scales the DC output voltage down by the same amount to produce a scaled DC output voltage αVout. The first comparator  1102  compares the scaled AC input voltage αVin to the scaled DC output voltage αVout, producing a high output voltage when Vin&gt;Vout and a low output voltage when Vin&lt;Vout. The inverting amplifier  1106  inverts the scaled DC output voltage αVout to produce a scaled and inverted DC output voltage −αVout. The second comparator  1104  compares the scaled and inverted DC output voltage −αVout to the scaled AC input voltage αVin, producing a high output voltage when Vin&lt;−Vout and a low output voltage when Vin&gt;−Vout. 
     The controller  614  of the AC/DC converter  600  also includes a switch control circuit  1200 , shown in  FIG. 12 , which controls the switching of the first, second, third, and fourth switches  602 ,  604 ,  606  and  608 . The switch control circuit  1200  comprises an error amplifier  1202 , a pulse-width modulator (PWM)  1204 , and switches  1206 - 1216  having on/off states that control the switching of the first, second, third and fourth switches  602 ,  604 ,  606  and  608 . The error amplifier  1202  compares the DC output voltage Vout to a precise reference voltage Vref that is equal to and defines the desired DC output voltage Vout and produces an error signals based on the difference between Vref and Vout. The PWM  1204  generates the aforementioned switch drive signal (labeled “D” in  FIG. 12 ) and complementary switch drive signal (labeled “1−D” in  FIG. 12 ) and modulates D based on the error signal c, thereby providing the switch control circuit  1200  the ability to regulate the DC output voltage Vout. The switches  1206 - 1216  are controlled by the outputs of the first and second comparators  1102  and  1104  of the comparator circuit  1100  in  FIG. 11  and control the switching states of the first, second, third and fourth switches  602 ,  604 ,  606  and  608 , in accordance with the switching table in  FIG. 8 . 
     In the exemplary embodiment above, the switch control circuit  1200  is described as controlling the opening and closing of the switches  606 ,  604 ,  606  and  608 , according to the switching table in  FIG. 8 . In another embodiment, the controller  614  is alternatively or further configured to hold switch  608  open during light load conditions. (What defines the light load condition is dependent on the application and established and set during design.) The remaining switches  602 ,  604  and  606  are configured to operate according to the switching table in  FIG. 8 , or are configured to not switch at all, with no effect on the load  616 . Hence, during light load conditions, the capacitor  612  serves as the power supply for the load  616 . 
     As discussed above, the output voltage of the AC/DC converter  600  can be regulated by adjusting over time (i.e., pulse-width modulating) the duty cycle D of the switch drive signals applied to the switches  602 ,  604 ,  606  and  608 , based on comparisons of the DC output voltage V out  to the AC input voltage V in . Since the AC/DC converter  600  operates at essentially constant power, i.e., P out ≈P in , the input current i in  drawn from the AC mains decreases as the input voltage V in  increases. This inverse dependency of the input current i in  on the input voltage V in  adversely affects the power factor of the system. In applications in which the load is constant and maintaining a high power factor is of primary concern, the power factor can be maintained at a high value by holding the duty cycle D of the switch drive signals constant, although, of course, at the expense of no regulation. Holding the duty cycle D constant results in essentially unity power factor since the input current i in  drawn by the AC/DC converter  600 =i in =D 2 V in /R load  is sinusoidal and in phase with the input voltage V in . In most applications, however, a power factor of unity is unnecessary and some level of regulation is desired. Accordingly, in one embodiment of the invention a desired combination of output regulation and power factor is realized by configuring the controller  614  of the AC/DC converter  600  so that it adjusts the duty cycle D of the switch drive signals, based on comparisons of the DC output voltage V out  to the AC input voltage V in , but only within a range that allows a minimum power factor to be maintained. 
     In applications in which the load is variable, the power factor can be maintained by simply adjusting the frequency f of the switch drive signals to the switches  602 ,  604 ,  606  and  608 . Assuming that the capacitor  612  is large enough to maintain a constant voltage across its terminals during each commutation cycle T, the average current that flows through the capacitor  612  is zero. The average input current i in,avg  drawn by the AC/DC converter  600  from the AC mains is therefore equal to the average inductor current i L,avg . In other words: 
                 i     in   ,   avg       =       i     L   ,   avg       =       1   T     ⁢       ∫   0   T     ⁢         i   L     ⁡     (   t   )       ⁢           ⁢     ⅆ   t               ,         
where T=1/f represents the period of the switch drive signals and i L (t) is the current through the inductor  610 .
 
     As illustrated in in  FIG. 13 , for each period T of the inductor current i L (t), the inductor current i L (t) rises linearly between 0 and Δt, falls linearly between Δt and 2Δt, and is zero for the remainder of the period. When i L (t) is integrated over these three intervals, it can be shown that:
 
 i   in,avg   =i   L,avg   ∝D   2   /f.  
 
In other words, the average input current i in,avg  is proportional to the duty cycle D (more specifically, to D 2 ) and inversely proportional to the frequency f of the switch drive signals applied to the switches  602 ,  604 ,  606  and  608 . Exploiting this dependency of the average input current i in,avg  on the switching frequency f, in one embodiment of the invention the controller  614  of the AC/DC converter  600  is configured to set and hold D of the switch drive signals to a constant value and adjust the switching frequency f of the switch drive signals as changes in the load occur. Setting D to a constant value allows a desired nominal DC output voltage V out  to be maintained, and adjusting f in response to changes in the load allows the power factor to be maintained despite variations in the load. When the load increases the controller  614  operates to lower the switching frequency f of the switch drive signals, thereby making the load resistance appear lower and the input current i in  drawn from the AC mains to be higher. Conversely, when the load decreases the controller  614  operates to increase the switching frequency f of the switch drive signals, thereby making the load resistance appear higher and the input current i in  drawn from the AC mains to be lower. By controlling the switching frequency f of the switch drive signals in this manner, the average input current i in,avg  is forced to adapt to changes in the load and the power factor is maintained, as a result.
 
     While various embodiments of the present invention have been described, they have been presented by way of example and not limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail may be made to the exemplary embodiments without departing from the true spirit and scope of the invention. Accordingly, the scope of the invention should not be limited by the specifics of the exemplary embodiments. Rather, the scope of the invention should be determined by the appended claims, including the full scope of equivalents to which such claims are entitled.