Abstract:
Presented is a single-stage power converter topology allowing for power factor correction during operation. The topology utilizes an integration of Ćuk converter and single-ended primary inductance converter (SEPIC) topologies to provide AC-to-AC, AC-to-DC, DC-to-AC, and AC/DC-to-AC operation. Shared use of the Ćuk converter&#39;s output inductor by the SEPIC-type circuit elements provides continuous output current, typically unknown to a SEPIC converter. Application of the single-stage power converter topologies may be had in a line conditioner circuit, a battery charger circuit, and as an uninterruptible power supply (UPS).

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates generally to single-stage power converters, and more particularly to single-stage power converters having an AC, a DC, or an AC/DC input and providing power factor correction.  
         BACKGROUND OF THE INVENTION  
         [0002]    As the number of computers and sophisticated electronics continues to grow, so to does the requirement for high-quality electric power to supply the growing demand presented by these devices. Unfortunately, the ability of electric power utilities to supply the power quality required by such equipment at the point of utilization has simply not kept up with the need for such power. As a result, there is a greatly increased need for power conversion and conditioning equipment that is capable of supplying reliable, high-quality power for use by these devices. Additionally, despite the essential role that such power conversion and conditioning equipment plays, there is increased market pressure for such equipment to be as low-cost and complexity as possible to achieve the required reliability demanded by the consuming public.  
           [0003]    Uninterruptible power supplies (UPSs) make up one class of such power conversion and conditioning equipment in increased demand. These UPSs are needed to supply reliable high-quality power to the consumer and commercial electronics industries to maintain operation of this equipment in the face of degraded or absent utility power. A typical UPS includes power conversion circuitry capable of conditioning poor quality AC voltage from the utility line, as well as generating high-quality output power from electric storage batteries. The typical UPS utilizes a multistage power converter to fulfill all of the requirements of the UPS system. These requirements may include maintaining the charge of the electric storage batteries during AC power line usage, performing power factor correction on the input power draw from the utility line, and generating an AC output voltage from the electric storage batteries during periods of loss or sufficient degradation of the power quality of the utility line voltage. Unfortunately, such increased complexity also tends to drive up the cost of such UPS equipment.  
           [0004]    Various single-stage power converter topologies are known in the power conversion industry. Unfortunately, most are limited to simple DC-to-DC conversion. One such single-stage power converter, which has found wide application as a DC-to-DC converter, is known as the Ćuk converter after its inventor, Dr. Slobodan Ćuk of the California Institute of Technology. This single-stage switched mode DC-to-DC converter  400  illustrated in FIG. 4 operates from a DC input voltage source  402  and delivers an output DC voltage to the connected load  416 . The DC source  402  is coupled through input inductor  404  and coupled across switching device  406 . The converter  400  includes capacitor  408  and diode  410 , as well as an output inductor  412  and filter capacitor  414 . The control and operation of the Ćuk converter  400  is well known, and has an output transfer function defined by the following equation:  
           V   OUT     =       -     V   IN       ×       T   ON       T   OFF           ,                         
 
           [0005]    where T on  and T off  are the on and off times of the switching element  406 .  
           [0006]    As will be recognized by those skilled in the art, this converter may be operated in a boost or buck fashion. As will also be recognized, the Ćuk converter  400  is inverting, that is the output voltage is of opposite polarity to the input voltage. It is of interest to note that the Ćuk converter  400  allows for a continuous input current and a continuous output current. However, limitations on this circuit require that the input voltage be equal to or greater than zero, thereby constraining its operation to applications having DC input voltages only. As such, the Ćuk converter  400  has not found applicability where AC input voltage is used.  
           [0007]    Another switched mode single-stage power converter that has found wide applicability is the single ended primary induction converter (SEPIC)  500  as illustrated in FIG. 5. As with the Ćuk converter  400 , the SEPIC converter  500  is a DC-to-DC converter. Unlike the Ćuk converter  400 , the SEPIC converter  500  is non-inverting. Its output transfer function is defined by  
           V   OUT     =       V   IN     ×       T   ON       T   OFF           ,                         
 
           [0008]    where T ON  and T OFF  are the on and off times of switching element  506 .  
           [0009]    The actual construction of the SEPIC converter  500  is also very similar to the Ćuk converter, utilizing a DC input voltage source  502  coupled through an input inductor  504  across switching element  506  and across inductor  508 . Unlike the Ćuk converter  400 , the SEPIC converter  500  utilizes an output diode  512 , coupling inductor  510  between capacitor  508  and diode  512  to ground. The output capacitor  514  is coupled in parallel with the output load  516 . While the input current from the positive DC voltage source  502  may be continuous, the structure of the SEPIC converter  500  results in an output current that is discontinuous. This discontinuity in the output current tends to increase the output distortion, and further limits application of the SEPIC converter to applications that can tolerate such increased output waveform distortion. This SEPIC converter  500  is also limited to applications that have only a positive DC input voltage, therefore prohibiting its application where AC line voltage must be used.  
         SUMMARY OF THE INVENTION  
         [0010]    In view of the above, it is an object of the present invention to provide a new and improved single-stage power converter. More particularly, it is an object of the present invention to provide a new and improved single-stage switched mode power converter providing power factor correction.  
           [0011]    In one embodiment of the present invention a single-stage power converter comprises an input that receives an electric power input from an external source, a first circuit portion coupled to the input and operative during a first phase to produce an output power to an external coupled load. The converter further includes a second circuit portion coupled to the input and operative during a second phase to produce the output power. In this embodiment, the first circuit portion is configured as either a Ćuk converter or a single-ended primary inductance converter (SEPIC) and the second circuit portion is configured as either a Ćuk converter or a single-ended primary inductance converter (SEPIC). Unlike a typical SEPIC converter, the output power produced by each of the first and the second circuit portions is continuous.  
           [0012]    In an embodiment of the present invention, the first circuit portion includes an output inductor through which the output power is supplied during the first phase. The second circuit portion shares this output inductor. For an embodiment where the input electric power is AC and the output electric power is DC, the first circuit portion is configured as a Ćuk converter and the second portion is configured as a SEPIC converter. In this embodiment the first circuit portion is operative during a negative half-cycle of the AC input electric power, and the second circuit portion is operative during a positive half-cycle of the AC input electric power.  
           [0013]    For an embodiment where the input electric power is AC and the output electric power is also AC, the first circuit portion is configured as a Ćuk converter and the second circuit portion is also configured as a Ćuk converter. The first circuit portion is operative during a positive half-cycle of the AC input electric power, and the second circuit portion is operative during a negative half-cycle of the AC input electric power. With this embodiment the output electric power is inverted relative to the input electric power.  
           [0014]    For an embodiment where the input electric power is AC and the output electric power is AC, the first circuit portion is configured as a SEPIC converter and the second circuit portion is also configured as a SEPIC converter. In this embodiment the first circuit portion is operative during a positive half-cycle of the AC input electric power, and the second circuit portion is operative during a negative half-cycle of the AC input electric power. The output electric power is non-inverted relative to the input electric power.  
           [0015]    For and embodiment where the input electric power is DC and the output electric power is AC, the first circuit portion is configured as a Ćuk converter and the second circuit portion is configured as a SEPIC converter. In this embodiment the first circuit portion is operative to construct a negative half-cycle of the AC output electric power, and the second circuit portion is operative to construct a positive half-cycle of the AC output electric power.  
           [0016]    In a further embodiment of the present invention, the converter further comprises a third circuit portion coupled to the input and operative during a third phase of operation to produce the output power. The third circuit portion is configured as one of a Ćuk converter and a single-ended primary inductance converter (SEPIC) sharing the output inductor through which the output power is supplied. In this embodiment, at least one of the first, second, and third circuit portions is configured as a Ćuk converter and at least one other of the first, the second, and the third circuit portions is configured as a SEPIC converter.  
           [0017]    Preferably in this embodiment the input is adapted to selectively receive an electric power input from a first external source of DC electric power and a second external source of AC electric power. When the input receives the electric power input form the first external source of DC electric power and when the output electric power is AC, the first circuit portion is configured as a Ćuk converter and the second circuit portion is configured as a SEPIC converter. In this configuration the first circuit portion is operative to construct a negative half-cycle of the AC output electric power. The second circuit portion is operative to construct a positive half-cycle of the AC output electric power.  
           [0018]    In this embodiment when the input receives the electric power input form the second external source of AC electric power and when the output electric power is AC, the second circuit portion is configured as a SEPIC converter and the third circuit portion is also configured as a SEPIC converter. In this configuration, the second circuit portion is operative to construct a positive half-cycle of the AC output electric power, and the third circuit portion is operative to construct a negative half-cycle of the AC output electric power.  
           [0019]    In an alternate embodiment a single-stage AC-to-AC converter comprises an input adapted to receive AC electric power from an external source and a first circuit portion. This first circuit portion forms a first Ćuk converter having an input inductor, an output inductor, and a line capacitor. The AC-to-AC converter also includes a second circuit portion forming, in conjunction with the input inductor, the output inductor, and the line capacitor, a second Ćuk converter. This second Ćuk converter is oriented in opposite polarity to the first Ćuk converter. Preferably, the first Ćuk converter is operative during a positive half-cycle of the AC electric power from the external source to generate a negative half-cycle of an output AC electric power. The second Ćuk converter is operative during a negative half-cycle of the AC electric power from the external source to generate a positive half-cycle of the output AC electric power.  
           [0020]    In one embodiment, the first Ćuk converter includes a first power switching device and a first series connected diode. Likewise, the second Ćuk converter includes a second power switching device and a second series connected diode. The first power switching device and the first series connected diode are coupled in opposite parallel orientation to the second power switching device and the second series connected diode.  
           [0021]    In a further embodiment, a single-stage AC-to-DC converter is presented comprising a first circuit portion forming a Ćuk converter having an input inductor and an output inductor, and a second circuit portion forming, in conjunction with the input inductor of the Ćuk converter, a single-ended primary inductance converter (SEPIC). The SEPIC converter is coupled to the output inductor of the Ćuk converter to maintain a constant output current during operation of the SEPIC converter. Further, the SEPIC converter is oriented in opposite polarity to the Ćuk converter.  
           [0022]    Preferably, reverse power flow through the first circuit portion and the second circuit portion is prohibited by blocking diodes. Additionally, in one embodiment the first circuit portion is operative during a negative half-cycle of the AC electric power to produce a positive DC output, and the second circuit portion is operative during a positive half-cycle of the AC electric power to produce a positive DC output. The shared output inductor ensures constant output current during operation of the second circuit portion.  
           [0023]    In a further embodiment of the present invention, an uninterruptible power supply (UPS) operative to supply AC output power to connected loads from both AC line power and backup DC battery power in the event of loss or severe degradation of AC line power is presented. This UPS comprises input source selection circuitry that is adapted to receive the AC line power and the backup DC battery power. The UPS also includes a first circuit portion forming a Ćuk converter having an input inductor and an output inductor. A second circuit portion is included that forms, in conjunction with the input inductor of the Ćuk converter, a first single-ended primary inductance converter (SEPIC). The first SEPIC converter is also coupled to the output inductor of the Ćuk converter to maintain a constant output current during operation of the first SEPIC converter. This first SEPIC converter is oriented in opposite polarity to the Ćuk converter. A third circuit portion is also included. This third circuit portion forms, in conjunction with the input inductor of the Ćuk converter, a second single-ended primary inductance converter (SEPIC). The second SEPIC converter is also coupled to the output inductor of the Ćuk converter to maintain a constant output current during operation. This second SEPIC converter is oriented in like polarity to the Ćuk converter.  
           [0024]    In this embodiment when the input source selection circuitry selects the AC line power, the second SEPIC converter is operative during a positive half-cycle of the AC line power to produce a positive half-cycle of the AC output power through the shared output inductor of the Ćuk converter. The first SEPIC converter is then operative during a negative half-cycle of the AC line power to produce a negative half-cycle of the AC output power through the shared output inductor of the Ćuk converter. When the input source selection circuitry selects the backup DC battery power, the second SEPIC converter is operative to produce a positive half-cycle of the AC output power through the shared output inductor of the Ćuk converter. The Ćuk converter is then operative to produce a negative half-cycle of the AC output power.  
           [0025]    Other objectives and advantages of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]    The accompanying drawings incorporated in and forming a part of the specification illustrate several aspects of the present invention, and together with the description serve to explain the principles of the invention. In the drawings:  
         [0027]    [0027]FIG. 1 is a simplified single line schematic diagram illustrating an embodiment of an AC-to-DC single-stage power converter having power factor correction constructed in accordance with the teachings of the present invention;  
         [0028]    [0028]FIG. 2 is a simplified single line schematic diagram illustrating an embodiment of an AC-to-AC single-stage power converter having power factor correction constructed in accordance with the teachings of the present invention;  
         [0029]    [0029]FIG. 3 is a simplified single line schematic diagram illustrating an embodiment of an DC-to-AC or AC-to-AC single-stage power converter having power factor correction constructed in accordance with the teachings of the present invention;  
         [0030]    [0030]FIG. 4 is a simplified single line schematic diagram of a typical Ćuk switched mode power converter; and  
         [0031]    [0031]FIG. 5 is a simplified single line schematic diagram of a typical single-ended primary inductance converter (SEPIC). 
     
    
       [0032]    While the invention will be described in connection with certain preferred embodiments, there is no intent to limit it to those embodiments. On the contrary, the intent is to cover all alternatives, modifications and equivalents as included within the spirit and scope of the invention as defined by the appended claims.  
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0033]    An embodiment of a single-stage power converter having an AC input in accordance with the present invention is illustrated in FIG. 1. In this embodiment of the present invention, the single-stage converter  100  utilizes an AC input  102  that may be from, for example, a utility line input voltage. This single-stage converter  100  produces a DC output to be used by the connected load  130 . In addition to this converter&#39;s applicability as a DC power supply, it is also particularly applicable to battery charging applications for use, for example, in uninterruptible power supplies (UPSs). In such an application, the utility line voltage is used to maintain a charge on the batteries in the UPS so that, upon a loss of the utility line voltage or a degradation in the power quality, the batteries are of sufficient charge to maintain the power supply to critical electronic equipment.  
         [0034]    Operation of converter  100  during the positive half-cycle of the input AC voltage source  102  proceeds with power flow through input inductor  104 , diode  106 , capacitor  114 , inductor  118 , diode  132 , capacitor  122 , diode  124 , output inductor  126 , capacitor  128 , and load  130 . The control of the output voltage generation is accomplished by control of switching element  110  during this positive half-cycle. As will be recognized by those skilled in the art, the power flow circuitry during this positive half-cycle of the AC input voltage utilize components constructed similarly to a SEPIC converter as discussed above with regard to FIG. 5. As such, the output transfer function of the converter  100  during the positive half-cycle of the input AC voltage source  102  is governed by  
         V   OUT     =       V   IN     ×         T   ON       T   OFF       .                             
 
         [0035]    However, unlike the typical SEPIC converter illustrated and discussed above with regard to FIG. 5, the converter  100  illustrated in FIG. 1 has a continuous output current during this positive half-cycle. As will be recalled, the typical SEPIC converter has a discontinuous output current which significantly adds to the output waveform distortion, and renders the application of such topology inappropriate for many applications that require low harmonic distortion on the output. However, converter  100  overcomes this problem through the use of the shared output inductor  126 . This provides a significant advantage, enabling the application of the simple single-stage SEPIC type topology in applications that require low output harmonic distortion.  
         [0036]    Steering diodes  106  and  124  also provide protection to the SEPIC type circuitry during the negative half-cycle of the input AC voltage source  102 . As discussed above with regard to the typical SEPIC converter illustrated in FIG. 5, this circuitry absolutely required that the input voltage source be positive. While such a requirement is not a problem in applications that utilize a DC voltage, such requirement absolutely precluded the use of this topology converter in applications that utilized voltage sources that could become negative. In the embodiment of the present invention illustrated in FIG. 1, steering diodes  106  and  124  block the negative voltage from being impressed across the circuit elements of the SEPIC type circuitry, and therefore prevent any reverse voltage damage to switching element  110 .  
         [0037]    During the negative half-cycle of the input AC voltage source  102 , the power flow in converter  100  utilizes the inductor  104 , diode  108 , capacitor  116 , diode  120 , output inductor  126 , filter capacitor  128 , and output load  130 . The control of the output voltage is accomplished through switching element  112 . As will be recognized by those skilled in the art from the above discussion, during this phase of converter  100  operation, a single-stage topology similar to the Ćuk converter discussed above with regard to FIG. 4 is realized. However, unlike the typical Ćuk converter that requires a positive input voltage, the configuration of switching element  112  and diode  120  enable operation with the negative input voltage from source  102 . As with the typical Ćuk converter, the output transfer function during this phase of operation is governed by  
         V   OUT     =       -     V   IN       ×         T   ON       T   OFF       .                             
 
         [0038]    That is, the transfer function is inverting, resulting in the generation of a positive output DC voltage. This, in combination with the non-inverting transfer function of the circuitry utilized during the positive half-cycle of voltage source  102  results in the output generation of a positive DC voltage.  
         [0039]    The overall transfer function, therefore, of converter  100  is as follows:  
         V   OUT     =       ABS        [       V   IN     ×       T   ON       T   OFF         ]       .                           
 
         [0040]    That is, the converter  100  provides a rectification of the input AC voltage source  102  to generate a regulated output DC voltage to the connected load  130 . As with steering diodes  106 ,  124 , steering diode  108  provides protection of switching element  112  from reverse voltage damage.  
         [0041]    Advantageously, converter  100  utilizes a continuous input current and generates a continuous output current, thereby minimizing the distortion induced on the line and generated at the output. This allows the increased applicability for applications that cannot tolerate the inducement or generation of high harmonic distortion caused by discontinuous current flow. Indeed, the switching elements  110 ,  112  may utilize typical power factor correction control methodologies to ensure that the current draw from source  102  is in phase with the voltage waveform to provide power factor correction in this single-stage AC-to-DC converter  100 . This provides improved power efficiency and reduced cost over the typical multistage converters required when an AC input is to be used.  
         [0042]    Further, any switched mode control methodology may be utilized to control this converter  100 , including a constant on time, constant off time, constant frequency duty cycle control, hysteretic current loop control, etc. Further, the control may be used with or without inner current loop control and/or feedback. As such, its operation is very robust and may be applied in applications that require advanced, sophisticated control or in applications that are driven to a very simplified control.  
         [0043]    An alternate embodiment of a single-stage power converter having an AC input in accordance with the teachings of the present invention is illustrated in FIG. 2. In this embodiment, the converter  200  utilizes an AC input voltage source  202  and generates an output AC voltage for the load  228 . As such, this single-stage power converter  200  acts as a line conditioner, providing a constant output power quality regardless of the distortions that may be present from the input power source  202 .  
         [0044]    During the positive half-cycle of the AC line voltage source  202 , power flow through converter  200  utilizes the input inductor  204 , capacitor  214 , diode  220 , and silicon controlled rectifier (SCR)  222 , output inductor  224 , and output filter capacitor  226 . The control of the output voltage generation takes place through the diode  206  and switching element  208 . As will be recognized by those skilled in the art, this configuration approximates a Ćuk converter topology with the addition of diode  206  and SCR  222 . As such, the output voltage delivered to the load  228  is inverted from that of the input voltage source  202 . As with the typical Ćuk converter, the output transfer function during this positive half-cycle of the input voltage is  
         V   OUT     =       -     V   IN       ×         T   ON       T   OFF       .                             
 
         [0045]    Diode  210  serves to protect switching element  212  from a reverse voltage condition.  
         [0046]    During the negative half-cycle of the input voltage source  202 , power flow through converter  220  proceeds through the same shared input  204  and output  224  inductors and through capacitor  214  and  226 , but now utilizes the diode  216  and the SCR  218 . Likewise, the control of the output voltage generation during this negative half-cycle of the input voltage is controlled through diode  210  and switching element  212 . As with the circuit elements utilized during the positive half-cycle of the input voltage waveform, during the negative half-cycle the converter  200  topology approximates a reverse Ćuk converter. As such, the output voltage generated across load  228  is also inverted from the input voltage waveform, and is governed by the output transfer function  
         V   OUT     =       -     V   IN       ×         T   ON       T   OFF       .                             
 
         [0047]    During this half-cycle, diode  206  protects switching element  208  from a reverse voltage condition.  
         [0048]    As is apparent from the foregoing discussion, this embodiment of the present invention utilizes shared input and output inductors,  204  and  224 , respectively. In this way, both the input and the output current are continuous, even during transitions through the zero cross of the input voltage waveform. Additionally, switching elements  208  and  212  may be controlled in a known manner to provide power factor correction by ensuring that the power draw from the voltage source  202  is in phase with the voltage waveform. As such, this converter  200  provides an improved power efficiency and reduced cost over prior multistage converters. Further, this simple single-stage topology enables the utilization of known switched mode control methods such as constant on time, constant off time, constant frequency duty cycle control, hysteretic current loop control, etc. Further, the control may operate with or without inner current loop feedback.  
         [0049]    If the single-stage converter of the present invention were to be used in an uninterruptible power supply application requiring both AC and DC inputs to generate an AC output, the embodiment of the invention illustrated in FIG. 3 may be used. This embodiment of a single-stage converter  300  may operate from both an AC voltage source  302  and a battery source or other source of DC power  308  to generate an output AC voltage for the connected load  352 . As will become apparent from the following discussion of this embodiment  300 , the topology utilized to achieve single-stage AC/DC to AC conversion approximates a double SEPIC/single Ćuk converter.  
         [0050]    Such a topology allows an output transfer function that does not invert the AC line voltage during the AC power phase of operation, and therefore allows an additional phase of bypass operation to be included in the UPS design. Specifically, if the input power from the utility source  302  is of sufficient quality to be supplied directly to the load  352 , the converter  300  operates to supply the line voltage directly to the load  352  without additional conversion. This is accomplished by gating SCRs  304 ,  306 ,  330 , and  332  on during appropriate half-cycles of the voltage input source  302 . In this way, a high efficiency bypass mode of operation may be accomplished whereby no additional conditioning of the input voltage is accomplished to minimize circuit loss and increase efficiency.  
         [0051]    While operating in a bypass mode of operation, the converter  300  may begin power conditioning upon sensing a loss of power quality of the input power from source  302  by controlling switching elements  316 ,  320 , and gating appropriate SCRs  304 ,  306 ,  324 ,  326 ,  340 ,  342 , and  330 ,  332 . While using this AC input source  302  in this phase of operation, SCRs  346  and  310  are not gated into conduction. During the positive half-cycle of the AC input source  302 , power flow uses SCR  304 , input inductor  312 , capacitor  322 , inductor  328 , anti-parallel connected SCRs  324  and  326 , SCR  330 , diode  334 , capacitor  338 , anti-parallel connected SCRs  340  and  342 , output inductor  348 , and output filter capacitor  350 .  
         [0052]    As discussed briefly above, the usage of the shared output inductor  348  allows for this SEPIC type configuration to have a continuous output current, thereby extending the applicability of this embodiment  300  to applications requiring continuous output current and low harmonic distortion. The control of the output voltage waveform generation is accomplished through diode  314  and switching element  316 . Diode  318  serves to protect switching element  320  from a reverse voltage condition during this positive half-cycle.  
         [0053]    During the negative half-cycle of the input AC voltage source  302 , power flow uses SCR  306 , input inductor  312 , capacitor  322 , inductor  328 , anti-parallel coupled SCRs  324  and  326 , SCR  332 , diode  336 , capacitor  338 , anti-parallel connected SCRs  340  and  342 , output inductor  348 , and output filter capacitor  350 . Control of the voltage output generation is accomplished through diode  318  and switching element  320 . Diode  314  is utilized to protect switching element  316  from a reverse voltage condition during this negative half-cycle. As with operation during the positive half-cycle of the input voltage waveform, the usage of the output inductor  348  ensures a continuous output current flow to load  352  which otherwise would be impossible with the SEPIC type topology utilized. As discussed above, this allows for the usage of this topology in applications that require a continuous output current and low harmonic distortion.  
         [0054]    As indicated above, the embodiment of the present invention  300  may also use a DC voltage input, for example, from battery  308 . Such operation may be required in situations where the input AC voltage source  302  is unavailable, or is of such a poor power quality that the generation of an output of sufficient quality cannot be maintained by using this AC input source  302 . To allow such operation in a single-stage converter  300 , the SCRs  304 ,  306  are not gated into conduction, while SCR  310  is. The power flow from the DC voltage source  308  to construct an output positive half-cycle of the generated AC waveform utilizes the SCR  310 , input inductor  312 , capacitor  322 , inductor  328 , anti-parallel coupled SCRs  324 ,  326 , SCR  330 , diode  334 , output inductor  348 , and output capacitor  350 . Control of the output voltage waveform generation is accomplished through diode  314  and switching element  316 . As will be recognized by one skilled in the art, the topology of the circuit during this phase of operation approximates a SEPIC type converter having a non-inverting transfer characteristic.  
         [0055]    To generate the negative half-cycle of the output AC waveform from the input DC source  308 , the power flow uses SCR  310 , inductor  312 , capacitor  322 , SCR  332 , diode  336 , output inductor  348 , and output filter capacitor  350 . As will be recognized, the topology during this phase of operation approximates a Ćuk converter having an inverting power transfer characteristic. As such, the output to the connected load  352  will form the negative half-cycle of the output waveform controlled by switching element  316 .  
         [0056]    As will now be understood, operation of the single-stage converter  300  approximates a SEPIC/SEPIC type converter when the input is to be derived from the AC voltage source  302 . Further the operation approximates a SEPIC/Ćuk converter when the output AC voltage is to be generated from the input DC voltage source  308 . However, even during operation in the SEPIC type configuration, the converter  300  of the present invention utilizes a shared output inductor  348  required by the Ćuk converter topology to maintain a continuous output current. This provides a significant advantage over the typical SEPIC topology converter in that the output harmonic distortion is greatly decreased.  
         [0057]    As with the prior embodiments of the present invention, power factor correction can be accomplished through appropriate control of switching elements  316  and  320  by maintaining the input current draw in phase with the input current voltage during AC line voltage operation from source  302 . This provides improved power efficiency and reduced cost over the typical multistage converters required when input AC voltage is to be utilized. Likewise, converter  300  may be operated with any known switched mode control method, including constant on time, constant off time, constant frequency duty cycle control, hysteretic current loop control, etc. Further, any of these control methods can be used with or without inner current loop feedback. The control of the gating of the various SCRs utilized in converter  300  for the operating modes discussed above is illustrated in the following truth table:  
                                                   AC-to-   AC-to-   DC-to-   DC-to-       Component No.   AC +cycle   AC −cycle   AC +cycle   AC −cycle                   304, 306   ON   ON   OFF   OFF       310   OFF   OFF   ON   ON       316, 320   PWM   PWM   PWM   PWM       324, 326   ON   ON   ON   OFF       340, 342   ON   ON   ON   OFF       332   OFF   ON   OFF   ON       330   ON   OFF   ON   ON       346   OFF   OFF   OFF   ON                  
 
         [0058]    The foregoing description of various preferred embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obvious modifications or variations are possible in light of the above teachings. The embodiments discussed were chosen and described to provide the best illustration of the principles of the invention and its practical application to thereby enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.