Abstract:
A circuit for implementing tracking supply alternating current (“AC”) regeneration is described. In one embodiment, the circuit comprises a line synchronization device for converting an incoming AC signal to a square wave, wherein the square wave is precisely in phase with the incoming AC signal; a processor for processing the square wave to synthesize a sine wave therefrom; a digital to audio converter (“DAC”) to convert the synthesized sine wave into an analog signal, wherein the analog signal is precisely in phase with the incoming AC signal; and an amplifier for amplifying the audio signal to a desired voltage level for driving a load.

Description:
CROSS-REFERENCE UNDER 35 U.S.C. §119(e) 
   This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/649,387, filed Feb. 2, 2005, which is hereby incorporated by reference in its entirety. 

   BACKGROUND 
   This invention relates to a system and method for regenerating alternating current for the purpose of producing a cleaner AC waveform, eliminating noise on the AC line and providing regulation and, more particularly, to such a system and method utilizing a tracking supply. 
   In many applications utilizing high-precision electrical components, it is desirable, if not necessary, that the source of AC power used to drive the components is at a constant voltage, so as not to compromise the performance of the components. In cases where the components are driven by ordinary house current that purports to supply 120 volts, variations in demands on the power grid supplying the voltage often cause this value to vary considerably. 
   Therefore, voltage regulator systems have evolved, many of which utilize autotransformers controlled by a motor or by switched taps. However, these systems are not without problems. For example, they can only correct the voltage in discrete steps and therefore do not completely eliminate the error between the desired voltage and the actual output. Also, they are inherently slow, and can only correct the voltage after it has been measured and determined to be wrong. Further, they do nothing to reduce harmonic distortion or noise and their circuitry increases the impedance of the power source. 
   Other regulation systems utilize power regenerators that create AC power from fixed DC supplies. However, these systems suffer primarily from extremely low efficiency, since the DC supplies are fixed, and have to be fairly high in magnitude in order to generate 120 VAC directly. The result is a great deal of voltage drop on the output devices. Also, the high voltages make MOSFET based switching designs impractical. Practical limitations (including efficiency and weight) make high power (&gt;1000 W) systems unmanageable for the consumer electronics market. 
   Other regulation systems create a correction signal that is summed in with the AC input to create a “correct” output power waveform. These systems generally use an output transformer to sum the error signal, which increases the source impedance. They also rely on a measurement of the incoming voltage as a means of creating the error signal. This means the correction is limited by the capabilities of the error circuitry. 
   Therefore what is needed is a voltage regulation system that eliminates the above problems. 
   SUMMARY 
   One embodiment is a circuit for implementing tracking supply alternating current (“AC”) regeneration. The circuit comprises a line synchronization device for converting an incoming AC signal to a square wave, wherein the square wave is precisely in phase with the incoming AC signal; a processor for processing the square wave to synthesize a sine wave therefrom; a digital to audio converter (“DAC”) to convert the synthesized sine wave into an analog signal, wherein the analog signal is precisely in phase with the incoming AC signal; and an amplifier for amplifying the audio signal to a desired voltage level for driving a load. 
   Another embodiment is a method of implementing tracking supply alternating current (“AC”) regeneration. The method comprises converting an incoming AC signal to a square wave that is precisely in phase with the incoming AC signal, wherein the square wave is at a first peak-to-peak voltage level; processing the square wave signal to generate a digital representation of a sine wave; converting the digital representation of the sine wave to an analog signal at a second peak-to-peak voltage level; and amplifying the analog signal to a desired level. 
   Another embodiment is a system of implementing tracking supply alternating current (“AC”) regeneration. The system comprises means for converting an incoming AC signal to a square wave that is precisely in phase with the incoming AC signal, wherein the square wave is at a first peak-to-peak voltage level; means for processing the square wave signal to generate a digital representation of a sine wave; means for converting the digital representation of the sine wave to an analog signal at a second peak-to-peak voltage level; and means for amplifying the filtered analog signal to a desired level. 
   Still another embodiment is a tracking supply alternating current regeneration method comprising superimposing a dual direct current (“DC”) supply voltage on a source of alternating current (“AC”) power, and synchronizing the dual DC supply voltage with the AC power to produce a range of available tracking power supply voltages sufficient to feed a power amplifier producing a desired output voltage. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a graph depicting a typical waveform that is delivered from the power company, along with several other AC waveforms. 
       FIG. 2  is a schematic diagram depicting a regulator circuit according to an embodiment of the present disclosure for implementing tracking supply AC power regeneration. 
       FIG. 3  is a flowchart depicting the operation of the regulator circuit of  FIG. 2  according to an embodiment of the present disclosure. 
   

   DETAILED DESCRIPTION 
   Referring to  FIG. 1 , a waveform L 1  represents a typical waveform of AC power as it is delivered from a power provider. As can be seen from  FIG. 1 , the waveform L 1  is generally not a perfect sine wave and the waveform shown has 5% of the 3rd harmonic thereof added in. Further, the 3rd harmonic lags the fundamental, creating an asymmetrical half cycle. Finally, the top of the waveform L 1  is truncated to represent the distortions caused by rectifier/capacitor power supply loads. 
   Waveforms L 2  and L 3  represent the positive rails and the negative rails, respectively, of a tracking power supply. In this example, DC supplies are 18V, plus and minus, and referenced to the incoming AC signal. This creates a “window” of available power supply voltage from which a “correct” output power is regenerated. A waveform L 4  represents an output in the form of a perfect 117VAC sine wave that extends between lines L 2  and L 3 , which means that there is sufficient voltage available to create a perfect output waveform. 
     FIG. 2  is a schematic diagram of a regulator circuit  200  in accordance with one embodiment for implementing tracking supply AC power regeneration. The circuit  200  includes a primary tracking power supply stage  202  comprising a transformer T 1 , a bridge rectifier D 7 , and capacitors C 1 -C 4 . This circuit arrangement embodies the waveforms described in  FIG. 1 . A “Line In” signal on a line  203  is the distorted, unregulated mains voltage, corresponding to the waveform L 1  in  FIG. 1 . For convenience, the other voltages are measured with respect to this voltage (hence the “signal-ground” symbol). The circuit nodes  204 ,  206 , labeled “+15” and “−15”, respectively, correspond to the waveforms L 2  and L 3 , respectively, in  FIG. 1 . The transformer T 1  itself is too large to be mounted directly on a printed circuit board comprising the circuit  200 , thus terminals TB 1 -TB 5  are used for these connections as well as the main input/output connections. 
   In accordance with one embodiment, the circuit  200  is used to digitally synthesize a very low distortion sine wave. The synthesized sine wave is precisely matched (within 0.001 Hz) to the frequency and phase of the incoming AC signal on the line  203 ; however, the voltage and wave shape of the incoming power are completely ignored. This signal is amplified to approximately 117VAC with respect to the AC neutral line and is then input to a power stage that supplies the current to a load (not shown) via a “Line Out” signal on a line  207 . This signal is represented in  FIG. 1  by the waveform L 4 . 
   Referring again to  FIG. 2 , a device U 7 , along with resistors R 16  and R 20  and diode D 9 , function as a line synchronization device  208  to convert the incoming AC voltage into a 5 volts peak-to-peak (“VPP”) square wave that is precisely in phase with the incoming AC signal. The 5VPP signal is fed into a microprocessor U 3 . The microprocessor U 3  performs three primary functions for the circuit  200 , including a phase locked loop function, sine wave synthesis, or oscillator, function, and a system routines function. 
   The phase locked loop function performed by the microprocessor U 3  is a software implementation of a 12-state, state machine phase comparator. This function has primarily 2 input signals and 2 output signals. The input signals consist of the incoming AC signal mentioned above and a similar square wave representation of the internal oscillator signal. It will be recognized that the sine wave does not truly exist as a real waveform, rather a virtual representation of it is within registers of the microprocessor U 3 . The state machine compares the two signals in a way that allows it to determine which signal is higher in frequency and which signal is advanced in phase, with respect to the other. The two output signals can be described simply as “speed up” and “slow down”, which are fed to the sine wave synthesizer. 
   The sine wave synthesizer uses Direct Digital Synthesis (“DSS”) techniques to create a variable frequency sine wave. DDS employs a pointer used to index a lookup table that contains sine wave data. At fixed intervals, the pointer is incremented by an amount proportional to the desired frequency. The larger the increment amount, the faster the pointer completes one cycle through the lookup table. Since the table consists of one complete sine wave, cycling through the table faster corresponds to a higher frequency. Using this technique, it is possible to vary the oscillator frequency within less than 0.001 Hz. The lookup table uses 16-bit precision, which allows the digital sine wave a distortion figure less than 0.01%. The digital signal generated in this manner is fed to a digital-analog converter (“DAC”) U 4 . 
   The system routines performed by the microprocessor U 3  include, at a minimum, monitoring the status of the phase locked loop function and operation of a bypass relay K 1 B, which connects the load directly to the incoming AC signal if the oscillator has not yet “locked” to the frequency of the incoming AC signal or if any other anomalous conditions are detected that warrant disconnecting the circuit  200  from the load. 
   The DAC U 4  and its associated components, including device U 5 , resistor R 10 , and capacitor C 10 , function as an oscillator  210  to take the digital representation of a sine wave generated by the microprocessor U 3  and convert it to an analog “real world” voltage signal at a level of approximately 2.5VPP. This 2.5VPP signal is precisely in phase with the incoming AC signal, but has extremely low harmonic distortion and is fixed in amplitude; i.e., it is not related to the amplitude of the incoming AC signal. 
   The 2.5VPP signal output from the DAC U 4  is very low in harmonic distortion; however, it does have a DC component, as well as significant levels of noise at the DDS oscillator sample frequency (typically 6.4 KHz). A DAC output filter circuit  212  comprising an op-amp U 1 A, capacitors C 11 -C 14 , and resistors R 1 -R 4  functions as an output filter that removes the DC component from the signal output from the DAC U 4  and filters the sample noise to an acceptable level. 
   Op-amps U 1 B and U 2  comprise a voltage amplifier stage  214 , which amplifies the 2.5VPP (0.88VRMS) signal output from the DAC output filter  212  by approximately 133 in order to create a sine wave of 117VAC. This final voltage is adjustable somewhat by a variable resistor R 17 . 
   Op-amp U 2  also serves as a first stage of a current amplifier stage  216 , as it can supply output current up to 200 mA. Additionally, the high voltage capabilities thereof allow more headroom for and better utilization of the available main supply voltages. The current amplifier stage  216  also includes transistors Q 1 -Q 4  and resistors R 1 -R 4 , which create a bipolar emitter-follower style current amplifier. The current amplifier stage  216  is enclosed within a feedback loop of DAC U 4  in order to compensate for the voltage drops associated with the current stage components. Diodes D 1 -D 6 , together with capacitors C 5 -C 8 , comprise a voltage doubler type power supply  218 , which creates the higher voltages required by U 2 . 
     FIG. 3  is a flowchart depicting the operation of the regulator circuit of  FIG. 2  according to an embodiment of the present disclosure. In step  300 , the incoming AC signal is converted to a 5 volts peak-to-peak (“VPP”) square wave that is precisely in phase with the incoming AC signal. In step  302 , the 5VPP signal is processed to generate a digital representation of a sine wave. In the embodiment described herein, the processing of step  302  is performed by a microprocessor. Details of the processing are provided above in connection with the description of  FIG. 2 . In step  304 , the digital representation of the sine wave generated by the microprocessor U 3  is converted to an analog “real world” voltage signal at a level of approximately 2.5VPP. As previously indicated, the 2.5VPP signal is precisely in phase with the incoming AC signal, but has extremely low harmonic distortion and an amplitude that is not related to the amplitude of the incoming AC signal. In step  306 , the 2.5VPP signal is then filtered to remove the DC component thereof and to filter the sample noise to an acceptable level. In step  308 , the filtered 2.5VPP signal is amplified to a desired level. 
   As a result of the above, the output signal can be adjusted (if desired) infinitely to any voltage, since the output waveform on the line  207  is not simply a multiple of the input power, as is the case with an autotransformer, as discussed above. Also, since the output waveform is not dependant on the input waveform, shape, distortion, noise, and other anomalies can be eliminated. Further, since the DC voltages are superimposed on the incoming AC signal, the voltages seen by the power devices are much smaller than standard regenerators, thus improving efficiency and thus allowing higher power systems to be achieved in a smaller, lighter package than traditional regenerators. Finally, since the active output stage drives the load directly, without the use of transformers, the source impedance is very low. The active output stage can have a source impedance that is actually lower than the original AC power source. 
   Variations may be made in the foregoing without departing from the scope of the invention. Examples of variations include, but are not limited to, the following:
         The power supplies may utilize other topologies and technologies. Although the above example uses a simple transformer-rectifier-capacitor supply, it is understood that other power supply designs, including switching type supplies, could be substituted.   The amplifier stage may utilize other topologies and technologies, such as class-D amplifier designs.   Parameters such as the tracking supply voltage, the DDS oscillator sample frequency, the resolution of the D/A converter, and the number and the type of output transistors, can be modified to meet the performance constraints of a particular application.   In addition to the “single-ended” implementation shown, in which the line voltage is regulated with respect to a fixed and unaltered neutral connection, multiple instances of the invention may be implemented in a “balanced” configuration, in which both line and neutral are regulated with respect to a common ground.       

   Although only a few exemplary embodiments of this invention have been described in detail above, those skilled in the art will readily appreciate that many other modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention as defined in the following claims.