Abstract:
Methods and apparatuses for providing a solution for incompatibility between nonsinusoidal waveform uninterruptible power supply (UPS) systems and active power factor correction (PFC) loads are disclosed. An embodiment of the invention includes generating a nonsinusoidal signal waveform (e.g., a voltage waveform), to be delivered to the load, with a pulse width modulation (PWM) duty width, sampling the nonsinusoidal signal waveform to collect output signal samples, and adjusting the duty width to control the nonsinusoidal signal waveform as a function of the output signal samples to deliver a desired signal characteristic (e.g., RMS signal level) to the load. In embodiments of the invention, the output duty width is adjusted differently in cases of rising and falling power consumption, respectively, by the load. Techniques disclosed herein find broad applicability in electronic devices, such as servers, computers, UPS systems and inverters and improve efficiency and reliability for end users and utility providers.

Description:
RELATED APPLICATIONS 
     This application is a Continuation of U.S. application Ser. No. 12/512,709 filed Jul. 30, 2009, claims the benefit of U.S. Provisional Application No. 61/201,502, filed on Dec. 10, 2008. The entire teachings of the above applications are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     Energy transmission via electric power systems is critical to modern civilization. Since numerous electric devices draw alternating current (AC) power from utility sources (i.e., line power from the electric power grid), power failures (i.e., outages or interruptions) can be a serious concern. The use of uninterruptible power supplies (UPSs) having battery back-up systems to provide regulated, uninterrupted power for sensitive and/or critical loads, such as computer systems, and other data processing systems is well known. 
     Uninterruptible power supply (UPS) systems are commonly used in conjunction with a variety of electric devices to safeguard against power failures. UPS systems are available in various configurations to provide varying functionality, but they all typically provide power in the event of a power interruption. 
     A UPS system may be connected to line power so that a device connected to the UPS system (referred to as the load) may use line power during normal conditions. UPS systems typically include a battery to store energy, a rectifier to convert AC to direct current (DC) for passing through the battery, and an inverter to convert DC battery power back to AC when backup power is generated. 
     Ideally, the voltage waveform corresponding to the UPS output voltage (i.e., the electrical signal representing the voltage delivered to the load) is sinusoidal, since electric devices draw AC power that is sinusoidal. Nonsinusoidal waveforms (e.g., square waveforms) typically have greater occurrence of transient oscillations due to sharp voltage changes. However, certain nonsinusoidal waveforms, such as square waves, are easier to generate (and hence less expensive) than sinusoidal waveforms and are therefore commonly used in UPS systems. 
     SUMMARY OF THE INVENTION 
     An embodiment of the invention is a method, or corresponding apparatus, of controlling an output signal to be delivered to a load. A nonsinusoidal signal waveform that is to be delivered to the load is generated with a pulse width modulation (PWM) duty width. The nonsinusoidal signal waveform is sampled to collect output signal samples. A determination is made based on the output signal samples as to whether the load is demanding more or less power over time. The duty width is adjusted to control the nonsinusoidal signal waveform as a function of the output signal samples. In this way, a desired signal characteristic is delivered to the load. 
     In an embodiment, the output signal, nonsinusoidal signal waveform, and output signal samples correspond to voltages. 
     The desired signal characteristic may be a desired root mean square (RMS) signal level. 
     In a case of falling output signal samples, adjusting the duty width may include increasing the duty width. In a case of rising output voltage samples, adjusting the duty width may include decreasing the duty width, for example, to match the RMS signal level of output voltage samples to a desired RMS signal level. 
     In one embodiment, the nonsinusoidal signal waveform is a square signal waveform. The nonsinusoidal signal waveform may have a zero value for a nonzero time duration between each half-cycle of the nonsinusoidal signal waveform. 
     The method may also include controlling an electromagnetic field in a transformer according to the PWM and controlling power switches configured to control power delivery to the load according to the PWM. 
     The method may also include delivering the nonsinusoidal signal waveform to a load including an active power factor correction (PFC) controller. 
     An embodiment includes executing software to control the nonsinusoidal signal waveform. 
     Embodiments may be employed in an inverter or an uninterruptible power supply (UPS). 
     In one embodiment, sampling includes sampling the nonsinusoidal signal waveform on a cycle-to-cycle basis, and determining whether consecutive cycles indicate that the load is demanding more power over time is based on consecutive cycles. In that case, the duty width is increased to compensate for the load&#39;s demanding more power. 
     The output signal sample from a given cycle may be obtained at a point offset from the start of the given cycle by about one-eighth of the period of the given cycle, less than one-half of the period of the given cycle, or less than one-quarter of the period of the given cycle. 
     Determining whether consecutive cycles indicate that the load is demanding more power over time may include comparing a change in samples representing the nonsinusoidal signal waveform from cycle-to-cycle against a threshold, e.g., 10 volts. 
     In an embodiment, generating the nonsinusoidal signal waveform includes calculating the duty width. Increasing the duty width may include re-calculating the duty width to determine a re-calculated duty width, based on the output signal samples and further based on averaging the output signal samples from a given cycle and one or more cycles preceding the given cycle, if the load is less than a threshold. If the load is not less than the threshold, the output signal sample from the given sample may be used. Calculating the duty width may further be based on resetting the duty width to a value higher than the re-calculated duty width in a way that yields a zero signal level for a nonzero time duration between each half-cycle of the nonsinusoidal signal waveform. The value to which the duty width is reset may be 108% of the re-calculated duty width. 
     In an embodiment, the nonsinusoidal signal waveform is sampled at multiple instants within a given cycle, and determining whether the load is demanding less power over time is based on the sampling within the given cycle. Adjusting the duty width may include zeroing the nonsinusoidal signal waveform when or after a predetermined condition is satisfied. The determination as to whether the load is demanding less power over time may include a comparison of a change in samples representing the nonsinusoidal signal waveform during the given cycle against a threshold, e.g., a difference of 5 volts. 
     Another embodiment is a system that includes a processor for performing computing functions, an active power factor correction (PFC) controller, and an uninterruptible power supply (UPS) for delivering a generated nonsinusoidal signal waveform with a desired signal characteristic to a load including the processor and the active PFC controller. The UPS adjusts a duty width of the nonsinusoidal signal waveform based on a determination that the load is demanding more or less power over time. The determination is based on output signal samples of the nonsinusoidal signal waveform. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing will be apparent from the following more particular description of example embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating embodiments of the present invention. 
         FIG. 1  is an example server room configuration in accordance with an embodiment of the invention. 
         FIG. 2  is a block diagram of an uninterruptible power supply (UPS) in accordance with an embodiment of the invention. 
         FIG. 3  is a flow diagram in accordance with an embodiment of the invention. 
         FIG. 4  is a signal diagram depicting sampling in accordance with an embodiment of the invention. 
         FIG. 5  is a detailed flow diagram in accordance with an embodiment of the invention. 
         FIG. 6  is a circuit diagram corresponding to low frequency transformer operation in accordance with an embodiment of the invention. 
         FIG. 7  is a signal diagram illustrating a condition associated with connecting a lightly loaded active PFC to a UPS. 
         FIG. 8  is a circuit diagram corresponding to high frequency transformer operation in accordance with an embodiment of the invention. 
         FIGS. 9 a - c    are signal traces showing signal waveforms in accordance with an embodiment of the invention. 
         FIG. 10  is a block diagram of a prior art standby (offline) topology for a UPS used to provide regulated uninterrupted power. 
         FIG. 11  is a circuit diagram of a typical prior art inverter circuit. 
         FIG. 12  is a block diagram of a line interactive UPS topology for providing AC power to a load. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A description of example embodiments of the invention follows. 
     Uninterruptible Power Supply (UPS) systems with nonsinusoidal waveforms (in particular, square waveforms) have been found to be incompatible with a particular class of load characterized as an active power factor correction (PFC) load (i.e., having an active PFC controller). A general description of square waveforms and power factor correction, including active PFC, follows. 
     A square waveform (square wave) is a periodic waveform that typically includes, in one cycle, a “high” (constant) signal level for a given duration (duty width), followed by a region of zero signal level, and then a “low” signal level. Pulse width modulation is typically employed with square waves in UPS systems to modulate the duty width to achieve a desired output characteristic, such as a desired root mean square (RMS) voltage. 
     The power factor of a system, a dimensionless quantity between 0 and 1, is the ratio of the system&#39;s true power to its apparent power. True power (or real power), measured in Watts, is the actual amount of power dissipated (i.e., used), indicative of the capacity of the system to perform work over time. True power is a function of the resistive (dissipative) elements of a circuit (e.g., resistors). Apparent power (measured in Volt-Amperes) is the product of the current and voltage of the system. When reactive elements (e.g, inductors or capacitors) are present, the current and voltage are not in phase, causing the power to be negative for a portion of each cycle. Thus, some power delivered during a portion of each cycle is returned during another portion, causing true power to be less than apparent power. In other words, apparent power, which is a function of impedance, is greater than true power if reactive elements are present in the system. 
     Reactive loads (systems) are burdensome to utility providers in terms of infrastructure requirements, since power is wasted in the resistance of electric lines between the generator (power grid) and the (reactive) load. Therefore, power factor correction (PFC) techniques have been developed to increase the power factor of reactive loads. 
     Passive PFC uses passive circuit elements (e.g., inductors or capacitors) to compensate for the reactance of the load. For example, a load&#39;s inductive reactance can be compensated by adding another load drawing an equal and opposite amount of reactive power (e.g., a capacitive reactance, such as with a capacitor added in parallel). This technique results in the circuit&#39;s total impedance being nearly equal to the resistance, resulting in a power factor near 1. 
     Active PFC, in contrast, uses a separate circuit (not just passive circuit elements) to compensate for the reactive load. Active PFC draws power resistively from the line in order to supply power to the reactive load. This technique isolates the load from the line and results in better power factor correction than with passive PFC. 
     The use of PFC load design has become more popular recently, and many regulations require the use of PFC. However, the use of active PFC loads introduces challenges for uninterruptible power supply (UPS) technologies using square waveforms. Embodiments of the present invention address these challenges and are applicable to the UPS topologies of  FIGS. 1 and 3 , inverter configurations as in  FIG. 2 , as well as other UPS and inverter configurations. 
       FIG. 1  is an example server room configuration in accordance with an embodiment of the invention. The server room  100  includes several devices, e.g., computing devices, including server A  110   a , and server B  110   b . In order to protect high-value equipment, such as servers, against power outages, a UPS  150  providing sinusoidal waveforms is connected to server A  110   a  and server B  110   b . The UPS  150  draws AC power  160  from mains (grid)  170 . In case of a power outage, the sinusoidal waveform UPS  150  delivers a sinusoidal waveform  115  to server A  110   a . As shown by expanded view  116 , delivered voltage  117  and current  118  are in phase with one another. 
     Lower-value equipment, such as terminals, may be protected via less expensive UPS systems, which may provide nonsinusoidal waveforms, such as square waves.  FIG. 1  shows a square wave UPS connected to a terminal  120 , representing a passive PFC load, and to a terminal  130  with an active PFC controller not shown. The square waveform UPS  140  also draws AC power  160  from the mains  170 . In the example embodiment of  FIG. 1 , the square wave  125  delivered to the passive PFC terminal  120  and the square wave  135  delivered to the active PFC terminal  130  are shown in respective expanded views  126  and  136 ; this situation corresponds to functionality during power blackouts, for example. The output voltage  127  and current  128  delivered to the passive PFC terminal  120  in a power outage condition are well-behaved square waves. The delivered output current  128  does not exceed a maximum current rating  129 , as indicated by a horizontal dashed line. 
     Active PFC loads, such as an active PFC controller, are typically designed to operate well under nominal line conditions. However, when an active PFC load is connected to a square wave UPS in battery mode (e.g., in the case of a power outage), the active PFC load will typically start demanding (i.e., consuming) power, resulting in a decrease in the output voltage  137  of the square wave UPS  140 . In other words, the load represented by the terminal  130  with active PFC controller draws the current  138  rapidly during this time (from the UPS) in order to recover from the abnormal conditions (power shortage) and maintain a power characteristic (e.g., according to a specified power requirement of the load, as regulated by the active PFC controller). In fact, the current may exceed the maximum tolerance  143 , possibly damaging the UPS or causing the UPS to disable its output to protect itself, resulting in dropped load or damage to user equipment. Devices with higher power ratings may be unaffected, but higher power ratings result in higher cost of hardware design. The load may then continue to overdraw current, for example, to charge an internal capacitor of the load (not shown). The load reduces power consumption during an interval  139  when the load does not need to draw a large current from the square wave UPS  140  anymore. During the interval  139  of low power draw, the output voltage  137  rises back to a level based on the battery voltage of the UPS. 
     According to an embodiment of the present invention, a case of falling voltage  137  is detected, and the duty width of the voltage signal  137  is extended ( 141 ) in compensation. A case of rising voltage (e.g., during interval  139 ) is detected, and the duty width of the voltage signal is ( 142 ) decreased in compensation, e.g., by zeroing the voltage signal. 
     It should be understood that, although the example embodiment described above is presented with respect to voltages, currents may also be used in embodiments of the invention. As described herein, the term “signal” may refer to the case of either voltage or current. One of ordinary skill in the art should appreciate that technologies such as current-controlled voltages and sense resistors may be employed to interchange between voltage embodiments and current embodiments in order to adapt the presently described techniques to different implementations. 
     The foregoing behavior of an active PFC load connected to a square wave UPS in the event of a power shortage results in numerous problems. As mentioned earlier, overloaded current can damage the connected device. Also, rapid output voltage changes in each half cycle of the waveform can be problematic, resulting in peak voltage fluctuation (unstable voltage regulation). Power consumption by the load can be unpredictable in this situation, as some PFC loads have been observed to stop and resume consumption of power in unpredictable manners. Finally, square wave UPS systems may be designed to shutdown in such situations in order to protect themselves from dangerous conditions, defeating the purpose of a UPS system. 
     Embodiments of the present invention address the foregoing deficiencies of prior art UPS systems with nonsinusoidal voltage waveforms connected to active PFC loads. Square waveforms are discussed herein, although other nonsinusoidal waveforms are applicable as well. 
     Embodiments of the invention maintain stable output power from a square wave UPS to active PFC loads by stabilizing output true RMS voltage to the loads. 
     An embodiment of the invention monitors the square wave RMS output voltage of each pulse and compares it with the nominal RMS output voltage. If a pulse&#39;s RMS output voltage is lower than the nominal RMS output voltage, the output pulse duty width is increased to make the RMS output voltage reach (or get closer to) the nominal RMS output voltage. If a pulse&#39;s RMS output voltage is greater than the nominal RMS output voltage, the output pulse duty is turned off so that the RMS output voltage is limited to the nominal RMS output voltage. In this embodiment, all pulses are monitored and adjusted when needed. 
     Another embodiment of the invention monitors a square wave pulse type and the RMS output voltage of each pulse. For pulses with rising output peak voltage, the output pulse duty is turned off when the pulse&#39;s RMS output voltage has reached the nominal RMS output voltage. A rise of 5 V for a 120 V system may be used to identify this type of pulse (rising output peak voltage). For pulses with falling output peak voltage, the ouptut pulse duty width is increased to reach (or get closer to) the nominal RMS output voltage. A fall of 5 V for a 120 V system may be used to identify this type of pulse (falling output peak voltage). 
       FIG. 2  is a block diagram of an uninterruptible power supply (UPS) in accordance with an embodiment of the invention.  FIG. 2  shows a nonsinusoidal waveform UPS  240  which may be connected to an active PFC load according to techniques described herein without the negative effects described above in the context of conventional technology. The UPS  240  includes a waveform generator  250 , the output of which is sampled by a sampler  260  The resulting output signal samples  265  (current or voltage) are sent to a demand module that determines, based on the output signal samples  265 , whether the load is demanding more or less power over time. Based on that determination, a duty width adjustment unit  270  may adjust the duty of the waveform. A microcontroller  290  or other processor or logic may control the components described above to contribute to the delivery of an output signal  295  that does not suffer from the deficiencies of conventional technology, as described previously. 
       FIG. 3  is a flow diagram  300  in accordance with an embodiment of the invention. In the process  300 , a nonsinusoidal signal waveform is generated ( 310 ). The nonsinusoidal signal waveform may be a nonsinusoidal voltage waveform or a nonsinusoidal current waveform. Output samples, e.g., output voltage samples, are collected ( 320 ). The process  300  includes determining whether the load is demanding more or less power over time ( 330 ). The duty width of the nonsinusoidal signal is adjusted ( 340 ) to deliver a desired signal characteristic, e.g., a root mean square (RMS) signal level, to the load. 
       FIG. 4  is a signal diagram depicting sampling in accordance with an embodiment of the invention. Each period of the periodic waveform  435  may be subdivided into 16 time slices (shown generally as  440 ) or 32 ticks for purposes of sampling. 
       FIG. 5  is a detailed flow diagram in accordance with an embodiment of the invention. In an embodiment, the nonsinusoidal waveform is generated using pulse width modulation (PWM). The duty width of the waveform is initially calculated ( 505 ) as 
     
       
         
           
             D 
             := 
             
               
                 ( 
                 
                   Vrms 
                   Vpeak 
                 
                 ) 
               
               2 
             
           
         
       
     
     where D is the duty width (also known as duty cycle) of the waveform, Vpeak is peak voltage, and Vrms is the desired RMS voltage to be delivered to the load. 
     Depending on the configuration of the UPS system&#39;s inverter, the PWM may be used to control an electromagnetic field in a transformer or to control power switches configured to control power delivery to the load according to the PWM. 
     After output of the square waveform is enabled ( 510 ), the output voltage is sampled, and the duty width is adjusted to control the nonsinusoidal voltage waveform as a function of the output voltage samples to deliver a desired root mean square (RMS) voltage to the load. The method of adjustment varies depending on the results of the sampling, as described below. 
     On a cycle-to-cycle basis (preferably once per cycle but possibly once every other cycle or according to other inter-cycle sampling employed), samples are gathered ( 515 ) at a point offset from the start of each cycle according to one of the following rules: 1) less than one-half of the period of the cycle; 2) less than one-quarter of the period of the cycle; or 3) about one-eighth of the period of the cycle (plus or minus 5 percent of the period or some other percentage consistent with operational parameters or other criteria). For example, the sampling time corresponding to option (3) above is the second time slice. Sampling the output voltage early in the cycle is useful, but not at the very beginning, since fluctuations may be present at the very beginning. 
     When the load is determined to be relatively light ( 520 ), e.g., less than 20% of the maximum power rating for the UPS system, it may be necessary to protect a transformer component of the UPS against saturation, since drawing higher current suddenly can cause the output voltage to drop sharply. This case applies to low frequency (LF) design. In this case, re-calculating the duty width based on an average of the present cycle&#39;s output voltage sample and the previous cycle&#39;s sample ( 525 ) mitigates against effects of voltage fluctuations. In other words, the following formulas may be used: 
     
       
         
           
             Vpeak 
             := 
             
               
                 Vprevious_peak 
                 + 
                 
                   V 
                   ⁢ 
                   present_peak 
                 
               
               2 
             
           
         
       
       
         
           
             D 
             := 
             
               
                 ( 
                 
                   Vrms 
                   Vpeak 
                 
                 ) 
               
               2 
             
           
         
       
     
     Other averaging or filtering techniques may be used as well, for example, averaging the past three cycles. 
     If the present cycle&#39;s peak output voltage is not less than the previous cycle&#39;s peak output voltage ( 530 ), then the above averaging technique is used to re-calculate the output duty width. Otherwise, the output duty width is re-calculated using the present peak voltage ( 535 ). 
     If the output voltage is obviously dropping ( 540 ), e.g., dropping by more than a threshold such as a difference of 10 V, then embodiments of the invention compensate for the steep voltage drop by increasing the duty width ( 545 ). The duty width may be increased within hardware limitations so as to preserve a region of zero voltage between high and low voltage regions of the waveform. In this way, the output voltage delivered to a load may be brought closer to a desired root mean square (RMS) voltage (since increasing the duty width compensates for the falling peak voltage). In one embodiment, the duty width is reset to 108% of the re-calculated duty width. The duty width may be increased by other factors as well. 
     The square wave output is disabled when the required duty width, i.e., the reset duty width, is reached ( 550 ). This condition may be detected based on a timer compare value which indicates when to disable the output voltage signal. 
       FIG. 6  is a circuit diagram corresponding to low frequency UPS operation in accordance with an embodiment of the invention. This circuit diagram is an example hardware implementation, although other hardware implementations are envisioned as well. For example, sense resistors may be used for current-based implementations, as would be appreciated by one of ordinary skill in the art, as described previously. In  FIG. 6 , a microcontroller  610  provides control signals, supply output A  615  and supply output B  620  to a pair of switches  625 - a  and  625 - b , which may be implemented with MOSFET technology, coupled to a push-pull transformer  630 . Push-pull (buck boost) transformers are generally used in power engineering to change the voltage of a DC power supply, i.e., for DC-to-DC conversion. The voltage associated with a battery  635  may be fed back to the microcontroller  610  for processing, e.g., to adjust the duty width according to embodiments of the invention. A load  635  draws an output voltage  645 , which may be fed back ( 640 ) to the microcontroller  610 , e.g., at a port  642  configured to receive such a signal, for further processing, e.g., for sampling and duty width adjustment. In another embodiment, an output current  650  may be fed back to the microprocessor  610  for further processing. Thus, embodiments of the invention may sample the output voltage, the output current, or the battery voltage and use the samples to adjust duty width. 
       FIG. 7  is a signal diagram illustrating a condition associated with transformer saturation during low frequency operation in accordance with an embodiment of the invention. With a light load, for some active PFC controllers, the load consumes power quickly, resulting in an unbalanced output voltage as shown in  FIG. 7 . Also, this condition can result in unbalanced usage of inverter MOSFETs in the UPS system. 
     For high frequency (HF) design, there are no comparable transformer saturation considerations. Therefore, the following formulas may be used: 
     
       
         
           
             Vpeak 
             := 
             
               V 
               ⁢ 
               
                   
               
               ⁢ 
               present_peak 
             
           
         
       
       
         
           
             D 
             := 
             
               
                 ( 
                 
                   Vrms 
                   Vpeak 
                 
                 ) 
               
               2 
             
           
         
       
     
     Using only the present cycle&#39;s voltage sample in this case, instead of an average relating to the present cycle and past cycles, helps charge the load&#39;s internal DC bus faster. 
       FIG. 8  is a circuit diagram corresponding to high frequency transformer operation in accordance with an embodiment of the invention. This circuit diagram is an example hardware implementation, although other hardware implementations are envisioned as well. A microcontroller  810  generates a nonsinusoidal waveform, samples output voltage, and adjusts duty width according to embodiments of the invention. The microcontroller provides between two and six outputs  820  in embodiments of the invention. A battery  830 , a high-frequency transformer  840 , and an H-bridge  850  are used in this implementation. In the example configuration shown in  FIG. 8 , microcontroller outputs  820  labeled A, B, C, and D are coupled to corresponding switches  845  of the H-bridge  850  labeled A, B, C, D. Different pairs of switches  845  diagonally across the load  835  (e.g., A/D or B/C) in the H-bridge  850  may be turned on or off to change the polarity of power delivered to a load  835 . It should be understood by one of ordinary skill in the art that less than 2 or greater than six microcontroller outputs could also be used to control the switches  845 , depending on different control signal configurations used to control the load. 
     Thus, embodiments of the invention apply respectively to low frequency and high frequency UPS designs. 
     Referring back to  FIG. 5 , in other embodiments of the invention, if the output pulse is still on ( 555 ), the output voltage is sampled at multiple instants within a given cycle, e.g., at every tick, although other intra-cycle sampling methods may be used ( 560 ). The present half-cycle output RMS voltage, i.e., the RMS voltage of the output voltage samples encountered so far in the given cycle, is calculated ( 565 ). If the output voltage detected to be rising ( 570 ), e.g., based on a difference in samples greater than a threshold such as a 5 V difference, then the PFC load is known to be demanding less power over time, i.e., reducing power consumption. In this case, the voltage waveform is zeroed (set to zero voltage) if the present half-cycle output RMS voltage has reached the desired (nominal) output RMS voltage ( 575 ). 
     Thus, when the PFC load stops drawing a large current from the square wave UPS, embodiments of the invention track the output voltage and disable it (e.g., zero it) when it reaches the nominal output voltage (e.g., 115 V in the United States) precisely. 
     In this situation, the internal DC bus of the load is not at a low voltage level, and embodiments of the present invention maintain the output voltage at the nominal voltage precisely so that the load will not be over-charged. 
     With this strategy, both the PFC load and the square waveform UPS output become stable. Thus, embodiments of the present invention resolve the incompatibilities of square wave UPS with active PFC loads. 
       FIGS. 9A-C  are signal traces showing signal waveforms in accordance with an embodiment of the invention. The techniques described herein have been tested with square wave UPS systems and active PFC loads, and  FIGS. 9A-C  demonstrate the efficacy of embodiments of the invention.  FIG. 9A  shows output voltage  910 - a  and output current  920 - a  when an active PFC load draws power from a square wave UPS after a power outage  930 , without applying embodiments of the present invention. The output voltage is seen to fluctuate (indicative of poor voltage regulation), and the output current is seen to surge.  FIG. 9B  shows the result of applying embodiments of the invention. Output voltage  910 - b  and output current  920 - b  are seen to be better regulated.  FIG. 9C  shows the result of applying embodiments of the invention and waiting until circuit behavior has stabilized. Output voltage  910 - c  and output current  920 - c  are seen to be well-regulated periodic waveforms. 
     Embodiments of the invention may be implemented in hardware, firmware, or software to enable compatibility between nonsinusoidal UPS systems and active PFC loads. In some embodiments, no additional hardware components are required beyond square wave UPS systems already available, thus minimizing cost. In other words, software may be executed to control the nonsinusoidal voltage waveform. The software may be in the form of instructions stored on a computer-readable medium, which, when executed, cause a processor to perform the processes described above and in  FIGS. 3 and 5 . 
     With embodiments of the invention, the unstable transient state of the UPS system is shortened, stabilizing the UPS output. Thus, the capability of the UPS system to support PFC loads is improved. 
     While this invention has been particularly shown and described with references to example embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims. 
     The disclosed embodiments can be applied to any UPS topology, such as off-line, line-interactive, or double conversion. 
       FIG. 10  is a block diagram of a typical prior art UPS  10  used to provide regulated uninterrupted power in an off-line (standby) UPS topology. The UPS  10  includes an input filter/surge protector  12 , a transfer switch  14 , a controller  16 , a battery  18 , a battery charger  19 , an inverter  20 , and a DC-DC converter  23 . The UPS also includes an input  24  for coupling to an AC power source and an outlet  26  for coupling to a load (not shown). 
     The UPS  10  operates as follows. The filter/surge protector  12  receives input AC power from the AC power source through the input  24 , filters the input AC power and provides filtered AC power to the transfer switch  14  and the battery charger  19 . The transfer switch  14  receives the AC power from the filter/surge protector  12  and also receives AC power from the inverter  20 . The controller  16  determines whether the AC power available from the filter/surge protector  12  is within predetermined tolerances, and if so, controls the transfer switch  14  to provide the AC power from the filter/surge protector  12  to the outlet  26 . A rectifier (not shown) is used to convert alternating current (AC) to direct current (DC) for passing through the battery  18 . 
     If the AC power from the rectifier is not within the predetermined tolerances, which may occur because of “brown out,” “high line,” or “black out” conditions, or due to power surges, then the controller  16  controls the transfer switch to provide the AC power from the inverter  20 . The DC-DC converter  23  is an optional component that converts the output of the battery  18  to a voltage that is compatible with the inverter  20 . Depending on the particular inverter and battery used the inverter  20  may be operatively coupled to the battery  18  either directly or through a DC-DC converter. 
     The inverter  20  of the prior art UPS  10  receives DC power from the DC-DC converter  23 , converts the DC voltage to AC voltage, and regulates the AC voltage to predetermined specifications. The inverter  20  provides the regulated AC voltage to the transfer switch  14 . Depending on the capacity of the battery  18  and the power requirements of the load, the UPS  10  can provide power to the load during brief power source “dropouts” or for extended power outages. In typical medium power, low cost inverters, such as inverter  20  of UPS  10 , the Z waveform of the AC voltage has a rectangular shape rather than a sinusoidal shape. 
       FIG. 11  is a circuit diagram of a typical prior art inverter circuit  1100  coupled to a DC voltage source  1118   a  and coupled to a typical load  1126  comprising a load resistor  1128  and a load capacitor  1130 . The DC voltage source  1118   a  may be a battery (not shown). 
     Typical loads have a capacitive component due to the presence of an EMI filter in the load. The inverter circuit  1100  includes four switches S 1 , S 2 , S 3 , and S 4  in a similar configuration as the switches  845  of  FIG. 8 . Each of the switches S 1 -S 4  is implemented using power MOSFET devices which include respective transistors  1106 ,  1112 ,  1118 ,  124  having respective intrinsic diodes  1104 ,  1110 ,  1116 , and  1122 . Each of the transistors  1106 ,  1112 ,  1118  and  1124  has a gate, respectively  1107 ,  1109 ,  1111 , and  1113 . As understood by those skilled in the art, each of the switches S 1 -S 4  can be controlled using a control signal input to its gate. 
       FIG. 12  is a block diagram of a line interactive UPS topology for providing AC power to a load. One embodiment of an uninterruptible power supply in accordance with the present invention will now be described with reference to  FIG. 12 . The UPS  1210  includes an input  1212  to receive AC power from an AC power source, an output  1214  that provides AC power, a battery  1222  coupled to a DC to DC converter  1215 , an inverter  1218  operatively coupled to the DC to DC converter  1215  to receive DC power and to provide AC power, a transfer relay  1216  selectively coupled to the UPS input  1212  and the inverter  1218 , a UPS controller  1217 , an EMI/Surge filter  1221 , a battery charger  1225  and an automatic voltage regulation (AVR) transformer  1220  coupled to the transfer relay  1216 , and at least one AVR relay  1233 . The UPS  1210  further includes a UPS controller  1217  that monitors and controls the operation of the UPS  1210 . The AVR  1220  and its associated relays are optional devices used in at least one embodiment to allow the UPS  1210  to operate over a wider range of input voltages. 
     The UPS  1210  operates as follows. The UPS  1210  receives input AC power from the AC power source through the input  1212 , filters the input AC power and provides filtered AC power to the transfer relay  1216 . The transfer relay  1216  receives the filtered power and also receives power from the DC/AC inverter  1218 . The controller  1217  determines whether the power available from the AC power source is within predetermined tolerances, and if so, controls the transfer relay to provide the power from the AC power source to the output. If the power from the AC power source is not within the predetermined tolerances, which may occur because of “brown out” or “black out” conditions, or due to power surges, then the controller  1217  controls the transfer relay  1216  to provide the power from the inverter  1218 . 
     The inverter  1218  of the UPS  1210  receives DC power from the DC-DC converter  1215  and converts the DC power to AC power and regulates the AC power to predetermined specifications. Depending on the capacity of the battery  1222  and the power requirements of the load, the UPS  1210  can provide power to the load during brief power source dropouts or for extended power outages. 
     Using data stored in associated memory, the controller  1217  performs one or more instructions and monitors and controls operation of the UPS  1210 . In some examples, the controller  1217  may include one or more processors or other types of controllers. In one example, the controller  1217  is a commercially available, general purpose processor. In another example, the controller  1217  performs a portion of the functions disclosed herein on a general purpose processor and performs another portion using an application-specific integrated circuit (ASIC) tailored to perform particular operations. As illustrated by these examples, embodiments in accordance with the present invention may perform the operations described herein using many specific combinations of hardware and software and the invention is not limited to any particular combination of hardware and software components. 
     The associated memory of the controller  1217  includes data storage that stores computer readable and writable information required for the operation of the UPS  1210 . This information may include, among other data, data subject to manipulation by the controller  1217  and instructions that are executable by the controller  1217  to manipulate data. The data storage may be a relatively high performance, volatile, random access memory such as a dynamic random access memory (DRAM) or static memory (SRAM) or may be a nonvolatile storage media such as magnetic disk or flash memory. Various embodiments in accord with the present invention can organize the data storage into particularized and, in some cases, unique structures to perform the aspects and functions disclosed herein. In addition, these data structures may be specifically configured to conserve storage space or increase data exchange performance.