AC power supply apparatus with economy mode and methods of operation thereof

[0002] The present invention relates to power supply apparatus and methods of operation thereof, and more particular, to AC power supply apparatus and methods.

[0003] Power supply circuits are commonly used in equipment such as uninterrupted (or “uninterruptible”) power supplies (UPSs), motor drives, and other applications. Conventional UPSs use a variety of different circuit topologies, including standby, line-interactive and on-line topologies. Generally, each of these topologies has advantages and disadvantages and, accordingly, selection of a particular topology is typically governed by the needs of the application.

[0004] For example, a typical standby UPS topology includes a transfer switch that directly connects the load to a primary AC power source under normal conditions and that transfers the load to a secondary AC source derived from a battery or other auxiliary source when the primary AC power source fails. Due to the time needed to operate the transfer switch, such a standby UPS often exhibit a significant interruption in power delivered to the load, which may be unacceptable for data processing or other critical tasks. In addition, standby UPSs often do not compensate for power quality, e.g., voltage degradation, harmonic distortion and low power factor. Nonetheless, the standby topology is often used for low-cost UPSs, as it is often cheaper to produce than other topologies. A line interactive UPS may use a transfer switch arrangement similar to that of the standby topology, but may also include means for regulating and conditioning the AC power source to improve power quality.

[0005] Many UPSs use an online topology. For example, a typical online series train UPS includes a series train AC/AC converter that produces an AC output voltage at a load from an AC input voltage provided by an AC power source such as a utility, but includes a DC link that is used to isolate the load from disturbance and other degradation of the AC power source. The intermediate DC bus is typically coupled to an inverter that inverts the DC voltage on the DC bus to produce an AC output. Other circuits, such as filters and regulators, may be included in the path with the rectifier and the inverter. Typically, the DC bus is also coupled to an auxiliary source of power, such as a battery, which maintains the DC voltage on the DC bus in the event the AC power source fails. Some online UPSs use circuit topologies other than a series train, including more complex topologies, such as delta converters, or other techniques.

[0006] Under normal operating conditions, online UPS's supply power to a load through a rectifier/inverter chain or similar regulating circuitry, providing relatively clean and regulated power at the output of the UPS. When the AC power source fails, the UPS may achieve an uninterrupted transition to battery power, as there typically is no need to change the state of a transfer switch. Some online series train UPSs also include a bypass feature such that, in the event of a failure, the inverter is prevented from supplying power to the load (e.g., by disconnecting it from the load or placing it into an inactive standby state) while the load is connected to the AC power source via a bypass path. Such a feature may also be used to provide an “economy” mode of operation, as power dissipation associated with the operation of the rectifier/inverter chain may be reduced when the load is transferred to the bypass path.

[0007] However, such an approach may be compromised if the AC power source is subject to small but frequent excursions outside of the power quality criteria, as may be the case, for example, in systems powered by an unsteady AC power source, Such as an auxiliary generator set. In such a case, the UPS may be forced to power the load through the inverter in order to meet the power quality requirements of the load, thus reducing the opportunity for greater efficiency through bypass operation.

[0008] According to embodiments of the invention, improved efficiency may be achieved in a power supply apparatus (e.g., a UPS circuit) that incorporates an AC/DC converter that produces an intermediate DC voltage from an AC input voltage. The power supply apparatus includes a multi-mode DC/AC converter circuit that provides a first component of power, for example, a real power component, while a bypass circuit provides a second component of power, for example, a harmonic power component and/or a reactive power component, to the load from an AC power source. The DC/AC converter circuit may include a current mode controlled inverter that provides reactive and harmonic currents to the load such that the bypass circuit predominantly transfers real power between the AC source and the load. In this manner, power factor and other power quality parameters at the AC source may be maintained at desired levels.

[0009] In particular, in embodiments of the invention, a power supply apparatus includes an AC input port, an output port, and a bypass circuit that couples the AC input port to the output port. An AC/DC converter circuit, coupled to the AC input port, produces a DC voltage from an AC input voltage at the AC input port. A DC/AC converter circuit, coupled to the AC/DC converter circuit, controls current transfer between the output port and the AC/DC converter circuit responsive to a control input such that respective first and second component currents of a current delivered to a load coupled to the output port pass via respective ones of the bypass circuit and the DC/AC converter circuit. The DC/AC converter circuit may be operated such that current passing through the bypass circuit is constrained to be substantially in phase with the AC input voltage.

[0010] In other embodiments of the invention, a power supply apparatus includes an AC input port, first and second voltage busses, and an output port. An AC/DC converter circuit, coupled to the AC output port and to the first and second voltage busses, produces a DC voltage across the first and second voltage busses from an AC input voltage at the AC input port. A first switching circuit selectively couples the AC input port and the output port responsive to a first control input. A second switching circuit selectively couples the output port and the first and second voltage busses responsive to a second control input. A control circuit generates the first and second control inputs such that, in a first mode of operation, the first switching circuit decouples the AC input port from the output port while the second switching circuit supplies power to the output port. In a second mode of operation, the first switching circuit couples the AC input port to the output port and supplies a first component of a power at the output port while the second switching circuit supplies a second component of the power at the output port. In the second mode of operation, the control circuit may generate the second control input such that the current passing through the first switching circuit is substantially in phase with the AC input voltage.

[0011] In yet other embodiments of the invention, an uninterruptible power supply (UPS) apparatus includes an AC input port, an AC output port, and a bypass circuit that couples and decouples the AC input port to and from the AC output port responsive to a first control input. A series train AC/AC conversion circuit, coupled to the AC input port and to the AC output port, transfers power between the AC input port and the AC output port such that, in a first mode of operation, the series train AC/AC conversion circuit transfers a first component of power from the AC input port to the AC output port while the bypass circuit transfers a second component of power from the AC input port to the AC output port. In a second mode of operation, the series train AC/AC conversion circuit transfers power from the AC input port to the AC output port while the bypass circuit decouples the AC input port from the AC output port. In the first mode of operation, the series train AC/AC conversion circuit may be operated such that it causes the bypass circuit to predominantly transfer real power from the AC input port to the AC output port. The series train AC/AC conversion circuit may include an AC/DC converter circuit, such as a rectifier circuit, that produces a DC voltage from an AC voltage at the AC input port, and a DC/AC converter circuit, such as an inverter circuit, that controls current transfer between the AC/DC converter circuit and the AC output port. The series train AC/DC converter circuit may also include an auxiliary DC input port that provides DC power to the DC/AC converter circuit.

[0012] According to still other embodiments of the invention, a UPS apparatus includes an AC input port, an AC output port, and a bypass circuit that couples and decouples the AC input port to and from the AC output port responsive to a first control input. A rectifier circuit, coupled to the AC input port, produces a DC voltage at a rectifier circuit output from an AC input voltage at the AC input port. An inverter circuit, coupled to the rectifier circuit output and to the AC output port, transfers power between the rectifier circuit and the AC output port such that, in a first mode of operation, the inverter circuit transfers a first component of power to the AC output port while the bypass circuit transfers a second component of power to the AC output port. In a second mode of operation, the inverter circuit transfers power to the AC output port while the bypass circuit decouples the AC input port from the AC output port. In the first mode of operation, the inverter circuit may be operated such that it causes the bypass circuit to predominantly transfer real power from the AC input port to the AC output port. The UPS apparatus may further include an auxiliary DC power source that provides a DC voltage to the inverter circuit.

[0013] According to method embodiments of the invention, AC power is provided to a load. An AC power source is coupled to the load through a bypass path. The AC power source is also coupled to an intermediate bus such that a DC voltage is generated at the intermediate bus. Current transfer between the intermediate bus and the load is controlled such that respective first and second component currents of a current delivered to the load pass via respective ones of the bypass path and the intermediate bus. Current transfer between the intermediate bus and the load may be controlled such that predominantly real power is delivered to the load via the bypass path, and such that predominantly non-real power components are delivered to the load via the intermediate bus.

[0014] The apparatus and methods of the present invention may provide improved ways for providing AC power to a load. For example, in a UPS application according to embodiments of the invention, improved efficiency may be provided by operating a series train UPS in an economy mode wherein a bypass circuit provides real power and the series train provides reactive and harmonic power. This may provide more power quality and security than offered by conventional bypass modes. It will be appreciated that the invention is applicable to a variety of single and polyphase AC applications.

[0015] FIG. 1 is a schematic diagram illustrating a power supply apparatus according to embodiments of the invention.

[0016] FIG. 2 is a schematic diagram illustrating a power supply apparatus according to other embodiments of the invention.

[0017] FIG. 3 is a schematic diagram illustrating a three-phase power supply apparatus according to embodiments of the invention.

[0018] FIGS. 4 and 5 are schematic diagrams illustrating a multi-mode DC/AC converter circuit according to embodiments of the invention.

[0019] FIG. 6 is a schematic diagram illustrating alternative control configurations according to embodiments of the invention.

[0020] FIG. 7 is a schematic diagram illustrating a current mode controlled inverter circuit according to embodiments of the invention.

[0021] FIG. 8 is a schematic diagram illustrating an uninterruptible power supply according to embodiments of the invention.

[0022] The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, like numbers refer to like elements.

[0023] FIG. 1 illustrates a power supply apparatus 100 according to embodiments of the present invention. The power supply apparatus 100 includes an AC/DC converter circuit 110, such as rectifier circuit, that produces a DC voltage V_DCfrom an AC input voltage ν_in, produced by an AC power source 10 coupled to an AC input port 101 of the power supply apparatus 100. The power supply apparatus 100 also includes a bypass circuit 130 that is operative to couple and decouple the AC input port 101 to an output port 102 of the power supply apparatus 100 responsive to a first control input 131. The power supply apparatus 100 further includes a multi-mode DC/AC converter circuit 120 that, in a first mode in cooperation with bypass circuit 130, controls current transfer between the AC/DC converter circuit 110 and the output port 102 responsive to a control input 121 such that respective first and second component currents i₁, i₂of a current i_outdelivered to a load 20 coupled to the output port 102 are supplied by respective ones of the bypass circuit 130 and the multi-mode DC/AC converter circuit 120 when the AC input port 101 and the output port 102 are coupled by the bypass circuit 130. As described in greater detail below, when operating in this mode, the multi-mode DC/AC converter circuit 120 preferably controls the current i₂such that the bypass circuit 130 predominantly delivers real power to the load 20 while the multi-mode DC/AC converter circuit 120 predominantly supplies reactive and harmonic power components associated with the load 20, thus generally acting as a tracking line filter. In a second mode, the AC input port 101 is decoupled from the output port 102 by the bypass circuit 130, and the current i_outis preferably supplied substantially solely by the multi-mode DC/AC converter circuit 120, operating in an on-line series train fashion.

[0024] The AC/DC converter circuit 110 may comprise any of a number of different circuits that are operative to produce a DC voltage from an AC input voltage, such as passive and active rectifier circuits. The multi-mode AC/DC converter circuit 120 may comprise any of a number of different types of circuits capable of controlling current flow between the AC/DC converter circuit 110 and the output port 102, such as current mode controlled inverter circuits. The bypass circuit 130 may comprise any of a number of switching circuits, such as those employing solid state or mechanical relays, insulated gate bipolar transistors (IGBTs), and silicon control rectifiers (SCRs). As described below, the bypass circuit 130 preferably employs an antiparallel (back to back) SCR configuration that is controlled such that a relatively seamless transition between modes of operation of the power supply apparatus can be achieved.

[0025] FIG. 2 illustrates a power supply apparatus 200 according to other embodiments of the invention. The power supply apparatus 200 includes an AC/DC converter circuit 210 that produces a DC voltage V_DCfrom an AC input voltage ν_inand a bypass circuit 230 that couples and decouples an AC input port 201 from an output port 202 responsive to a mode control signal 231. The power supply apparatus 200 also includes a multi-mode DC/AC converter circuit 220 that controls a current i₂passing between the output port 202 and the AC/DC converter circuit 210 responsive to an output current i_outpassing between the output port 202 and a load 20 sensed by a current sensor 240, an output voltage ν_outat the output port 202, and the AC input voltage v_in.

[0026] The multi-mode DC/AC converter circuit 220 is also operative to transition between first and second operating modes responsive to the mode control signal 231. In a first mode, the multi-mode DC/AC converter circuit 220 and the bypass circuit 230 operate such that respective first and second component currents i₁, i₂of the output current i_outare supplied via respective ones of the multi-mode DC/AC converter circuit 220 and the bypass circuit 230. In a second mode, the multi-mode DC/AC converter circuit 220 and the bypass circuit 230 preferably operate such that the load 20 is exclusively supplied via the multi-mode DC/AC converter circuit 220.

[0027] In either of the first and second modes, the multi-mode DC/AC converter circuit 220 preferably determines a differential power demand at the output port 202 responsive to a sensed output current i_out, a sensed output voltage signal v_outand a sensed input voltage signal ν_indetermines a level for the current i₂that supports the determined power demand and controls the current i₂to provide the determined level. In the second mode, the multi-mode DC/AC converter circuit 220 determines a total power demand and, preferably, determines a level for the current i₂that meets this total power demand. In the first mode the multi-mode DC/AC converter circuit 220 preferably determines a level for the current i₂that meets a non-real component of the total power demand at the output port 202, i.e., a component associated with harmonic and reactive currents needed to support the total power demand at the output port 202, while the bypass circuit 230 supplies real power to the output port 202, i.e., such that the component current i₁is substantially in phase with the AC input voltage ν_in.

[0028] FIG. 3 illustrates an exemplary three-phase power supply apparatus 300 according to yet other embodiments of the invention, in particular, a power supply apparatus configured for use in a UPS. The power supply apparatus 300 includes an AC input port 301 that is coupled to an input of an AC/DC converter circuit 310, here shown as a three-phase passive rectifier circuit. The AC/DC converter circuit 310 produces a DC voltage V_DCacross first and second voltage busses 311, 312. The AC input port 301 is also coupled to a bypass circuit 330, here shown as including a three-phase array 334 of antiparallel-connected SCRs controlled by an SCR gating control circuit 332 responsive to a mode control signal 331 and the AC input voltage ν_in. An auxiliary DC power source 370 is coupled to the first and second voltage busses 311, 312 such that an alternative means for maintaining the DC voltage V_DCis provided in the event, for example, that an AC source coupled to the AC input port 301 fails.

[0029] The power supply apparatus 300 also includes a multi-mode DC/AC converter circuit 320 that controls current flow between the first and second voltage busses 311, 312 and an output port 302. As shown, the multi-mode DC/AC converter circuit 320 includes a switching circuit 326 configured in a three-phase inverter configuration, controlled by a control circuit 325 and switchably coupled to the output port 302 by a switch 360. The control circuit 325 includes a current mode control circuit 324 that controls the switching circuit 326 responsive to a reference current signal i_ref(current command) and a sense inverter current signal i_invertergenerated by a first current sensor 350. The control circuit 325 also includes a voltage control circuit 322 that generates the reference current signal i_refresponsive to a sensed output voltage signal ν_outat the output port 302, a sensed output voltage signal ν_inverterproduced by the switching circuit 326, the sensed AC input voltage signal ν_in, and a sensed output current signal i_outgenerated by a second current sensor 340.

[0030] The reference current i_refis generated by the voltage control circuit 322 depending on the mode control signal 321. In a first, “economy” mode, the voltage control circuit 322 generates the reference current signal i_refsuch that respective first and second power components are delivered to the output port 302 by respective ones of the bypass circuit 330 and the switching circuit 326, preferably such that the bypass circuit 330 predominantly delivers real power to the output port 302 while the switching circuit 326 delivers predominantly reactive and harmonic power components. In a second, “normal” mode, the bypass circuit 330 decouples the input port 301 from the output port 302, and the voltage control circuit 322 produces the reference current i_refsuch that the total power demand at the output port 302 is served by the switching circuit 326.

[0031] In normal mode, i.e., when the switch 360 is closed and the bypass circuit 330 is open, the DC/AC converter circuit 320 supplies power to the load. To initiate transition to the economy mode, the voltage control circuit 322 first matches the phase and amplitude of the inverter output voltage ν_inverterto the AC input voltage ν_in, and then the SCR gating control circuit 332 commences gating of the SCRs of the bypass circuit 330. The voltage control circuit 322 determines a real current demand at the output port 302 responsive to the sensed output voltage signal ν_outand the output current signal i_out, and produces the reference current signal i_refbased on this determined current demand and synchronized with the AC input voltage signal ν_in. To transition back to normal mode, the SCR gating control circuit 332 ceases gating the SCRs, thus decoupling the input port 301 from the output port 302.

[0032] The SCR gating control circuit 332 may be operated such that a substantially seamless transition between the two operating modes can be achieved under conditions that might otherwise cause a significant disruption of the voltage ν_outproduced at the output port 302. Preferably, the SCRs are gated such that each SCR receives a gate pulse only when its anode is positive with respect to its cathode. For example, when the SCR gating circuit 332 is operated in such a manner, if the AC input port 302 is shorted to ground during a half-cycle of the AC input voltage ν_inwhen operating in economy mode, conduction between the output port 302 and the AC input port 301 can be avoided because the currently conducting SCR of each antiparallel connected pair will automatically turn off and the other SCR of the pair will remain inactive. Accordingly, the power supply apparatus 300 can transition the load 20 to the DC/AC converter circuit 320 with reduced disruption at the output port 302.

[0033] It will be appreciated that the multi-mode DC/AC converter circuit 320 is offered for exemplary purposes, and that, in general, any of a number of other circuit configurations may be used with the invention. For example, additional regulation components may be included in the path of the AC/DC converter 310 and the DC/AC converter 320, such as DC voltage regulators and neutral regulators. It will also be appreciated that any of number of different circuits capable of controlling current flow between the first and second voltage busses 311, 312 and the output port 302 may be used with the invention. The current mode control circuit 324 may comprise any of a number of different controller designs, and preferably has bandwidth sufficient to allow the DC/AC converter circuit 320 to modulate the inverter current i_inverterat frequencies that are multiples of the fundamental frequency of the AC input voltage ν_in,. The power control circuit 322 may also take any of a number of different control configurations. It will be appreciated that, in general, the current mode control circuit 324 and the voltage control circuit 324 may be implemented using analog and/or digital control circuits, and may be implemented in special purpose hardware, as software or firmware executing on a general or special purpose computing device, or as combinations thereof.

[0034] FIG. 4 illustrates an exemplary multi-mode DC/AC converter circuit 400 according to embodiments of the invention. The DC/AC converter circuit 400 includes a hardware current mode control circuit 420 that operates a switching circuit 410 as a current controlled inverter. The switching circuit 410 is switchably coupled by a switch 490 to an output port 403 to which a load and a bypass circuit may be connected. The current mode control circuit 420 controls the current between the first and second voltage busses 401, 402 (across which a DC voltage V_DCis maintained) and the output port 403 responsive to a reference current signal i_refgenerated by a voltage control circuit 430 and to an inverter current signal i_invertergenerated by a current sensor 485.

[0035] The voltage control circuit 430 is here shown as including a plurality of modules executing on a combination of a data processor (e.g., a microprocessor, digital signal processor (DSP), or combination thereof) and an associated memory. The modules include an RMS voltage calculation module 441 that computes an RMS voltage V_rmsfrom an output voltage signal ν_outsensed at the output port 403, an economy mode control module 440 that generates a first reference current signal i_ref1, a normal mode control module 450 that generates a second reference current signal i_ref2, and a selector module 470 that applies one of the first and second reference current signals i_ref1, i_ref2, to the current mode control circuit 420 responsive to a mode control signal 471.

[0036] The economy mode control module 440 includes a power calculation module 442 that calculates the power P_outresponsive to the sensed output voltage signal ν_outand to a sensed output current signal i_outgenerated by a second current sensor 480. A divider module 443 divides the calculated power P_outby the calculated RMS voltage V_rms, producing a calculated current magnitude I^*that represents an estimate of the magnitude of a real current component (i.e., a current substantially in phase with the AC input voltage ν_in,) of the output current signal i_out. The computed current magnitude I^*is modulated at a multiplier module 444 by a reference signal r generated by a phase lock loop 460 from the output voltage signal ν_outand the AC input voltage signal ν_in. The resulting current signal i^*represents a desired current to be provided to the output port via the bypass circuit (not shown) such that current carried by the bypass circuit is substantially in phase with the AC input voltage signal ν_inand the bypass circuit predominantly delivers the real power demand at the output port 403. The current signal i^*is subtracted from the sensed output current signal i^outat a summing module 445 to determine the first reference current signal i_ref1. In an economy mode commanded by a mode control signal 471, the first reference current signal i_ref1is selected by the selector module 470 as the reference current signal i_reffor the current mode control circuit 420.

[0037] Referring to FIG. 5, in the normal mode, a reference current signal i_ref2generated by the normal mode control circuit 450 responsive to the output voltage signal ν_outis selected for application to the current mode control circuit 420. The normal mode control module 450 includes a first summing module 451 that subtracts the computed RMS voltage V_rmsfrom a reference voltage signal V_refwhich may be provided by any of a variety of control circuits. The resulting rms voltage error signal V_eis then processed by an RMS voltage control module 452 and multiplied by the reference signal r in a multiplier module 453, and the resulting signal is subtracted from a sensed inverter output voltage ν_inverterin a summing module 454 to produce an error voltage signal ν_e. The error voltage signal ν_eis processed by a voltage control module 455 to generate the second reference current signal i_ref2.

[0038] Preferably, the processing operations described with reference to FIGS. 4 and 5 are partitioned between a microcontroller, such as a Siemens ST10167, and a DSP, such as a DSP from Texas Instruments TMS320XX family of DSP chips. More preferably, computationally intensive operations, such as RMS voltage calculations and power calculations, are performed by the DSP chip, while other functions, such as the phase lock loop and the selector 470, are performed by the microcontroller. However, it will be appreciated that other digital signal processing architectures may be used with the present invention. It will also be appreciated that many of the digital signal processing functions may be implemented using corresponding analog signal processing architectures.

[0039] It will be appreciated that, although FIGS. 3-5 and the accompanying description refer to a three-phase UPS application, the invention is applicable to other AC power applications. In general, it will be appreciated that the invention is applicable to single and polyphase systems, and to AC power applications other than UPSs.

[0040] The invention also is not limited to the control architectures illustrated in FIGS. 2-5. As illustrated in FIG. 6, a power supply apparatus 600 includes an AC/DC converter circuit 610 that produces a DC voltage V_DCfrom an AC input voltage ν_inat in AC input port 601, and a bypass circuit 630 that couples and decouples the AC input port to and from an output port 602 responsive to a mode control signal 631. The power supply apparatus 600 also includes a multi-mode DC/AC converter circuit 620 that controls a current flowing between the AC/DC converter circuit 610 and the output port 602 such that, when the bypass circuit 630 is closed, respective first and second current components i₂, i₁of an output current i_outdelivered to a load 20 are carried by respective ones of the bypass circuit 630 and the multi-mode DC/AC converter circuit 620. Here, however, the current i₂may be controlled responsive to the current i₁passing through the bypass circuit 630 and sensed by a current sensor 640. For example, the multi-mode DC/AC converter circuit 620 may compare the sensed current i₁to an ideal current waveform (e.g., a sinusoidal current waveform in phase with the AC input voltage ν_in), and generate a current reference signal based on this comparison that may be used by a current mode control circuit (not shown) within the multi-mode DC/AC converter circuit 620 to control the current i₂. It will be appreciated that, in general, a variety of different state variables associated with the power supply apparatus may be used to generate a control input that produces a desired waveform for the current i₂.

[0041] FIG. 7 illustrates an exemplary current mode control circuit according to embodiments of the invention, in particular, a hysteretic current control circuit 700 that can provide high-bandwith and favorable switching frequency characteristics. The control circuit 700 controls the operation of an inverter transistor pair 735 that is connected between positive and negative voltage busses +V, −V and has an output connected to an RC output filter including an inductor 740 and a capacitor 745. The control circuit 700 includes a hysteresis limit generating circuit 705 that generates first and second hysteresis limit signals i_refhi, i_refloresponsive to a reference current signal i_refand a filtered output voltage ν_fproduced by the RC output filter circuit. Preferably, the limit signals i_refhi, i_refloare generated such that they track the input reference current i_refand are separated by a voltage that varies based on the filtered output voltage ν_f.

[0042] A current feedback signal if from a current sensor 741 is compared to the limit signals i_refhi, i_refloby first and second comparators 710, 715. Respective output signals produced by the comparators 710, 715 are fed to respective ones of a pair of cross-coupled NAND gates 720, 725 that act as a set-reset (SR) flip-flop. Complementary output signals Q₁, Q₂produced by the NAND gates 720, 725 are provided to a dead-band control circuit 730 that conditions the NAND gate output signals such that corresponding respective gate signals G₁, G₂applied to the transistors of the inverter transistor pair 735 operate the transistors in a break-before-make fashion. With respect to the generation of the upper and lower hysteresis limit signals i_refhi, i_reflo, the following relationship has been found to produce desirable performance: $H=\frac{\left[1-{\left(2*\frac{v_f}{v_{\mathrm{in}}}\right)}^2\right]}{\left[\frac{4*L}{V_{\mathrm{in}}*f}\right]},$

[0043] where H represents a difference between the reference current signal i_refand each of the upper and lower hysteresis limit signals i_refhi, i_reflo, V_inis the DC voltage applied across the transistor pair 735, ν_fis an instantaneous value of the feedback voltage, L is the inductance of the inductor 740, and 𝑓 is a desired switching frequency for the transistors 735.

[0044] With appropriate control of circuit parameters and other factors, the hysteretic current mode control circuit 700 can constraint the actual frequency at which the transistors 735 operate to be substantially near the desired frequency 𝑓. This is a potential advantage of the hysteretic current mode control circuit 700 with respect to many conventional hysteretic current mode control circuits, which may not provide such frequency control. Preferably, the hysteretic current mode control circuit 700 is operated such that the transistors 700 switch at a frequency that varies over a range, spreading the energy of interference generated by the switching across a range of frequencies and thus potentially aiding compliance with regulatory requirements. Although FIG. 7 illustrates current control for a transistor pair configuration, the hysteretic current mode control technique illustrated in FIG. 7 is also applicable to single-ended transistor configurations. It will also be appreciated that a wide variety of other control circuits may be used with the invention, including, for example, average current mode control circuits using fixed frequency pulse-width modulation (PWM) techniques, as well as non-current mode control circuits that may be operated such that they provide control of current transfer through a DC/AC converter (e.g., inverter) circuit.

[0045] FIG. 8 illustrates an exemplary UPS 800 according to an embodiment of the invention. The UPS includes a power supply circuit 801 that produces a three-phase AC output at terminals L1′, L2′, L3′ from an AC input provided at terminals L1, L2, L3, N or a DC input provided by a battery 865. The power supply circuit 801 is monitored and controlled by a control circuit 802. The power supply circuit 801 includes a series train AC/AC conversion circuit 803 connected to the input terminals L1, L2, L3 by a contactor 805. The series train AC/AC converter circuit 803 includes a passive three-phase rectifier circuit 810, a boost regulator circuit 815 and a three-phase inverter circuit 830. The rectifier circuit 810 produces a DC output voltage that is supplied to the boost regulator circuit 815, which provides a boosted and regulated DC output voltage to the inverter circuit 830. Also included in the power supply circuit 801 is a neutral regulator circuit 820 that balances the DC voltage produced by the boost regulator circuit 815 around the voltage of the neutral terminal N, and a DC filter circuit 825. The output of the inverter 830 is filtered by an output filter 835, which is switchably coupled to the output terminals L1′, L2′, L3′ by a second contactor 840. The power supply circuit 801 also includes a bypass circuit including a third contactor 845 that couples a three-phase SCR bypass circuit 850 to the input terminals L1, L2, L3. The output of the SCR bypass circuit 850 is coupled to the output terminals L1′, L2′, L3′. The power supply circuit 801 further includes a start-up circuit 855, and a battery charger/coupling, circuit 860 that couples the battery 865 to the output of the rectifier circuit 810.

[0046] The control circuit 802 is operative to monitor various voltages and currents within the power supply circuit 801, and to responsively generate appropriate signals for controlling the boost regulator circuit 815, the neutral regulator circuit 820, the inverter circuit 830, the SCR circuit 850, the battery charger/coupling circuit 860 and the contactors 805, 840, 845. In particular, the control circuit 802 is operative to monitor and control these components of the power supply circuit 801 along the lines described above to provide a normal series train mode of operation and an economy mode of operation in which the inverter circuit 830 and the SCR bypass circuit 850 both provide power to the output terminals L1′, L2′, L3′.

[0047] It will be appreciated that the UPS 800 may provide superior functionality in comparison to conventional online UPSs, including those with a conventional bypass capability. The UPS 800 may be operated in an economy mode, as described herein, wherein the inverter circuit 830 is operated as a tracking line filter, generally providing only reactive and harmonic currents at the output terminals L1′, L2′, L3′ while real power is transferred to the output terminals L1′, L2′, L3′ via the SCR bypass circuit 850. The decision as to whether to operate in this economy mode or in a normal series train mode can be conditioned upon power quality at the input terminals L1, L2, L3, as well as on other factors, such as the degree of security desired by the operator of the UPS 800. For example, for periods in which the load served by the UPS 800 is less critical, e.g., during nights or weekends, the operator may choose to exercise the economy mode to realize power savings while accepting an added degree of risk. During periods in which the load is more critical, the operator may choose to disable the economy mode such that the UPS 800 is operating at its highest level of security. Other criteria may also be utilized, such as adaptive criteria that condition use of economy mode based on the conditions experienced by the UPS 800. For example, the UPS 800 may monitor the AC power applied to the input terminals L1, L2, L3, looking for patterns that suggest an increased likelihood of power quality degradation, and may base its decision as to whether to employ the economy mode on this analysis.

[0048] In the drawings and specification, there have been disclosed typical preferred embodiments of the invention and, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being set forth in the following claims.