Abstract:
At least one aspect is directed to an uninterruptible power supply that includes a first input to receive a first input voltage from a first voltage source, a second input to receive a second input voltage from a second voltage source, and a boost circuit coupled to the first input and the second input. The boost circuit is configured to provide a positive output DC voltage and a negative output DC voltage derived from at least one of the first input voltage and the second input voltage. The uninterruptible power supply is configured such that in a back-up mode of operation, the negative output DC voltage is derived from the second input voltage using a circuit that includes at least two controlled switches coupled in series.

Description:
BACKGROUND OF INVENTION 
   1. Field of Invention 
   Embodiments of the invention relate generally to power supplies and more specifically, at least one embodiment relates to a method and apparatus for generating an output voltage derived from an input voltage. 
   2. Discussion of Related Art 
   Uninterruptible power supplies (UPS) for providing power to critical loads are well known.  FIG. 1  provides a block diagram of a typical on-line UPS  100  that provides regulated power as well as back-up power to a load  140 . UPS&#39;s similar to that shown in  FIG. 1  are available from American Power Conversion (APC) Corporation of West Kingston, R.I. The UPS  100  includes a rectifier/boost converter  110 , an inverter  120 , a controller  130  and a battery  150 . The UPS has inputs  112  and  114  to couple respectively to line and neutral of an input AC power source and has outputs  116  and  118  to provide an output line and neutral to the load  140 . 
   In line mode of operation, under control of the controller, the rectifier/boost converter  110  receives the input AC voltage and provides positive and negative output DC voltages at output lines  120  and  122  with respect to a common line  124 . In battery mode of operation, upon loss of input AC power, the rectifier/boost converter  110  generates the DC voltages from the battery  150 . The common line  124  may be coupled to the input neutral  114  and the output neutral  118  to provide a continuous neutral through the UPS  100 . The inverter  120  receives the DC voltages from the rectifier/boost converter  110  and provides an output AC voltage at lines  116  and  118 . 
   Further details of the rectifier/boost converter  110  and the battery  150  are shown in  FIGS. 2A and 2B  with  FIG. 2A  showing the UPS in line mode of operation and  FIG. 2B  showing the UPS in battery mode of operation. The rectifier/boost converter  110  includes input diodes  160 ,  162 , input capacitors  164 ,  166 , relays  168  and  170 , inductors  172  and  174 , boost transistors  176  and  178 , diode  177 , output diodes  180 ,  182 , and output capacitors  184 ,  186 . In addition, the rectifier/boost converter includes a transistor  188  that, as described below functions as part of a buck-boost circuit in the battery mode of operation. 
   In line mode of operation, relays  168 ,  170  are configured as shown in  FIG. 2A  to couple an input AC line voltage at inputs  112 ,  114  to inductors  172  and  174 , such that positive and negative rectified voltages are respectively provided to inductors  172  and  174 . Inductor  172  operates in conjunction with transistor  176  and diode  180  as a positive boost circuit under the control of the controller  130  using pulse width modulation to provide a positive DC voltage across capacitor  184 . Similarly, inductor  174  operates in conjunction with transistor  178  and diode  182  as a negative boost circuit under the control of the controller  130  using pulse width modulation to provide a negative DC voltage across capacitor  186 . The controller may control operation of the boost circuits to provide power factor correction at the input of the uninterruptible power supply, with the input current substantially in phase with the input voltage. 
   In battery or backup mode of operation, for example, upon failure of the AC voltage source, the relays  168 ,  170  are moved, under the control of the controller, to the positions shown in  FIG. 2B  to couple the battery  150  to inductors  172  and  174 . In the battery mode of operation, the positive boost circuit operates as discussed above using the battery voltage to generate the DC voltage across capacitor  184 . To generate the negative voltage across the capacitor  186  in battery mode, the transistor  188 , under the control of the controller, in conjunction with inductor  174  and diode  182  functions as a buck-boost circuit with transistor  188  being cycled off and on. In one version, during each cycle, transistor  178  is turned on immediately prior to transistor  188  being turned on to reduce the voltage across transistor  188  at the time of turn-on to approximately the battery voltage. The drive signal to transistor  178  remains on for the duration of the on time of transistor  188 . There is no current flow in transistor  178  due to the fact that the emitter of transistor  178  is at the battery voltage. When transistor  188  is turned off, transistor  178  is again forward biased and the inductor current flows through diode  177  and transistor  178 . Transistor  178  stays on for 0.5 microseconds to allow transistor  188  to turn off totally, and is then turned off. 
   SUMMARY OF INVENTION 
   At least one aspect of the invention is directed to an improved uninterruptible power supply and method for providing uninterruptible power. The uninterruptible power supply includes a first input to receive a first input voltage from a first voltage source, a second input to receive a second input voltage from a second voltage source, and a boost circuit coupled to the first input and the second input and configured to provide a positive output DC voltage and a negative output DC voltage derived from at least one of the first input voltage and the second input voltage, wherein the uninterruptible power supply is configured such that in a back-up mode of operation, the negative output DC voltage is derived from the second input voltage using a circuit that includes at least two controlled switches coupled in series. 
   The uninterruptible power supply may further include a control circuit coupled to the boost circuit, wherein the boost circuit includes a positive boost circuit and a negative boost circuit, wherein the control circuit is coupled to the positive boost circuit to control the positive boost circuit to generate the positive DC output voltage in a line mode of operation from at least the first input voltage, and wherein the control circuit is coupled to the negative boost circuit to control the negative boost circuit to generate the negative DC output voltage in the line mode of operation from at least the first input voltage. The at least two controlled switches may include a first controlled switch that forms a part of the negative boost circuit, and a second controlled switch coupled to the control circuit, the second input and to the negative boost circuit and configured such that in the back-up mode of operation, a current path is formed through the first controlled switch and the second controlled switch. The uninterruptible power supply may further include a third input to couple to a ground connection of at least one of the first voltage source and the second voltage source. The negative boost circuit may include a first diode coupled between the third input and a first node of the first controlled switch, an inductor having a first node and a second node with the second node coupled to a second node of the first controlled switch, and a second diode coupled to the second node of the first controlled switch. The second controlled switch may have a first node coupled to the second input and a second node coupled to the first node of the first controlled switch. The uninterruptible power supply may further include the second voltage source, and the second voltage source may include a battery coupled between the second input and the third input. The uninterruptible power supply may further include a first switching circuit coupled to the control circuit and operative to selectively couple an input of the positive boost circuit to one of the first input and the second input, and a second switching circuit coupled to the control circuit and operative to selectively couple the second node of the inductor to one of the first input and the third input. The uninterruptible power supply may further include an inverter coupled to outputs of the boost circuit, coupled to the third input and operative to generate an output AC voltage at first and second output nodes derived from the positive DC voltage and the negative DC voltage. The uninterruptible power supply may be configured to provide an uninterrupted neutral connection from the third input to the second output node, and each of the positive boost circuit and the negative boost circuit may be controlled to provide power factor correction. Each of the first controlled switch and the second controlled switch includes a transistor. 
   Another aspect of the invention is directed to a method of generating an output voltage from at least one of a first input voltage source providing a primary voltage and a secondary input voltage source providing a back-up voltage. The method includes in a line mode of operation, generating a positive DC voltage and a negative DC voltage derived from at least the primary voltage; and in a back-up mode of operation, generating a positive DC voltage and a negative DC voltage derived from at least the back-up voltage, wherein in the back-up mode of operation, the negative DC voltage is generated using a plurality of controlled switches operatively coupled in series to create a current path from the secondary input voltage source through each of the plurality of controlled switches. 
   The method may further include controlling each of the plurality of controlled switches using pulse width modulation, such that while in back-up mode of operation each of the plurality of controlled switches is switched between an off state having an off time and an on state having an on time with a ratio between the on time and the off time controlled to provide a regulated output voltage. In the method, the stage of controlling may include controlling the plurality of controlled switches, such that in back-up mode of operation, voltage across a first one of the controlled switches is not greater than the back-up voltage. The method may further include controlling draw of current from the first input voltage source to provide power factor correction, and converting the positive DC voltage and the negative DC voltage into an output AC voltage in both the line mode of operation and the back-up mode of operation. The method may further include rectifying the primary voltage to provide a positive rectified voltage and a negative rectified voltage. The plurality of controlled switches may include a first transistor and a second transistor. 
   Still another aspect of the invention is directed to an uninterruptible power supply. The uninterruptible power supply includes a first input to receive a first input voltage from a first voltage source, a second input to receive a second input voltage from a second voltage source, and means for providing a positive output DC voltage and a negative output DC voltage derived from at least one of the first input voltage and the second input voltage, wherein the means for providing is configured such that in a back-up mode of operation, the negative output DC voltage is derived from the second input voltage using a circuit that includes at least two controlled switches coupled in series. 
   The at least two controlled switches of the uninterruptible power supply may include a first transistor and a second transistor coupled in series, and the means for providing may include means for maintaining a voltage across the first transistor to be less than or equal to the second input voltage. The uninterruptible power supply may further include the second voltage source and the second voltage source may include a battery. The uninterruptible power supply may include means for generating an AC output voltage at an output of the uninterruptible power supply, and an uninterrupted neutral connection from the first input to the output. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
     The accompanying drawings, are not intended to be drawn to scale. In the drawings, each identical or nearly identical component that is illustrated in various figures is represented by a like numeral. For purposes of clarity, not every component may be labeled in every drawing. In the drawings: 
       FIG. 1  is a functional block diagram of an uninterruptible power supply; 
       FIG. 2A  is a schematic diagram of a prior art rectifier/boost converter used in the uninterruptible power supply of  FIG. 1  with the rectifier/boost converter in a first state of operation; 
       FIG. 2B  is a schematic diagram of the rectifier/boost converter of  FIG. 2A  in a second state of operation; 
       FIG. 3A  is a schematic diagram of a rectifier/boost converter in accordance with one embodiment of the invention with the rectifier/boost converter in a first mode of operation; 
       FIG. 3B  is a schematic diagram of the rectifier/boost converter of  FIG. 3A  in a second mode of operation; 
       FIG. 4A  is a plot of a control voltage waveform applied to a buck-boost transistor of the rectifier/boost converter of  FIG. 3B ; 
       FIG. 4B  is a plot of a control voltage waveform applied to a boost transistor of the rectifier/boost converter of  FIG. 3B ; 
       FIG. 4C  is a plot of a current waveform through a negative boost inductor of the rectifier/boost converter of  FIG. 3B ; 
       FIG. 4D  is a plot of a voltage waveform at a first location in the rectifier/boost converter of  FIG. 3B ; and 
       FIG. 4E  is a plot of a voltage waveform across the buck-boost transistor of the rectifier/boost converter of  FIG. 3B . 
   

   DETAILED DESCRIPTION 
   This invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having,” “containing”, “involving”, and variations thereof herein, is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. 
   At least one embodiment of the present invention provides an improved rectifier/boost converter circuit for use, for example, in the uninterruptible power supply of  FIG. 1 . However, embodiments of the present invention are not limited for use in uninterruptible power supplies, but may be used with other power supplies or other systems generally. 
   In the rectifier/boost circuit described above with reference to  FIGS. 2A and 2B , in battery mode of operation, the voltage across transistor  188  during its “off” state will be approximately equal to the battery voltage plus the absolute value of the negative output voltage. In one system, a 100V battery is used and the output negative voltage is −400 volts, resulting in a voltage of approximately 500 volts across transistor  188 . It is desirable to utilize an IGBT for transistor  188 . Typically, IGBT&#39;s are available with voltage ratings of 600 volts and 1200 volts, and because of tolerances and de-rating requirements, it may become necessary to utilize a 1200 volt IGBT for transistor  188 . Higher voltage parts tend to have higher losses, are more expensive and may prevent a design from operating at higher frequencies. 
   As will now be described, in at least one embodiment of the present invention, a rectifier/boost converter that may be used, for example, in a UPS, such as that shown in  FIG. 1 , is configured such that a transistor used in a buck-boost circuit in a back-up mode of operation may be implemented with a lower voltage device. A rectifier/boost converter  210  in accordance with one embodiment of the present invention is shown in  FIG. 3A  in a line mode of operation and in  FIG. 3B  in a battery or back-up mode of operation. The rectifier/boost converter  210  includes input diodes  260 ,  262 , input capacitors  264 ,  266 , relays  268  and  270 , inductors  272  and  274 , boost transistors  276  and  278 , diode  277 , output diodes  280 ,  282 , and output capacitors  284 ,  286 . In addition, the rectifier boost converter includes a transistor  288  that, as described below functions as part of a buck-boost circuit in the battery mode of operation. 
   In line mode of operation, relays  268 ,  270  are configured as shown in  FIG. 3A  to couple an input AC line voltage at inputs  112 ,  114  to inductors  272  and  274 , such that positive and negative rectified voltages are respectively provided to inductors  272  and  274 . Inductor  272  operates in conjunction with transistor  276  and diode  280  as a positive boost circuit under the control of a controller, such as controller  130 , using pulse width modulation to provide a positive DC voltage across capacitor  284 . Similarly, inductor  274  operates in conjunction with transistor  278  and diode  282  as a negative boost circuit under the control of the controller using pulse width modulation to provide a negative DC voltage across capacitor  286 . The controller may control operation of the boost circuits to provide power factor correction at the input of the uninterruptible power supply, with the input current substantially in phase with the input voltage. 
   In battery or backup mode of operation, for example, upon failure of an AC voltage source, the relays  268 ,  270  are moved, under the control of the controller, to the positions shown in  FIG. 3B  to couple the battery  250  to inductors  272  and  274 . In the battery mode of operation, the positive boost circuit operates as discussed above using the battery voltage to generate the DC voltage across capacitor  284 . To generate the negative voltage across the capacitor  286  in battery mode, the transistors  278  and  288 , under the control of the controller, in conjunction with inductor  274  and diode  282  function as a buck-boost circuit with transistor  288  being cycled off and on. In one version, during each cycle, transistor  278  is turned on immediately prior to transistor  288  being turned on and transistor  288  remains on for a short period of time after transistor  278  is turned off. 
   The rectifier/boost circuit  210  of  FIGS. 3A and 3B  is similar to the rectifier/boost circuit  110  of  FIGS. 2A and 2B  with at least one exception. In the circuit  210  the buck-boost transistor  288  is coupled between diode  277  and the negative boost transistor  278 , whereas in circuit  110 , the buck-boost transistor  188  is coupled between the negative boost inductor  174  and the negative boost diode  182 . The coupling of the transistor  288  in converter  210  provides a significant advantage in that it allows a lower voltage IGBT or a low voltage power MOSFET device to be used for transistor  288  for the same battery voltage and negative output voltage, as the voltage across transistor  288  during normal operation of the boost/rectifier converter  210  in the back-up mode does not exceed a value that is substantially equal to the battery voltage. 
   In one example, which will now be described, a PSPICE simulation was performed and a breadboard was made of the circuit of  FIG. 3B  with the component values shown in Table 1. 
                                                     TABLE 1                       Manufacturer/           Ref. No.   Device   Part No.   Value                                260, 262   Diode   General   35   amp/1000 volt               Semiconductor/               GBPC3510       264, 266   Capacitor   Illinois   10   uF/240 vac               Capacitor/106PHC400K       250   Battery       100   Volt       272, 274   Inductor   Falco/T23B16   430   uh       276, 278   IGBT   International   600   volt/20 amp               Rectifier/IRG4PC40U       280   Diode   International   600   volt/15 amp               Rectifier/HFA15PB60       282   Diode   International   1200   volt/16 amp               Rectifier/HFA16PB120       284, 286   Capacitor   Cornell   2200   uf/450 volts               Dubiller/400XE1197       288   Mosfet   International   200   volt/               Rectifier/IRF250       277   Diode   HFA15PB60   600   volt/15 amps       268, 270   Relays   Hasco/HAT901CSDC   30   amp/277 vac                    
The circuit in the simulation included a 100 volt battery and was controlled to provide positive and negative 400 volt output voltages at outputs  120  and  122 .  FIGS. 4A and 4B  provide timing diagrams of control signals  302  and  304  that were respectively applied to the gates of transistors  288  and  278  to control the turning on and off of these devices to provide the buck-boost mode of operation to generate the negative 400 volt output in back-up mode of operation. Each of the transistors is turned on with a high voltage (approximately 11.5 volts) applied to the gate and turned off with a low voltage (approximately 0.0 volts) applied to the gate.  FIG. 4C  provides a waveform  306  of the current through the inductor in the direction from point B to point A on  FIG. 3B ,  FIG. 4D  provides a waveform  308  of the voltage at point A with respect to a common point C, and  FIG. 4E  provides a waveform  310  of the voltage across transistor  288 .
 
   As indicated in  FIGS. 4A and 4B , transistors  278  and  288  are controlled such that transistor  278  is turned on approximately 0.5 microseconds before transistor  288  is turned on and transistor  288  is turned off approximately 0.5 microseconds before transistor  278  is turned off. As indicated in  FIG. 4C , with both transistors  278  and  288  turned on, the absolute value of the current through inductor  274  increases, and the absolute value of the current decreases when the transistors are turned off. Of particular significance is waveform  310  in  FIG. 4E , which indicates that the voltage across transistor  288  never exceeds approximately 100 volts. Accordingly, the buck-boost transistor need not be a high voltage device. 
   As discussed above, embodiments of the invention provide improved circuits for use in uninterruptible power supplies and other electronic devices, while maintaining advantages of prior art devices. In particular, power factor correction may be provided in embodiments of the invention and an uninterruptible neutral may be provided from an input of a UPS to an output of the UPS. 
   In embodiments of the invention discussed above, a rectifier/boost converter includes input capacitors and rectifier diodes. As understood by those skilled in the art, the input capacitors  264  and  266  need not be used in all embodiments, and for an input DC voltage, diodes  160  and  162  need not be included. Further, embodiments of the present invention are described as containing relays that are controlled to selectively couple to a primary voltage source or a backup voltage source. In other embodiments devices and switching circuits other than relays may be used including transistors and diodes, and in some embodiments, an uninterruptible power supply may be configured to derive power from both a primary and backup power source at substantially the same time. Embodiments of the invention may be used with single phase primary voltage sources and may also be used with multiphase sources of various voltages. 
   In describing devices of the invention, circuits and devices are described as having one or more voltage inputs and outputs. Each input and output may include multiple connections for coupling to, for example, respectively a voltage source and a load. 
   Having thus described several aspects of at least one embodiment of this invention, it is to be appreciated various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description and drawings are by way of example only.