Patent Publication Number: US-2022239146-A1

Title: Power supply device

Description:
TECHNICAL FIELD 
     The present disclosure relates to a power supply device. 
     BACKGROUND ART 
     Japanese Patent Laying-Open No. 2011-72155 (PTL 1) discloses an uninterruptible power supply device that supplies alternating-current (AC) power to a load and charges a battery through a power converter when an AC power supply is in a normal state. The power converter serves to convert AC power into direct-current (DC) power and converts the DC power into AC power. The uninterruptible power supply device is configured to supply electric power discharged from the battery to the load through the power converter when a power interruption occurs in the AC power supply. The uninterruptible power supply device stops discharging of the battery when the voltage discharged from the battery becomes equal to or lower than a discharge cut-off voltage. 
     CITATION LIST 
     Patent Literature 
     PTL 1: Japanese Patent Laying-Open No. 2011-72155 
     SUMMARY OF INVENTION 
     Technical Problem 
     In the above-mentioned uninterruptible power supply device, the power converter includes a voltage converter (a step-up/step-down chopper) for performing voltage conversion for a DC voltage on the battery, and a capacitor for smoothing the DC voltage generated by the voltage converter. During discharging of the battery, the capacitor is repeatedly charged and discharged in accordance with switching control of a switching element included in the voltage converter, and thereby, a ripple current that cyclically increases and decreases flows through the capacitor. When this ripple current is generated, power loss occurs in an equivalent series resistance (ESR) inside the capacitor, so that the capacitor generates heat. 
     When the electric power supplied to the load increases during discharging of the battery, the electric power associated with charging and discharging of the capacitor also increases, so that the ripple current in the capacitor also increases. As a result, heat generation increases in the ESR of the capacitor, and thereby, the temperature of the capacitor rises, which may accelerate performance deterioration in the capacitor. 
     The present disclosure has been made to solve the above-described problems. An object of the present disclosure is to suppress a temperature rise in a capacitor included in a power supply device and smoothing a DC voltage from a voltage converter. 
     Solution to Problem 
     A power supply device according to the present disclosure includes an inverter, a voltage converter, a capacitor, a current detector, a counter, and a controller. The inverter converts DC power into AC power and supplies the AC power to a load. The voltage converter performs voltage conversion for a DC voltage from a battery. The capacitor smoothes the DC voltage from the voltage converter and inputs the smoothed DC voltage to the inverter. The current detector detects a battery current flowing from the battery into the voltage converter. The counter measures a discharge time of the battery. The controller controls the voltage converter. The controller stops the voltage converter when the measured discharge time of the battery exceeds a discharge permissible time. When the battery current exceeds a threshold value during discharging of the battery, the controller sets the discharge permissible time based on an output from the current detector such that the discharge permissible time is shorter as the battery current is higher. 
     Advantageous Effects of Invention 
     The present disclosure can suppress a temperature rise in a capacitor included in a power supply device and smoothing a DC voltage from a voltage converter. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a circuit block diagram showing a configuration of an uninterruptible power supply device to which a power supply device according to an embodiment is applied. 
         FIG. 2  is a diagram for illustrating a flow of electric power during a power interruption of a commercial AC power supply. 
         FIG. 3  is a circuit block diagram showing a configuration example of a bidirectional chopper shown in  FIG. 1 . 
         FIG. 4  is a waveform diagram for illustrating the operation of the bidirectional chopper during a power interruption of the commercial AC power supply. 
         FIG. 5  is a circuit block diagram showing a configuration of a controller that controls the bidirectional chopper shown in  FIG. 1 . 
         FIG. 6  is a diagram schematically showing the relation between an average value of a battery current and a discharge permissible time. 
         FIG. 7  is a circuit block diagram showing a configuration of a controller that controls a bidirectional chopper in an uninterruptible power supply device according to the second embodiment. 
         FIG. 8  is a diagram schematically showing an example of discharge characteristics of a battery. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The following describes embodiments of the present disclosure in detail with reference to the accompanying drawings. In the following description, the same or corresponding portions in the accompanying drawings will be denoted by the same reference characters, and the description thereof will not be basically repeated. 
     First Embodiment 
     (Configuration of Uninterruptible Power Supply Device) 
       FIG. 1  is a circuit block diagram showing a configuration of an uninterruptible power supply device to which a power supply device according to an embodiment is applied. An uninterruptible power supply device  1  converts three-phase AC power from a commercial AC power supply  21  into DC power, then converts the DC power into three-phase AC power, and supplies the converted three-phase AC power to a load  22 .  FIG. 1  shows only circuits in a portion corresponding to one phase (for example, a U phase) among three phases (a U phase, a V phase, and a W phase) for simplicity of illustration in the drawings and description herein. 
     In  FIG. 1 , uninterruptible power supply device  1  includes an AC input terminal T 1 , an AC output terminal T 2 , and a battery terminal T 3 . AC input terminal T 1  receives AC power of a commercial frequency from commercial AC power supply  21 . AC output terminal T 2  is connected to load  22 . Load  22  is driven by AC power. Battery terminal T 3  is connected to battery  23 . Battery  23  stores DC power. 
     Uninterruptible power supply device  1  further includes electromagnetic contactors  2 ,  8 ,  14  and  16 , current detectors  3  and  11 , capacitors  4 ,  9  and  13 , reactors  5  and  12 , a converter  6 , a bidirectional chopper  7 , an inverter  10 , a semiconductor switch  15 , an operation module  17 , and a control device  18 . 
     Electromagnetic contactor  2  and reactor  5  are connected in series between AC input terminal T 1  and the input node of converter  6 . Capacitor  4  is connected to a node N 1  between electromagnetic contactor  2  and reactor  5 . Electromagnetic contactor  2  is turned on during use of uninterruptible power supply device  1 , and turned off during maintenance of uninterruptible power supply device  1 , for example. 
     The instantaneous value of an AC input voltage Vi appearing at a node N 1  is detected by control device  18 . Based on the instantaneous value of AC input voltage Vi, it is determined, for example, whether a power interruption occurs or not. Current detector  3  detects an AC input current Ii flowing through node N 1 , and supplies a signal Iif showing the detected value to control device  18 . 
     Capacitor  4  and reactor  5  constitute a low-pass filter for: allowing AC power of a commercial frequency to pass from commercial AC power supply  21  to converter  6 ; and preventing a signal of a switching frequency generated in converter  6  from passing through commercial AC power supply  21 . 
     Converter  6  is controlled by control device  18  to convert three-phase AC power into DC power (rectification) and output the converted DC power to a DC line L 1  in a normal state in which AC power is supplied from commercial AC power supply  21 . During a power interruption in which supply of AC power from commercial AC power supply  21  is stopped, the operation of converter  6  is stopped. The voltage output from converter  6  can be controlled to a desired value. 
     Capacitor  9  is connected to DC line L 1  and smoothes the voltage on DC line L 1 . The instantaneous value of a DC voltage VDC appearing on DC line L 1  is detected by control device  18 . DC line L 1  is connected to the node on the high voltage side of bidirectional chopper  7 . The node on the low voltage side of bidirectional chopper  7  is connected to battery terminal T 3  through electromagnetic contactor  8 . 
     Electromagnetic contactor  8  is turned on during use of uninterruptible power supply device  1 , and turned off during maintenance of uninterruptible power supply device  1  and battery  23 , for example. The instantaneous value of the voltage across terminals (hereinafter also referred to as a “battery voltage”) VB of battery  23  that appears at battery terminal T 3  is detected by control device  18 . 
     Bidirectional chopper  7  is controlled by control device  18  to supply the DC power generated by converter  6  to battery  23  to be stored therein in a normal state in which the AC power is supplied from commercial AC power supply  21 , and to supply the DC power of battery  23  to inverter  10  through DC line L 1  during a power interruption. Bidirectional chopper  7  corresponds to one example of a “voltage converter”. 
     When DC power is stored in battery  23 , bidirectional chopper  7  steps down DC voltage VDC on DC line L 1  and supplies the stepped-down DC voltage VDC to battery  23 . When the DC power of battery  23  is supplied to inverter  10 , bidirectional chopper  7  steps up battery voltage VB and outputs the stepped-up battery voltage VB to DC line L 1 . DC line L 1  is connected to an input node of inverter  10 . 
     Inverter  10  is controlled by control device  18  to convert the DC power supplied from converter  6  or bidirectional chopper  7  through DC line L 1  into AC power of a commercial frequency and output the converted AC power. In other words, in a normal state, inverter  10  converts the DC power supplied from converter  6  through DC line L 1  into AC power. Also, during a power interruption, inverter  10  converts the DC power supplied from battery  23  through bidirectional chopper  7  into AC power. The voltage output from inverter  10  can be controlled to a desired value. 
     Inverter  10  has an output node  10 a connected to one terminal of reactor  12 . Reactor  12  has the other terminal (a node N 2 ) connected to AC output terminal T 2  through electromagnetic contactor  14 . Capacitor  13  is connected to node N 2 . 
     Current detector  11  detects an instantaneous value of an output current Io from inverter  10  and supplies a signal Iof showing the detected value to control device  18 . The instantaneous value of an AC output voltage Vo appearing at node N 2  is detected by control device  18 . 
     Reactor  12  and capacitor  13  constitute a low-pass filter for: allowing the AC power of a commercial frequency generated by inverter  10  to pass through AC output terminal T 2 ; and preventing a signal of a switching frequency generated by inverter  10  from passing through AC output terminal T 2 . Inverter  10 , reactor  12 , and capacitor  13  constitute an inverter circuit. 
     Electromagnetic contactor  14  is controlled by control device  18  to be turned on in an inverter power feeding mode in which AC power generated by inverter  10  is supplied to load  22 , and to be turned off in a bypass power feeding mode in which AC power from commercial AC power supply  21  is supplied to load  22 . 
     Semiconductor switch  15  includes a thyristor and is connected between AC input terminal T 1  and AC output terminal T 2 . Electromagnetic contactor  16  is connected in parallel with semiconductor switch  15 . Semiconductor switch  15  is controlled by control device  18  to be turned off in a normal state and to be instantaneously turned on upon occurrence of a failure in inverter  10 , to thereby supply AC power from commercial AC power supply  21  to load  22 . Semiconductor switch  15  is turned off after a prescribed time has elapsed since semiconductor switch  15  was turned on. 
     Reactor  12  and capacitor  13  constitute a low-pass filter for allowing the AC power of a commercial frequency generated by inverter  10  to pass through AC output terminal T 2 , and preventing a signal of a switching frequency generated by inverter  10  from passing through AC output terminal T 2 . 
     Electromagnetic contactor  14  is controlled by control device  18  to be turned on in the inverter power feeding mode and to be turned off in the bypass power feeding mode. 
     Electromagnetic contactor  16  is turned off in the inverter power feeding mode and turned on in the bypass power feeding mode. When a failure occurs in inverter  10 , electromagnetic contactor  16  is turned on to supply the AC power from commercial AC power supply  21  to load  22 . In other words, when a failure occurs in inverter  10 , semiconductor switch  15  is instantaneously turned on for a prescribed time and electromagnetic contactor  16  is turned on. This is for the purpose of preventing semiconductor switch  15  from being overheated and thereby damaged. 
     Operation module  17  includes a plurality of buttons operated by a user of uninterruptible power supply device  1  and an image display unit on which various pieces of information are displayed. The user operates operation module  17  to thereby allow the power supply of uninterruptible power supply device  1  to be turned on and off, and allow one mode to be selected from the bypass power feeding mode and the inverter power feeding mode. 
     Control device  18  can be configured, for example, by a microcomputer and the like. By way of example, control device  18  incorporates a memory and a central processing unit (CPU), each of which is not shown, and is capable of executing a control operation (described later) by software processing implemented by the CPU executing a program stored in advance in the memory. Alternatively, the control operation can also be partially or entirely implemented by hardware processing using an incorporated and dedicated electronic circuit or the like in place of software processing. 
     Control device  18  controls the entire uninterruptible power supply device  1  based on signals from operation module  17 , AC input voltage Vi, AC input current Ii, DC voltage VDC, battery voltage VB, AC output current Io, AC output voltage Vo, and the like. In other words, based on the detected value of AC input voltage Vi, control device  18  detects whether a power interruption occurs or not, and controls converter  6  and inverter  10  in synchronization with the phase of AC input voltage Vi. 
     Further, in a normal state in which the AC power is supplied from commercial AC power supply  21 , control device  18  controls converter  6  such that DC voltage VDC reaches a desired reference voltage VDCr. Also, during a power interruption in which supply of the AC power from commercial AC power supply  21  is stopped, control device  18  stops the operation of converter  6 . 
     Further, in a normal state, control device  18  controls bidirectional chopper  7  such that battery voltage VB reaches a desired reference voltage VBr. Also, during a power interruption, control device  18  controls bidirectional chopper  7  such that DC voltage VDC reaches a desired reference voltage VDCr. 
     Then, the operation of uninterruptible power supply device  1  will be described. When the inverter power feeding mode is selected in a normal state in which AC power is supplied from commercial AC power supply  21 , semiconductor switch  15  and electromagnetic contactor  16  are turned off, and electromagnetic contactors  2 ,  8 , and  14  are turned on. 
     The AC power supplied from commercial AC power supply  21  is converted by converter  6  into DC power. The DC power generated by converter  6  is supplied by bidirectional chopper  7  to be stored in battery  23  and to be fed to inverter  10 . Inverter  10  converts the DC power supplied from converter  6  into AC power and supplies the converted AC power to load  22 . Load  22  is driven by the AC power supplied from inverter  10 . 
       FIG. 2  is a diagram for illustrating a flow of electric power during a power interruption of commercial AC power supply  21 . When supply of the AC power from commercial AC power supply  21  is stopped, that is, when a power interruption occurs, the operation of converter  6  is stopped, and the DC power of battery  23  is supplied by bidirectional chopper  7  to inverter  10 . Inverter  10  converts the DC power from bidirectional chopper  7  into AC power and supplies the converted AC power to load  22 . Thus, the operation of load  22  can be continued in a time period during which DC power is stored in battery  23 . 
     Specifically, control device  18  controls bidirectional chopper  7  to step up battery voltage VB and output the stepped-up battery voltage VB to DC line L 1 . Control device  18  further controls inverter  10  to convert the DC power supplied through DC line L 1  into three-phase AC power of a commercial frequency. Thereby, as indicated by an arrow in  FIG. 2 , the DC power of battery  23  is converted into three-phase AC power of a commercial frequency and then supplied to load  22  through electromagnetic contactor  14 . The operation of converter  6  is stopped. When the remaining capacity of battery  23  reaches a predetermined lower limit value, control device  18  stops the operations of bidirectional chopper  7  and inverter  10 . Thus, uninterruptible power supply device  1  ends power feeding to load  22 . 
       FIG. 3  is a circuit block diagram showing a configuration example of bidirectional chopper  7  shown in  FIG. 1 . In  FIG. 3 , a DC line L 1  on the positive side and a DC line L 2  on the negative side are connected between bidirectional chopper  7  and inverter  10 . Capacitor  9  is connected between DC lines L 1  and L 2 . 
     In a normal state in which AC power is supplied from commercial AC power supply  21 , bidirectional chopper  7  steps down DC voltage VDC between DC lines L 1  and L 2 , and applies the stepped-down DC voltage VDC to battery  23 . Bidirectional chopper  7  supplies the DC power generated by converter  6  to battery  23  to be stored therein. 
     On the other hand, when a power interruption occurs in commercial AC power supply  21 , bidirectional chopper  7  steps up battery voltage VB and applies the stepped-up battery voltage VB between DC lines L 1  and L 2 . Bidirectional chopper  7  supplies the DC power of battery  23  to inverter  10  through DC line L 1 . 
     Bidirectional chopper  7  includes input nodes  7   a  and  7   b,  output nodes  7   c  and  7   d,  insulated gate bipolar transistors (IGBT) Q 1  and Q 2 , diodes D 1  and D 2 , and a reactor  25 . The IGBTs and the diodes correspond to one example of a “switching element”. The switching element can be configured by connecting freewheeling diodes (FWD) in antiparallel to any self-arc-extinguishing type semiconductor switching element. 
     Input node  7   a  is connected to the positive electrode of battery  23 , and input node  7   b  is connected to the negative electrode of battery  23 . Input node  7   c  is connected to DC line L 1 , and input node  7   d  is connected to DC line L 2 . 
     IGBT Q 1  has a collector connected to DC line L 1  and an emitter connected to the collector of IGBT Q 2 . IGBT Q 2  has an emitter connected to DC line L 2 . Reactor  25  is connected between input node  7   a  and the emitter of IGBT Q 1  (the collector of IGBT Q 2 ). IGBT Q 1  and IGBT Q 2  are controlled by control device  18  to be turned on and off alternately at a prescribed switching frequency. 
       FIG. 4  is a waveform diagram for illustrating the operation of bidirectional chopper  7  during a power interruption of commercial AC power supply  21 . In  FIG. 4 , IB shows a battery current, and I 1  shows a current flowing through diode D 1 . I 2  shows a current input to inverter  10 , and  13  shows a current flowing through capacitor  9 .  FIG. 4  schematically shows changes over time in currents IB, I 1  to I 3  and DC voltage VDC, which occur when IGBT Q 2  is turned on or off. 
     When a power interruption occurs in commercial AC power supply  21 , bidirectional chopper  7  steps up battery voltage VB and applies the stepped-up battery voltage VB between DC lines L 1  and L 2 . Specifically, bidirectional chopper  7  steps up battery voltage VB in accordance with the time period during which IGBT Q 2  is turned on, and then, applies the stepped-up battery voltage VB between DC lines L 1  and L 2 . One cycle T during which IGBT Q 2  is turned on and off corresponds to the sum of a period t ON  during which IGBT Q 2  is turned on and a period t OFF  during which IGBT Q 2  is turned off. One cycle T is set depending on the switching frequency. The ratio of period t ON  in one cycle T is also referred to as an “on-duty”. 
     In period t ON  during which IGBT Q 2  is turned on, electric power is accumulated in reactor  25 . In period t OFF  during which IGBT Q 2  is turned off, the electric power accumulated in reactor  25  is applied between DC lines L 1  and L 2 . The on-duty of IGBT Q 2  is increased to thereby increase the electric power accumulated in reactor  25 , so that a higher voltage can be output. Thus, by controlling the on-duty of IGBT Q 2 , DC voltage VDC can be controlled to any voltage ranging from battery voltage VB as a lower limit value up to an upper limit value that is set based on the element breakdown voltage of the IGBT and the like. Thereby, voltage VDC input into inverter  10  can be variable in accordance with the operating state of load  22 . 
     In capacitor  9 , current I 1  reaches zero in period t ON  during which IGBT Q 2  is turned on. Thus, the electric power accumulated in capacitor  9  is supplied to inverter  10 . Due to such discharging of capacitor  9 , the voltage across the terminals of capacitor  9  (corresponding to DC voltage VDC) lowers. In period t ON , current I 3  and current I 2  have the same magnitude. 
     On the other hand, in period t OFF  during which IGBT Q 2  is turned off, capacitor  9  is charged with the electric power output from reactor  25 , and thereby, the voltage across the terminals of capacitor  9  (corresponding to DC voltage VDC) rises. In period t OFF , current I 1  becomes equal to the sum of current I 2  and current I 3 . 
     In the waveform of current I 3  flowing through capacitor  9 , an area S 1  corresponds to the electric charge accumulated in capacitor  9 , and an area S 2  corresponds to the electric charge discharged from capacitor  9 . Area S 1  is basically equal to area S 2 . 
     By repeating charging and discharging of capacitor  9  in accordance with switching control of IGBTs Q 1  and Q 2 , a ripple current that cyclically increases and decreases flows through capacitor  9 . The cycle in which the ripple current increases and decreases is equivalent to the cycle in which IGBTs Q 1  and Q 2  are controlled. Inside capacitor  9 , a voltage is generated that is given as the product of the equivalent series resistance (ESR) and the ripple current. This voltage is superimposed as a voltage variation on DC voltage VDC. When the ripple current occurs, power loss occurs in the ESR of capacitor  9 , which leads to heat generation in capacitor  9 . 
     During a power interruption of commercial AC power supply  21 , as the electric power supplied from inverter  10  to load  22  increases, the electric power associated with charging and discharging of capacitor  9  also increases. Bidirectional chopper  7  increases the on-duty of IGBT Q 2  to thereby increase the electric power accumulated in reactor  25 . In such a situation, since the ripple current in capacitor  9  also increases, the loss occurring in the ESR of capacitor  9  increases. Such a loss generates heat to thereby raise the temperature of capacitor  9 , which may accelerate performance deterioration in capacitor  9 . 
     Thus, uninterruptible power supply device  1  according to the present embodiment is configured to supply electric power to load  22  in consideration of the temperature rise in capacitor  9  during a power interruption of commercial AC power supply  21 . Thereby, the performance deterioration in capacitor  9  is suppressed. 
       FIG. 5  is a circuit block diagram showing a configuration of a controller that controls bidirectional chopper  7  shown in  FIG. 1 . The controller is included in control device  18 . In  FIG. 5 , uninterruptible power supply device  1  further includes a current detector  30  and voltage detectors  32  and  34 . 
     Current detector  30  detects an instantaneous value of a current IB flowing through battery  23  (hereinafter also referred to as a “battery current”), and supplies a signal IB showing the detected value to the controller. Voltage detector  32  detects an instantaneous value of DC voltage VDC that appears between DC lines L 1  and L 2 , and supplies a signal VDC showing the detected value to the controller. Voltage detector  34  detects an instantaneous value of battery voltage VB, and supplies a signal VB showing the detected value to the controller. 
     The controller includes a subtractor  50 , a compensator  52 , a duty ratio conversion circuit  54 , an averaging circuit (AVG)  60 , a setting module  62 , a discharge time counter  64 , and a comparator  66 . 
     Subtractor  50  calculates a deviation between a reference voltage VDCr and DC voltage VDC that is detected by voltage detector  32 . 
     Compensator  52  calculates a control amount that is applied for setting DC voltage VDC to be equal to reference voltage VDCr. Compensator  52  performs, for example, a control calculation including a proportional term and an integral term of the deviation calculated by subtractor  50 . Compensator  52  gives the calculated control amount to duty ratio conversion circuit  54  as a voltage command value. 
     Duty ratio conversion circuit  54  calculates a duty ratio used for setting DC voltage VDC at the voltage command value based on the voltage command value given from compensator  52 , signal VDC from voltage detector  32 , and signal VB from voltage detector  34 . Based on the calculated duty ratio, duty ratio conversion circuit  54  generates control signals G 1  and G 2  for turning on and off IGBTs Q 1  and Q 2  of bidirectional chopper  7 . Duty ratio conversion circuit  54  outputs the generated control signals G 1  and G 2  to IGBTs Q 1  and Q 2 , respectively. 
     Averaging circuit  60  receives signal IB from current detector  30 . Based on signal IB, averaging circuit  60  calculates an average value IB AVG  of battery current IB in one switching cycle T of bidirectional chopper  7 , and outputs the calculated average value IB AVG  to setting module  62 . 
     Based on average value IB AVG  given from averaging circuit  60 , setting module  62  sets a discharge permissible time DT lim . Discharge permissible time DT lim  is a limit value of the discharge time of battery  23  during a power interruption of commercial AC power supply  21 . Specifically, setting module  62  sets discharge permissible time DT lim  in accordance with the relation shown in  FIG. 6  between average value IB AVG  of the battery current and discharge permissible time DT lim . 
       FIG. 6  is a diagram schematically showing the relation between average value IB AVG  of the battery current and discharge permissible time DT lim . In  FIG. 6 , the horizontal axis shows average value IB AVG  of the battery current while the vertical axis shows discharge permissible time DT lim . 
     Referring to  FIG. 6 , when average value IB AVG  of the battery current exceeds a threshold value Ith set in advance, discharge permissible time DT lim  is shorter as average value IB AVG  is larger. This is based on the fact that, as the electric power supplied to load  22  increases, average value IB AVG  also increases. Specifically, in period t ON  during which IGBT Q 2  of bidirectional chopper  7  is turned on, battery current IB flows into capacitor  9  through reactor  25  and diode D 1  and is input to inverter  10 . Thus, as the electric power supplied to load  22  increases and current I 2  input to inverter  10  becomes higher, battery current IB also becomes higher. 
     Accordingly, the controller monitors average value IB AVG  of the battery current to determine whether the electric power supplied to load  22  increases or not. 
     The relation shown in  FIG. 6  is set such that, when battery current IB having average value IB AVG  continuously flows, the temperature of capacitor  9  that rises due to the loss occurring in the ESR does not exceed a prescribed permissible temperature at which the performance of capacitor  9  deteriorates. In the relation shown in  FIG. 6 , the discharge time is constant when average value IB AVG  is less than threshold value Ith. In other words, threshold value Ith is set such that the temperature of capacitor  9  does not exceed the permissible temperature even when battery current IB having average value IB AVG  equal to threshold value Ith continuously flows. 
     Setting module  62  has a storage area (not shown) in which the relation shown in  FIG. 6  between average value IB AVG  of the battery current and discharge permissible time DT lim  is stored in advance as a map for setting the discharge permissible time. Then, upon reception of average value IB AVG  from averaging circuit  60 , setting module  62  sets discharge permissible time DT lim  based on the map. 
     The relation shown in  FIG. 6  can be experimentally obtained based on the temperature of capacitor  9  detected in advance while battery current IB continuously flows and the temperature characteristics of the performance deterioration in capacitor  9 . Alternatively, the relation shown in  FIG. 6  may be analytically obtained by calculating the loss occurring in the ESR of capacitor  9 . 
     When setting module  62  sets discharge permissible time DT lim  based on the relation in  FIG. 6 , it outputs the set discharge permissible time DT lim  to comparator  66 . 
     Discharge time counter  64  measures discharge time DT of battery  23 . When a power interruption occurs in commercial AC power supply  21  and discharging of battery  23  is started, discharge time counter  64  measures discharge time DT and provides the measured discharge time DT to comparator  66 . 
     Comparator  66  determines whether or not discharge time DT measured by discharge time counter  64  exceeds discharge permissible time DT lim . At this time, when discharge time DT does not exceed discharge permissible time DT lim , comparator  66  determines that the temperature of capacitor  9  is lower than the prescribed permissible temperature and the performance of capacitor  9  is less likely to deteriorate. Then, comparator  66  outputs a signal STP inactivated to an L (logic low) level to duty ratio conversion circuit  54 . Signal STP is used for stopping the operation of bidirectional chopper  7 . 
     On the other hand, when discharge time DT exceeds discharge permissible time DT lim , comparator  66  determines that the temperature of capacitor  9  is equal to or higher than the prescribed permissible temperature and the performance of capacitor  9  is more likely to deteriorate. Then, comparator  66  outputs signal STP activated to an H (logic high) level to duty ratio conversion circuit  54 . 
     Upon reception of signal STP from comparator  66 , duty ratio conversion circuit  54  generates a control signal GB used for turning off IGBTs Q 1  and Q 2 , and then, outputs generated control signal GB to IGBTs Q 1  and Q 2 . Upon reception of control signal GB, IGBTs Q 1  and Q 2  each are turned off to thereby stop the stepping-up operation of bidirectional chopper  7 , and then, discharging of battery  23  is also stopped. 
     According to the relation in  FIG. 6 , when average value IB AVG  of the battery current exceeds threshold value Ith, discharge permissible time DT lim  is shorter as average value IB AVG  is larger. Thus, at a high load, average value IB AVG  increases, so that discharge time DT shortens. As a result, a temperature rise in capacitor  9  can be suppressed, so that the performance deterioration in capacitor  9  can be suppressed. 
     The map for setting the discharge permissible time is not limited to the map shown in  FIG. 6 , but may be any map as long as discharge permissible time DT lim  is shorter as average value IB AVG  of the battery current is larger. 
     As described above, the power supply device according to the first embodiment has a configuration including: bidirectional chopper  7  that performs voltage conversion for the DC voltage of battery  23 ; and capacitor  9  that smoothes the voltage output from bidirectional chopper  7  and outputs the smoothed voltage to inverter  10 . In such a configuration, when the battery current exceeds the threshold value, the discharge permissible time is shortened as the battery current increases. Thereby, as power loss in capacitor  9  increases, the discharge time of battery  23  is shortened. Thus, a temperature rise in capacitor  9  can be suppressed, so that the performance deterioration in capacitor  9  can be suppressed. 
     Second Embodiment 
       FIG. 7  is a circuit block diagram showing a configuration of a controller that controls a bidirectional chopper  7  in an uninterruptible power supply device according to the second embodiment. The controller is included in a control device  18 . 
     The controller shown in  FIG. 7  is the same as the controller shown in  FIG. 5  except that the controller shown in  FIG. 7  additionally includes a comparator  70 , a timer  72 , and an OR circuit  74 . Thus, detailed description of the same portions will not be repeated. 
     Comparator  70  determines whether or not battery voltage VB detected by voltage detector  34  falls below a discharge cut-off voltage VL that is set in advance. Discharge cut-off voltage VL can be set based on the minimum value of the discharge voltage at which discharging can be safely performed. Discharging to the level exceeding the minimum value may deteriorate the power storage performance of battery  23 . 
       FIG. 8  is a diagram schematically showing an example of discharge characteristics of battery  23 . In  FIG. 8 , the horizontal axis shows discharge time while the vertical axis shows battery voltage VB. 
     As shown in  FIG. 8 , when a power interruption occurs in commercial AC power supply  21 , electric power is discharged from battery  23  and supplied to load  22 . As discharging of battery  23  progresses, battery voltage VB gradually lowers. When battery  23  is discharged to some extent, battery voltage VB rapidly lowers. Waveforms k 1  to k 3  in the figure are different in the magnitudes of the currents discharged from battery  23 . Waveform k 1  shows the highest discharge current, and waveform k 3  shows the lowest discharge current. As the discharge current from battery  23  increases, battery voltage VB rapidly lowers. 
     As the current supplied to load  22  increases during a power interruption of commercial AC power supply  21 , the ripple current in capacitor  9  increases and the discharge current from battery  23  increases as described above. As the discharge current from battery  23  increases, battery voltage VB rapidly lowers. Thus, battery voltage VB may reach discharge cut-off voltage VL before discharge time DT reaches discharge permissible time DT lim . 
     Therefore, in the second embodiment, when discharge time DT of battery  23  exceeds discharge permissible time DT lim  set in accordance with average value IB AVG  of the battery current, or when battery voltage VB falls below discharge cut-off voltage VL, the stepping-up operation of bidirectional chopper  7  is stopped to thereby stop discharging of battery  23 . 
     Specifically, in  FIG. 7 , when battery voltage VB is higher than discharge cut-off voltage VL, comparator  70  outputs a signal DET at an L level. When battery voltage VB lowers and falls below discharge cut-off voltage VL, comparator  70  outputs signal DET activated to an H level. When signal DET output from comparator  70  transitions from an L level to an H level, timer  72  measures the time during which signal DET is maintained at an H level. When the value measured by timer  72  reaches a prescribed threshold value, signal DET at an H level is input to one terminal of OR circuit  74 . 
     The signal output from comparator  66  is input to the other terminal of OR circuit  74 . Based on the logical sum of signal DET output from timer  72  and signal STP output from comparator  66 , OR circuit  74  generates a signal STP 1  and outputs the generated signal STP 1  to duty ratio conversion circuit  54 . Signal STP 1  is used for stopping the operation of bidirectional chopper  7 . 
     Specifically, when signal STP output from comparator  66  is at an H level or when signal DET output from timer  72  is at an H level, OR circuit  74  outputs signal STP 1  activated to an H level. In other words, OR circuit  74  is configured to output signal STP 1  at an H level to duty ratio conversion circuit  54  when discharge time DT exceeds discharge permissible time DT lim  or when battery voltage VB falls below discharge cut-off voltage VL. 
     Upon reception of signal STP 1  from OR circuit  74 , duty ratio conversion circuit  54  generates a control signal GB for turning off IGBTs Q 1  and Q 2 , and then, outputs the generated control signal GB to IGBTs Q 1  and Q 2 . In response to control signal GB, IGBTs Q 1  and Q 2  each are turned off to thereby stop the stepping-up operation of bidirectional chopper  7 , and then, discharging of battery  23  is also stopped. 
     As described above, the power supply device according to the second embodiment is configured as follows. Specifically, when the battery current exceeds the threshold value, the discharge permissible time is set to be shorter as the battery current is higher. Also, when the discharge time of battery  23  exceeds the discharge permissible time or when the battery voltage falls below the discharge cut-off voltage, bidirectional chopper  7  is stopped to thereby stop discharging of battery  23 . 
     Thereby, a temperature rise in capacitor  9  resulting from discharging of battery  23  can be suppressed, and overdischarging of battery  23  can also be suppressed. As a result, performance deterioration in capacitor  9  and battery  23  can be suppressed. 
     It should be understood that the embodiments disclosed herein are illustrative and non-restrictive in every respect. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the meaning and scope equivalent to the terms of the claims. 
     REFERENCE SIGNS LIST 
       1  uninterruptible power supply device,  2 ,  8 ,  14 ,  16  electromagnetic contactor,  3 ,  11 ,  30  current detector,  4 ,  9 ,  13  capacitor,  5 ,  12 ,  25  reactor,  6  converter,  7  bidirectional chopper,  10  inverter,  15  semiconductor switch,  17  operation module,  18  control device,  21  commercial AC power supply,  22  load,  23  battery,  32 ,  34  voltage detector,  50  subtractor,  52  compensator,  54  duty ratio conversion circuit,  60  averaging circuit,  62  setting module,  64  discharge time counter,  66 ,  70  comparator,  72  timer,  74  OR circuit, T 1  AC input terminal, T 2  AC output terminal, T 3  battery terminal, L 1 , L 2  DC line, Q 1 , Q 2  IGBT, D 1 , D 2  diode.