Power supply apparatus

The power supply apparatus includes a first power device, a second power unit, and controlling means. Each of the first power device and the second power unit supplies DC power to a DC supply line. The first power device makes constant voltage control. The second power unit includes a second power device configured to make inclination control of monotonically decreasing its output voltage with an increase of its output current, and of monotonically increasing its output voltage with a decrease of its output current. Upon acknowledging that a measurement (a magnitude of a current flowing through the DC supply line) exceeds an optimal current magnitude (a magnitude of a current supplied to the DC supply line from the first power device operating at maximum conversion efficiency), the controlling means outputs an instruction such that a magnitude of a current supplied to the DC supply line from the second power unit is identical to a difference between the measurement and the optimal current magnitude. The second power device modifies a condition of the inclination control, thereby adjusting its output current to a current corresponding to the instruction without varying its output voltage.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to co-pending application: “ POWER SUPPLY APPARATUS” filed even date herewith in the names of Hiroaki KOSHIN and Takuya KAGAWA as a national phase entry of PCT/JP2010/060684 filed on Jun. 23, 2010, which application is assigned to the assignee of the present application and is incorporated by reference herein.

TECHNICAL FIELD

The present invention relates to power supply apparatus configured to operate simultaneously multiple power devices to supply DC power therefrom to a load device connected thereto.

BACKGROUND ART

In the past, there have been proposed various power supply apparatus which simultaneously operates plurality of power devices to supply DC power from the power devices to one or more load devices connected to the power devices.

As an instance of the prior power supply apparatus, power supply apparatus including two power devices each configured to decrease monotonically its output voltage with an increase of its output current (see Japanese patent laid-open publication No. 10-248253). In this power supply apparatus, the two power devices shows individual output current-output voltage characteristics of which lines have different gradient from each other. This means that, when the two power devices varies their output current by the same extent, one of the power devices shows a variation of the output voltage different from that of the other power device.

In this power supply apparatus, each of the power devices operates to reach a balance point determined by its output current-output voltage characteristics and the load current in accordance with a magnitude of a total consumed current (load current) of all the load devices. Therefore, each of the power devices can output the desired output voltage and output current.

However, in this power supply apparatus where the two power devices shows individual output current-output voltage characteristics of which lines have different gradient from each other, the output voltages (that is, supply voltages for the load current) of each of the power device are varied due to a magnitude of the load current. Therefore, the power supply apparatus fails to output the stable supply voltage. In this power supply apparatus, in order to keep the supply voltage to the load device at a constant level irrespective of changes of the output currents of the each of the power devices, both of the power supply apparatus have to vary their output current-output voltage characteristics. For satisfying this requirement, the power supply apparatus needs to have a more complex configuration.

Now, in order to solve the above problem, there has been proposed power supply system including the power devices which are operated simultaneously. One of the power devices is configured to make constant voltage control, and the remaining is configured to make inclination control of outputting its output voltage of a DC voltage decreased with an increase of its output current. In this power supply apparatus, the power device performing the inclination control supplies a current to the load device while having its output voltage identical to an output voltage (reference voltage) of the power device performing the constant voltage control. In this situation, the power device performing the constant voltage current compensates for a shortage a current supplied to the load device. Therefore, according to the power supply apparatus, the supplied voltage for the load device is kept constant voltage (the output voltage of the power device performing the constant voltage control) irrespective of the variation of the load current. Consequently, this power supply apparatus can successfully supply power to the load device.

For example, a commercial power source is used as a power source connected to the above power device. The power device to be connected to the commercial power source includes a DC/DC converter. The DC/DC converter suffers from internal loss such as conduction loss (loss caused by an on-resistance of a switching element, a parasitic resistance of an inductor, or the like). As a result, a variation of conversion efficiency of the power device (a proportion of output power of the power device to input power of the power device) with an output current is expressed by a characteristics line having an output current maximizing the conversion efficiency, as shown in (a) ofFIG. 9. With operating the power device connected to the commercial power source so as to supply an output current which is identical to a current supplied from the power device operating at the maximum conversion efficiency, it is possible to operate the power device efficiently.

However, in the prior power supply apparatus, each power device varies its output current depending on a magnitude of the load current. Therefore, the prior power supply apparatus is likely to operate inefficiently the power device connected to the commercial power.

DISCLOSURE OF INVENTION

In view of the above insufficiency, the present invention has been aimed to propose power supply apparatus capable of operating a power device to be connected to a commercial power source at maximum conversion efficiency.

The power supply apparatus in accordance with the present invention includes a first power device, a second power unit, load current measuring means, judging means, and controlling means. Each of the first power device and the second power unit is adapted to be connected to a DC supply line to be connected to a load device and is configured to supply DC power to the load device through the DC supply line. The load current measuring means is configured to measure a current flowing through the DC supply line and output a measurement indicative of a magnitude of the measured current. The judging means is configured to, upon obtaining the measurement from the load current measuring means, judge whether or not the obtained measurement exceeds an optimal current magnitude. The first power device is adapted to be connected to a commercial power source. The first power device is configured to convert power obtained from the commercial power source to the DC power so as to perform constant voltage control of supplying a constant voltage to the DC supply line irrespective of a current supplied therefrom to the DC supply line. The optimal current magnitude is defined as a magnitude of a current which is outputted to the DC supply line from the first power device operating maximum power-conversion efficiency thereof. The second power unit includes at least one of second power devices. The second power device is configured to perform inclination control of monotonically decreasing an output voltage supplied therefrom to the DC supply line with an increase of an output current supplied therefrom to the DC supply line, and of monotonically increasing the output voltage with a decrease of the output current. The controlling means is configured to, upon acknowledging that the judging means determines that the measurement exceeds the optimal current magnitude, send an instruction to the second power device such that a magnitude of a current supplied from the second power unit to the DC power line is identical to a difference between the measurement and the optimal current magnitude. The second power device includes adjusting means configured to adjust the output current on the basis of the instruction received from the controlling means. The adjusting means is configured to, upon receiving the instruction from the controlling means, modify the condition of the inclination control so as to adjust the output current to a current corresponding to the instruction without varying the output voltage.

In a preferred aspect, the controlling means is configured to, upon acknowledging that the judging means determines that the measurement does not exceed the optimal current magnitude, control the second power unit so as to terminate supplying the current to the DC supply line.

In a preferred aspect, the second power unit includes a plurality of the second power devices. The current supplied to the DC supply line from the second power unit is defined as the sum of the output currents respectively supplied from the second power devices.

BEST MODE FOR CARRYING OUT THE INVENTION

In the embodiment explained below, a house of a single-family dwelling is exemplified as a building where power supply apparatus3of the present invention is installed. The power supply apparatus3in accordance with the present invention may be installed in a housing complex. As shown inFIG. 2, there are a DC power supply unit101configured to supply DC power and DC devices (load devices)102placed in a house “H”. Each DC device102is a load activated by DC power. The DC power supply unit101has an output terminal connected to the DC devices102via a DC supply line Wdc and supplies DC power from the output terminal to the DC devices via the DC supply line Wdc. There is a DC breaker114interposed between the DC power supply unit101and the DC device102. The DC breaker114is configured to monitor a current flowing through the DC supply line Wdc. Upon detecting an abnormal state, the DC breaker114limits or terminates electrical power supply from the DC power supply unit101to the DC device102via the DC supply line Wdc.

The DC supply line Wdc is adopted as a power line for DC power as well as a communication line. For example, it is possible to communicate between devices connected to the DC supply line Wdc by means of superimposing on a DC voltage a communication signal used for transmitting data and made of a high-frequency carrier. This technique is similar to a power line communication technique where a communication signal is superimposed on an AC voltage applied to a power line for supplying an AC power.

The aforementioned DC supply line Wdc is connected to a home server116via the DC power supply unit101. The home server116is a primary device for constructing a home communication network (hereinafter referred to as “home network”). The home server116is configured to communicate with a subsystem constructed by the DC devices102in the home network, for example.

In the illustrated instance, an information system K101, lighting systems K102and K105, an entrance system K103, and a home alarm system K104are adopted as the subsystem. The information system K101includes the informational DC device102such as a personal computer, a wireless access point, a router, and an IP telephone transceiver. Each of the lighting systems K102and K105includes the lighting DC device102such as a lighting fixture. The entrance system K103includes the DC device102configured to respond to a visitor and to monitor an intruder. The home alarm system K104includes the alarming DC device102such as a fire alarm. The each subsystem is an autonomous distributed system, and operates by itself.

The aforementioned DC breaker114is associated with the subsystem. In the illustrated instance, the information system K101, a set of the lighting system K102and the entrance system K103, the home alarm system K104, and the lighting system K105are associated with the four single DC breakers114, respectively. A connection box121is provided to associate the single DC breaker114with a plurality of the subsystems. The connection box121is configured to divide a system of the DC supply line for each subsystem. In the illustrated instance, the connection box121is interposed between the lighting system K102and the entrance system K103.

The information system K101includes the DC device102connected to a DC socket131preliminarily provided to the house “H” (provided at the time of constructing the house “H”) as a wall outlet or a floor outlet, for example.

The lighting system K102includes the lighting fixture (DC device102) preliminarily provided to the house “H”. Meanwhile, the lighting system K105includes the lighting fixture (DC device102) connected to a ceiling-mounted hooking receptacle132preliminarily provided on a ceiling. It is noted that the lighting fixture is attached to the ceiling-mounted hooking receptacle132by a contractor at the time of constructing an interior of the house “H” or attached to the ceiling-mounted hooking receptacle132by a resident of the house “H”.

The lighting fixture of the lighting system K102can receive a control instruction from an infrared remote controller. Further, the control instruction can be sent by use of a communication signal transmitting to the lighting fixture from a switch141connected to the DC supply line Wdc. The lighting fixture of the lighting system K105can receive a control instruction from an infrared remote controller. Further, the control instruction can be sent by use of a communication signal transmitting to the lighting fixture from a switch142connected to the DC supply line Wdc. In short, each of the switches141and142has a function of communicating with the DC device102. The control instruction may be given by the manipulation of each of the switches141and142. In addition, the control instruction can be sent by use of a communication signal transmitted to the DC device102from the home server116or the other DC device102of the home network. The control instruction for the lighting fixture indicates such as turning on, turning off, dimming, and blinking.

Any DC device102can be connected to the DC outlet131or the ceiling-mounted hooking outlet132. Each of the DC outlet131and the ceiling-mounted hooking receptacle132supplies DC power to the connected DC device102. Therefore, the DC outlet131and the ceiling-mounted hooking receptacle132are hereinafter collectively referred to as “DC outlet”, if a distinction between the DC outlet131and the ceiling-mounted hooking receptacle132is unnecessary.

The DC outlet has a case which is provided with a connection slot (plug-in connection slot) for inserting a terminal of the DC device102. The case houses a terminal receiving member configured to directly contact to the terminal which is inserted into the case via the connection slot. In brief, the DC outlet with above mentioned configuration makes contact-type power supply. The DC device102with a communication function is capable of transmitting a communication signal via the DC supply line Wdc. The communication function is provided to the DC outlet in addition to the DC device102.

The home server116is connected to the home network as well as the wide area network NT constructing the Internet. While the home server116is connected to the wide area network NT, a user can enjoy service provided by a center server (computer server)200connected to the wide area network NT.

The center server200provides a service of monitoring or controlling a device (which is mainly the DC device102, but which may be other apparatus having a communication function) connected to the home network via the wide area network NT, for example. The service enables monitoring or controlling a device connected to the home network by use of a communication terminal (not shown) having a browsing function such as a personal computer, an internet TV, and a mobile telephone equipment.

The home server116has a function of communicating with the center server200connected to the wide area network NT and a function of communicating with a device connected to the home network. The home server116further has a function of collecting identification information (e.g. IP address) concerning a device connected to the home network.

The home server116utilizes the function of communicating with the center server200, thereby enabling the communication terminal connected to the wide area network NT to monitor and control the home device via the center server200. The center server200mediates a communication between the home device and the communication terminal on the wide area network NT.

When a user attempts to monitor or control the home device by use of the communication terminal, the user controls the communication terminal so as to store a monitoring request or a control request in the center server200. The device placed in the house establishes periodically one-way polling communication, thereby receiving the monitoring request or control request from the communication terminal. According to the aforementioned operation, it is possible to monitor or control the device placed in the house by use of the communication terminal.

When an event (such as fire detection) of which the home device should notify the communication terminal occurs, the home device notifies the center server200of occurrence of the event. When the center server200is notified of the occurrence of the event by the home device, the center server200notifies the communication terminal of the occurrence of the event by use of an e-mail.

A function of communicating with the home network of the home server116includes an important function of detecting and managing a device constructing the home network. By means of utilizing UPnP (Universal Plug and Play), the home server116automatically detects a device connected to the home network. The home server116further includes a display device117having a browsing function, and controls the display device117to display a list of the detected device. The display device117includes a touch panel or another user interface unit. Therefore, it is possible to select a desired one from options displayed on a screen of the display device117. Accordingly, a user (a contractor or a resident) of the home server116can monitor and control the device through the screen of the display device117. The display device117may be separated from the home server116.

The home server116manages information with relation to connection of devices. For example, the home server116stores a type or a function and an address of the device connected to the home network. Therefore, it is possible to make a linked operation between devices of the home network. As described in the above, the information with relation to connection of a device is automatically detected. In order to make the linked operation between the devices, it is sufficient that an association between devices is automatically made by an attribution of a device. An information terminal such as a personal computer may be connected to the home server116. In this instance, the association between devices can be made by use of a browsing function of the information terminal.

Each of the devices holds a relation with regard to the linked operations between the devices. Therefore, the devices can make the linked operation without requiring to access to the home server116. After establishing an association with regard to the linked operation of respective devices, a lighting fixture, which is one of the devices, is caused to turn on and off by manipulation of a switch, which is another of the devices, for example. Although the association with regard to the linked operation is made for the devices belonging to the same subsystem, the association with regard to the linked operation may be made for the devices belonging to the different subsystems.

Basically, the DC supply unit101is configured to generate DC power from AC power supplied from a commercial power source AC located outside. In the illustrated configuration, the commercial power source AC is connected to an AC/DC converter112including a switching regulator via a main breaker111. The main breaker111is embedded in a distribution board110. DC power outputted from the AC/DC converter112is supplied to each DC breaker114via a cooperation control unit113.

The DC supply unit101is provided with a secondary cell162in view of a period (an outage of the commercial power source AC) in which the DC supply unit101fails to receive electrical power from the commercial power source AC. For example, the secondary cell162may be a lithium ion secondary battery. In the DC supply unit101, a solar cell161and a fuel cell163configured to generate DC power can be used together with the secondary cell162. Each of the solar cell161, the secondary cell162, and the fuel cell163acts as a dispersed power source in view of a main power source including the AC/DC converter112configured to create DC power from AC power supplied from the commercial power source AC. Besides, the secondary cell162includes a charge controlling circuit (not shown).

The secondary cell162is charged by at least one of the commercial power source AC, the solar cell161, and the fuel cell163at a proper timing. The secondary cell162is discharged during a period in which the DC supply unit101fails to receive electrical power from the commercial power source AC. In addition, the secondary cell162is discharged at appropriate timing as necessary. The cooperation control unit113is configured to control discharge and charge of the secondary cell162and to make cooperation between the main power source and the dispersed power sources. In brief, the cooperation control unit113functions as a DC power control unit configured to control distributing to the DC device102electrical power from the main power source and dispersed power source constituting the DC supply unit101.

A drive voltage of the DC device102is selected from different voltages respectively suitable to individual devices of different voltage requirements. For this purpose, the cooperation control unit113is preferred to include a DC/DC converter configured to convert DC voltage from the main power source and dispersed power sources into a desired voltage. Normally, a fixed voltage is applied to one subsystem (or the DC device102connected to one particular DC breaker114). However, different voltages may be selectively applied to one subsystem by use of three or more lines. Use of two wired DC supply line Wdc can vary the voltage applied between wires with time. The DC/DC converter can be placed at plural points in a similar fashion as the DC breakers.

In the aforementioned configuration instance, only one AC/DC converter112is provided. However, a plurality of the AC/DC converters112may be connected in parallel to each other. When the plurality of the AC/DC converters112is provided, it is preferred to vary the number of the AC/DC converters112being activated in accordance with an amount of power required by loads.

Each of the AC/DC converter112, the cooperation control unit113, the DC breaker114, the solar cell161, the secondary cell162, and the fuel cell163is provided with a communication function. Therefore, the linked operation can be performed in response to status of each of the main power source, dispersed power sources, and loads including the DC device102. Like a communication signal used for the DC device102, a communication signal used by the communication function is transmitted by being superimposed on DC voltage.

In the aforementioned instance, in order to convert AC power outputted from the main breaker111to DC power, the AC/DC converter112is placed in the distribution panel110. However, the AC/DC converter112is not necessarily placed in the distribution panel110. For example, branch breakers (not shown) may be connected to an output side of the main breaker111in the distribution panel110such that a plurality of systems is branched off from an AC supply line, and an AC/DC converter may be provided to an AC supply line of each of the systems. That is, each system may be provided with an apparatus configured to convert AC power into DC power.

In this arrangement, it is possible to provide the DC supply unit101to each unit such as a floor or room of the house “H”. Accordingly, it is possible to manage the DC supply unit101for each system. In addition, it is possible to shorten a distance between the DC supply unit101and the DC device102configured to utilize DC power. Therefore, it is possible to reduce power loss caused by a voltage drop which occurs in the DC supply line Wdc. Alternatively, the main breaker111and branch breaker may be housed in the distribution panel110, and the AC/DC converter112, the cooperative control unit113, the DC breaker114, and the home server116may be placed in another panel different from the distribution panel110.

Next, an explanation referring toFIG. 1is made to the power supply apparatus3housed in the DC power supply unit101. The power supply apparatus3includes a plural (in the illustrated instance, four) power devices4(5,6) and a monitoring device7. The power devices4(5,6) are configured to operate simultaneously to supply a DC power to the DC device (load device)102. The monitoring device7is configured to monitor a whole system regarding the DC power supply.

The plural power devices4include a single first power device5and plural (in the illustrated instance, three) second power devices6(6ato6c).

The plural second power devices6(6ato6c) constitute a second power unit8. In the present embodiment, the second power unit8includes the three second power devices6a,6b, and6c. Therefore, a current supplied from the second power unit8to the DC supply line Wdc is defined as the sum (=Ioa+Iob+Ioc) of the output currents Io2respectively supplied from the three second power devices6.

In the present embodiment, the second power unit8includes the three second power devices6. By contrast, the second power unit8may include the single second power device6. In this arrangement, the current supplied from the second power unit8to the DC supply line Wdc is identical to the output current Io2of the single second power device6. Alternatively, the second power unit8may include the two second power devices6, or may include the four or more second power devices6.

The first power device5is configured to provide the output voltage Vo1of a DC voltage which is a constant voltage irrespective of a magnitude of the output current Io1(see (b) inFIG. 5). The first power device5receives a voltage supplied from the commercial power source AC as the input voltage Vi1(seeFIG. 3). That is, the first power source5is defined as a commercial power source dedicated power device configured to supply DC power to the DC device102on the basis of the voltage supplied from the commercial power source AC.

In brief, the first power device5is adapted to be connected to the commercial power source AC. The first power device5is configured to convert power obtained from the commercial power source AC to DC power so as to perform constant voltage control of supplying a constant voltage (output voltage Vo1) to the DC supply line Wdc irrespective of a current (output current Io1) supplied therefrom to the DC supply line Wdc.

In the present embodiment, as shown inFIG. 2, the first power device5is connected to the commercial power source AC via the AC/DC converter112. That is, the AC/DC converter112converts the AC voltage from the commercial power source AC to a predetermined DC voltage, and provides the resultant DC voltage to the first power device5. Thus, the input voltage Vi1is the DC voltage outputted from the AC/DC converter112. Alternatively, the input voltage Vi1may be an AC voltage supplied from the commercial power source AC. In this arrangement, the first power device5is provided with an AC/DC converter configured to convert the input voltage Vi1of the AC voltage to a DC voltage and supply the resultant DC voltage to the DC/DC converter52.

As shown inFIG. 3, the first power device5includes a voltage meter50, a switching controller51, and the DC/DC converter52. The voltage meter50is configured to measure the output voltage Vo1. The switching controller51is configured to generate a pulse width modulation signal S1which has its duty ratio selected based on a reference voltage V2and a detection voltage V1of the voltage meter50. The DC/DC converter52includes a switching device520. The switching device520is configured to be turned on and off in accordance with the duty ratio of the pulse width modulation signal S1outputted from the switching controller51.

The voltage meter50includes two resistors500and501connected in series and a voltage follower502configured to receive a divided voltage generated by the resistors500and501, thereby measuring the output voltage Vo1of the first power device5. The voltage meter50is configured to measure the output voltage Vo1and provide the detection voltage V1corresponding to the measured output voltage Vo1to the switching controller51.

The switching controller51includes a switching IC510configured to receive the detection voltage (an output voltage of the voltage follower502) V1of the voltage meter50as well as the reference voltage V2.

The switching IC510is configured to output to the switching device520the pulse width modulation signal S1which has its duty ratio selected such that a difference voltage (=V2−V1) between the detection voltage V1and the reference voltage V2is kept constant. That is, the switching IC510is configured to select the duty ratio of the pulse width modulation signal S1such that the output voltage Vo1(the detection voltage V1) is kept constant.

The DC-DC converter52includes a smoothing capacitor521, an inductor522, the switching device520, a diode523, and a smoothing capacitor524which are arranged in this order from its input side. The DC-DC converter52operates to turn on and off the switching device520for increasing the input voltage Vi1.

For example, the switching device520is a field-effect transistor. The switching device520has its gate receiving the pulse width modulation signal S1from the switching IC510via a resistor525. Therefore, the switching device520is turned on and off in accordance with the duty ratio of the pulse width modulation signal S1from the switching controller51. While the switching device520is turned on, the switching device520has its source electrically connected to its drain. Thereby, the inductor522continues to accumulate electromagnetic energy. Thereafter, when the switching device520is turned off, the inductor522discharges the accumulated electromagnetic energy. Thereby, the input voltage Vi1is increased. The increased input voltage Vi1is smoothed by the smoothing capacitor524and is supplied to the DC device102(seeFIG. 1) as the output voltage Vo1.

According to the aforementioned operation, the first power device5can make a feedback control to have the output current-output voltage characteristics of keeping the output voltage Vo1constant irrespective of the magnitude of the output current Io1, as shown in (b) ofFIG. 5.

The second power device6is configured to provide its output voltage Vo2(a voltage supplied to the DC supply line Wdc) decreasing monotonically as its output current Io2(a current supplied to the DC supply line Wdc) increases, as shown in (a) ofFIG. 5. A line indicative of output current-output voltage characteristics of the second power device6can be expressed as a relation of Vo2=−α*Io2+V0(α>0, V0>0). In this relation, V0is constant, and satisfies a relation V0=Vo2+α*Io2. It is noted that “α” may be different in each of the second power devices6and may be common to the second power devices6.

In other words, the second power device6is configured to perform inclination control of monotonically decreasing the output voltage Vo2supplied to the DC supply line Wdc with an increase of the output current Io2supplied therefrom to the DC supply line Wdc, and of monotonically increasing the output voltage Vo2with a decrease of the output current Io2.

As shown inFIG. 1, the second power devices6a,6b, and6care connected to the solar cell161, the secondary cell162, and the fuel cell163, respectively. The second devices6receive output voltages from the corresponding cells161to163as their input voltages Vi2(seeFIG. 4), respectively. In other words, the second power device6ais defined as a solar cell dedicated power device (PV converter) configured to supply DC power to the DC device102based on the supplied voltage from the solar cell161. The second power device6bis defined as a secondary cell dedicated power device (BAT converter) configured to supply DC power to the DC device102based on the supplied voltage from the secondary cell162. The second power device6cis defined as a fuel cell dedicated power device (FC converter) configured to supply DC power to the DC device102based on the supplied voltage from the fuel cell163. With regard to the first power device5, the second power devices6are treated as the other power devices.

As shown inFIG. 4, each second power device6includes a current meter60, a voltage meter61, a switching controller62, a DC-DC converter63, and adjustment means64. The current meter60is configured to measure the output current Io2. The voltage meter61is configured to measure the output voltage Vo2. The switching controller62is configured to generate a pulse width modulation signal S2which has its duty ratio selected on the basis of a detection voltage V5of the voltage meter61and a voltage V8outputted from the current meter60. The DC/DC converter63includes a switching device630. The switching device630is configured to be turned on and off in accordance with the duty ratio of the pulse width modulation signal S2outputted from the switching controller62. The adjusting means64is configured to adjust the output current Io2in accordance with an instruction from a control unit73(seeFIG. 1) as explained below.

The current meter60includes resistors600and “605”, a current IC601configured to measure a voltage across the resistor600, resistors602and603for dividing an output voltage V3of the current IC601, and a voltage follower604configured to receive a divided voltage generated by the resistors602and603. Thus, the current meter60is configured to measure the output current Io2.

The voltage meter61includes two resistors610and611connected in series and a voltage follower612configured to receive a divided voltage generated by the resistors610and611. The voltage meter61is configured to supply the detection voltage V5corresponding to the measured output voltage Vo2to the switching controller62.

The switching controller62includes a switching IC620configured to receive the detection voltage (output voltage of the voltage follower612) V5of the voltage meter61and the after-mentioned voltage V8.

The DC-DC converter63includes a smoothing capacitor631, an inductor632, the switching device630, a diode633, and a smoothing capacitor634which are arranged in this order from its input side. The DC-DC converter63operates to turn on and off the switching device620for increasing the input voltage V12.

The adjusting means64includes a CPU640, two resistors641and642for dividing an output voltage V6of the CPU640, and a non-inverting amplifier circuit643. The CPU640is configured to receive the instruction prescribing the magnitude of the output current Io2from the after-mentioned control unit73(seeFIG. 1). The non-inverting amplifier circuit643is configured to receive a divided voltage generated by the resistors641and642.

The CPU640is configured to vary the magnitude of the output current Io2on the basis of the instruction received from the control unit73while the power supply apparatus3is in operation (the power supply apparatus3supplies power to the DC device102).

As shown inFIG. 1, the monitoring device7includes a load current meter (load current detecting means)70, a remaining amount meter71, a judgment unit72, the control unit (controlling means)73. The load current meter70is configured to measure a load current ILsupplied to the DC device102. The remaining amount meter71is configured to determine an available power range of each of the solar cell161and the fuel cell163. Further, the remaining amount meter71is configured to measure a remaining amount of power in the secondary cell162. The judgment unit72is configured to judge whether or not the magnitude of the load current ILmeasured by the load current meter70exceeds an after-mentioned optimal current magnitude Im. The control unit73is configured to control the magnitude of the output current Io2of each second power device6.

The load current meter70is configured to measure the load current IL. The load current ILis defined as a total consumption current of the DC devices102. For example, the load current meter70is configured to measure a consumption current of each DC device102at a predetermined time interval while the power supply apparatus3is in operation (the power supply apparatus3supplies power to the DC device102). Further, the load current meter70is configured to determine the load current ILby calculating the sum of the measured consumption currents. The predetermined time interval may be a time interval (e.g., a few milliseconds) enough to enable a load-following operation. Thus, the load current meter70is configured to measure a magnitude (current value) I0of a current (load current IL) flowing through the DC supply line Wdc and output a measurement indicative of the measured magnitude.

The remaining amount meter71measures an output voltage and an output current of the secondary cell162at the above time interval while the power supply apparatus3is in operation (the power supply apparatus3supplies power to the DC device102). Further, the remaining amount meter71calculates the remaining amount of the power stored in the secondary cell162on the basis of a detection result (measurements of the output current and the output voltage).

The judgment unit72is configured to, upon receiving the measurement from the load current meter70, judge whether or not the received measurement exceeds the optimal current magnitude Im.

As described in the above, the judgment unit72judges whether or not the load current ILhas the magnitude greater than the optimal current magnitude Im. In addition, the judgment unit72refers to the remaining amount of the power in the secondary cell162obtained by the remaining amount meter71, and judges whether or not the secondary cell162stores enough power enabling the second power device (BAT converter)6bconnected to the secondary cell162to supply the output current Io2(Iob). For example, upon acknowledging that the remaining amount of the secondary cell162is equal to or more than a predetermined threshold, the judgment unit72judges that the power left in the secondary cell162is enough to enable the BAT converter6bto supply the output current Iob. By contrast, upon acknowledging that the remaining amount of the secondary cell162is less than the predetermined threshold, the judgment unit72judges that the power left in the secondary cell162is not enough to enable the BAT converter6bto supply the output current Iob.

The control unit73is configured to decide an amount of power to be supplied from each of the power devices (5,6) to the DC devices102with regard to a whole system, and adjusts an output of each power device (5,6) in response to the decided amount. The control unit73transmits the instruction prescribing the magnitude of the output current Io2, to the adjusting means64of each second power device6. Here, the instruction may be a numerical amount directly defining the magnitude of the output current Io2. Alternatively, the instruction may be a numerical amount defining a magnitude of a voltage converted from the magnitude of the output current Io2. Besides, the instruction is not limited to a numerical amount defining the magnitude of the output current Io2of the second power device6. The instruction may be a numerical amount defining output power of the second power device6.

The CPU640shown inFIG. 4is configured to output the output voltage V6having a magnitude corresponding to the instruction received from the control unit73(seeFIG. 1). The non-inverting amplifier circuit643is configured to increase its output voltage V7with an increase of the output voltage V6of the CPU640and to decrease its output voltage V7with a decrease of the output voltage V6of the CPU640.

The current detector60has a differential amplifier circuit606interposed between the voltage follower604and the resistor605. The differential amplifier circuit606is configured to supply, to the switching IC620, the voltage V8(=β*(V7−V4) (β>0)) which is proportional to a difference voltage (=V7−V4) between the output voltage V7of the non-inverting amplifier circuit643and the detection voltage V4(the output voltage of the voltage follower604) of the current meter60. Even if the detection voltage V4is not changed, the voltage V8is increased as the output voltage V6and the output voltage V7are increased in accordance with the instruction from the control unit73. By contrast, the voltage V8supplied to the switching IC620is decreased as the output voltage V6and the output voltage V7are decreased. It is noted that “β” is selected such that the switching IC620can make a calculation of the voltage V8and the detection voltage V5.

The switching IC620is configured to output the pulse width modulation signal S2to the switching device630. The duty ratio of the pulse width modulation signal S2is selected (varied) such that a difference voltage (=V8−V5) between the voltage V8and the detection voltage V5(i.e., a voltage (=β*V7−(V5+β*V4))) is kept constant. For instance, when the voltage (=β*V7−(V5+β*V4)) is increased from a preceding one, the switching IC620increases the duty ratio of the pulse width modulation signal S2to reduce the voltage (=β*V7−(V5+β*V4)) (to the preceding one). By contrast, when the voltage (=β*V7−(V5+β*V4)) is decreased from a preceding one, the switching IC620decreases the duty ratio of the pulse width modulation signal S2to increase the voltage (=β*V7−(V5+β*V4)) (to the preceding one).

For example, the switching device630is a field-effect transistor. The switching device630has its gate receiving the pulse width modulation signal S2from the switching IC620via a resistor635. While the switching device630is turned on, the switching device630has its source electrically connected to its drain. Thereby, the inductor632continues to accumulate electromagnetic energy. Thereafter, when the switching device630is turned off, the inductor632discharges the accumulated electromagnetic energy. Thereby, the input voltage V12is increased. The raised input voltage V12is smoothed by the smoothing capacitor634and is outputted to the DC device102(seeFIG. 1) as the output voltage Vo2.

When the output current Io2(the detection voltage V4) is increased, the voltage (=β*V7−(V5+β*V4)) is decreased from a preceding one. In response, the switching IC620decreases the duty ratio of the pulse width modulation signal S2to increase the voltage (=B*V7−(V5+β*V4)) to the preceding one. As a result, the output voltage Vo2(the detection voltage V5) is decreased. When the output current Io2(the detection voltage V4) is decreased, the voltage (=β*V7−(V5+β*V4)) is increased from a preceding one. In response, the switching IC620increases the duty ratio of the pulse width modulation signal S2to reduce the voltage (=β*V7−(V5+β*V4)) to the preceding one. As a result, the output voltage Vo2(the detection voltage V5) is increased.

As shown in (a) ofFIG. 5, the second power device6makes a feedback control to keep the voltage (=β*V7−(V5+β*V4)) constant, thereby having the output current-output voltage characteristics (a characteristics of keeping Vo2+α*Io2constant) of decreasing monotonically (linearly) the output voltage Vo2with an increase of the output current Io2.

The line indicative of the output current-output voltage characteristics of the second power device6has an intersection point with a line indicative of the output current-output voltage characteristics of the first power device5. Therefore, when the second power device6is used in combination with the first power device5, the output voltage Vo2is coordinated with the output voltage Vo1of the first power device5. Consequently, the output current Io2has its magnitude corresponding to the output voltage Vo2which has the same magnitude as the output voltage Vo1.

When the output current Io2decreases, the output voltage Vo2varies depending on the output current-output voltage characteristics shown inFIG. 6, thereby temporarily increasing (see (A) inFIG. 6). As seen from the above, the output current Io2is increased with an increase of the output voltage Vo2. As a result, the detection voltage V4also increases (see (B) inFIG. 6). The duty ratio of the pulse width modulation signal S2decreases because the voltage (=β*V7−(V5+β*V4)) decreases with an increase of the detection voltage V4. Consequently, the output voltage Vo2(the detection voltage V5) decreases (see (C) inFIG. 6). Thus, the output voltage Vo2becomes identical to the output voltage Vo1.

When the output current Io2increases, the output voltage Vo2varies depending on the output current-output voltage characteristics shown inFIG. 6, thereby temporarily decreasing (see (D) inFIG. 6). As seen from the above, when the output voltage Vo2decreases, the output current Io2decreases. As a result, the detection voltage V4also decreases (see (E) inFIG. 6). The duty ratio of the pulse width modulation signal S2increases because the voltage (=β*V7−(V5+β*V4)) increases with a decrease of the detection voltage V4. Consequently, the output voltage Vo2(the detection voltage V5) increases (see (F) inFIG. 6). Thus, the output voltage Vo2becomes identical to the output voltage Vo1.

Next, an explanation referring toFIG. 7is made to an instance where the second power device6receives the instruction from the control unit73. For example, when the total consumption current (load current IL) of the DC devices102increases, the control unit73provides to the second power device6the instruction so as to increase the output current Io2yet keep the output voltage Vo2(the detection voltage V5) constant. In response to the instruction, the output voltage V7and the voltage V8(=β*V7−V4)) are increased. Consequently, since the voltage (=β*V7−(V5+β*V4)) is increased, the duty ratio of the pulse width modulation signal S2is increased. As a result, the output voltage Vo2temporarily exceeds the output voltage Vo1(see (A) inFIG. 7). This operation means adding a predetermined voltage to the output voltage Vo2of the second power device6. When the output voltage Vo2is increased by the addition of the predetermined voltage, the output current Io2(the detection voltage V4) is also increased (see (B) inFIG. 7). Since the voltage (=β*V7−(V5+β*V4)) decreases with an increase of the detection voltage V4, the duty ratio of the pulse width modulation signal S2is decreased. Consequently, the output voltage Vo2is lowered (see (C) inFIG. 7). The second power device6repeats this operation. Thereby, the output voltage Vo2becomes identical to the output voltage Vo1in due course. As a result, the line indicative of the output current-output voltage characteristics of the second power device6is shifted in order to obtain the output current Io2at intersections with the line indicative of constant voltage characteristics (the output current-output voltage characteristics of the first power device5), thus obtained output current reaching the output current Io2corresponding to the instruction (the current magnitude I1).

For example, when the load current ILdecreases, the control unit73provides to the second power device6the instruction so as to decrease the output current Io2yet keep the output voltage Vo2(the detection voltage V5) constant. In response to the instruction, the output voltage V7and the voltage V8(=β*V7−V4)) are decreased. Consequently, since the voltage (=β*V7−(V5+β*V4)) is decreased, the duty ratio of the pulse width modulation signal S2is decreased. As a result, the output voltage Vo2temporarily falls below the output voltage Vo1(see (D) inFIG. 7). This operation means subtracting a predetermined voltage from the output voltage Vo2of the second power device6. When the output voltage Vo2is decreased by the subtraction of the predetermined voltage, the output current Io2(the detection voltage V4) is also decreased (see (E) inFIG. 7). Since the voltage (=β*V7−(V5+β*V4)) increases with a decrease of the detection voltage V4, the duty ratio of the pulse width modulation signal S2is increased. Consequently, the output voltage Vo2is raised (see (F) inFIG. 7). The second power device6repeats this operation. Thereby, the output voltage Vo2becomes identical to the output voltage Vo1in due course. As a result, the line indicative of the output current-output voltage characteristics of the second power device6is shifted in order to obtain the output current Io2at intersections with the line indicative of constant voltage characteristics (the output current-output voltage characteristics of the first power device5), thus obtained output current reaching the output current Io2corresponding to the instruction (the current magnitude I0).

As seen from the above, the adjustment means64is configured to, upon receiving the instruction from the control unit (controlling means)73, modify a condition of the inclination control so as to adjust the output current Io2to a current corresponding to the instruction without varying the output voltage Vo2. The adjusting means64varies the condition of the inclination control so as to shift the line indicative of the output current-output voltage characteristics. In other words, the adjusting means64makes a translational movement of the line indicative of the output current-output voltage characteristics.

Irrespective of shifting the output current-output voltage characteristics of the second power device6, the output voltage Vo2is coordinated with the output voltage Vo1of the first power device5. Therefore, the output current Io2has its magnitude corresponding to the output voltage Vo2which has the same magnitude as the output voltage Vo1.

According to the aforementioned configuration, each second device6is enabled to shift its output current-output voltage characteristics on the basis of the instruction received from the control unit73, as shown inFIG. 7. Even after the output current-output voltage characteristics are shifted, the second power device6provides its output voltage Vo2identical to the output voltage Vo1of the first power device5. Therefore, the output current Io2which is outputted from the second device6when the output voltage Vo2has the same magnitude as the output voltage Vo1can be provided to the DC device102. Consequently, even if the load current ILis varied, the power supply apparatus3can select the magnitude of the output current Io2for each second power device6in match with the load current IL. In addition, the output voltage Vo2can be kept constant because the second power device6has its output voltage Vo2kept identical to the output voltage Vo1of the first power device5even if the load current ILchanges its magnitude. Therefore, it is possible to make stable power supply for the DC device102.

The following explanation is made to an instance. InFIG. 5, (a) shows the output current-output voltage characteristics of the second power device6, and (b) shows the output current-output voltage characteristics of the first power device5. As shown in (c) ofFIG. 5, upon receiving the instruction prescribing the magnitude I11from the control unit73, the second device6translates the line indicative of the output current-output voltage characteristics as indicated by an arrow in (c) ofFIG. 5, thereby increasing the magnitude of the output current Io2of the second device6from the magnitude I12to the magnitude I11.

The second power device6has a configuration for monotonically decreasing the output voltage Vo2with an increase of the output current Io2. This configuration can be implemented by slight modification to the configuration of the first power device5only with exception of few additional parts.

The next explanation is made to the monitoring device7shown inFIG. 1. In the following explanation, the optimal current magnitude Im is defined as the magnitude of the output current Io1of the first power device5operating with its maximum power-conversion efficiency. In other words, the optimal current magnitude Im is defined as a magnitude of a current (the output current Io1) which is outputted to the DC supply line Wdc from the first power device5operating with its maximum power-conversion efficiency. For example, the conversion efficiency of the first power device5is defined as a proportion (=POUT/PIN) of power POUT to power PIN. The power POUT is defined as power supplied from the first power device5to the DC supply line Wdc. The power PIN is defined as power supplied from the commercial power source AC to the first power device5.

In the present embodiment, the control unit73is configured to, upon acknowledging that the judgment unit72determines that the measurement (the magnitude of the load current IL) exceeds the optimal current magnitude Im, send the instruction to the second power device6such that the current (=Ioa+Iob+Ioc) supplied from the second power unit to the DC power line Wdc is identical to the difference between the measurement and the optimal current magnitude Im.

In brief, upon acknowledging that the magnitude of the load current ILmeasured by the load current meter70exceeds the optimal current magnitude Im, the control unit73of the monitoring device7translates (shifts) the line indicative of the output current-output voltage characteristics by use of the adjusting means64(seeFIG. 4) of each second power device6such that the output current Io1of the first power device5has the same magnitude as the optimal current magnitude Im (i.e., the sum of the output currents Io2respectively outputted from the second power devices6having their output voltages Vo2equal to the output voltage Vo1of the first power device5has the same magnitude as the difference between the magnitude of the load current ILand the optimal current magnitude Im).

By contrast, upon acknowledging that the measurement I0falls below the optimal current magnitude Im, the control unit73controls the second power unit8to terminate supplying the current to the DC supply line Wdc. For example, upon acknowledging that the measurement I0falls below the optimal current magnitude Im, the control unit73sends an output termination signal to each second power device6of the second power unit8. The second power device6is configured to, upon receiving the output termination signal, terminate supplying the output current Io2.

Next, an explanation referringFIGS. 8 and 9is made to an operation of the power supply apparatus3in accordance with the present embodiment.

First, the load current meter70measures the magnitude I0of the load current IL(51inFIG. 8). Next, the judgment unit72judges whether or not the magnitude I0of the load current ILis greater than the optimal current magnitude Im (S2). When the magnitude I0is greater than the optimal current magnitude Im, the control unit73sends, to the PV converter6a, the instruction such that the magnitude of the output current Ioa of the PV converter6ais identical to the difference between the magnitude I0and the optimal current magnitude Im within the available power range of the solar cell161. (S3). Upon receiving the instruction from the control unit73, the PV converter6acontrols its adjusting means64to make the translation movement of the line indicative of the output current-output voltage characteristics of the PV converter6a, thereby supplying the output current Ioa having its magnitude identical to the difference (=I0−Im) to the DC device102.

As shown in (b) ofFIG. 9, when the maximum magnitude I1of the output current Ioa of the PV converter6ais less than the difference (=I0−Im) (i.e., I0−Im>I1) (S4), the control unit73sends the instruction to the BAT converter6bsuch that the output current Iob of the BAT converter6bhas the magnitude I2(I2=I0−Im−I1) (S5). Upon receiving the instruction from the control unit73, the BAT converter6bcontrols its adjusting means64to make the translation movement of the line indicative of the output current-output voltage characteristics of the BAT converter6b, thereby supplying the output current Iob having the magnitude I2to the DC device102as shown in (c) ofFIG. 9.

According to the steps S3to S5, the PV converter6ais used for compensating for the difference (=I0−Im) prior to the BAT converter6b. Therefore, it is possible to promote energy conservation.

A next explanation is made to an alternative example of the steps S4and S5. Upon receiving the difference (=I0−Im), the control unit73may make instant judgment on the basis of calculation. That is, the control unit73judges whether or not the maximum magnitude I1of the output current Ioa supplied from the PV converter6aat the present insolation condition is less than the difference. Upon acknowledging that the maximum magnitude I1is less than the difference, the control unit73calculates the current magnitude I2defining the magnitude of the output current Iob of the BAT converter6b. Then, the control unit73sends the instruction to the PV converter6asuch that the PV converter6asupplies the output current Ioa of the maximum magnitude I1as well as sends the instruction to the BAT converter6bsuch that the BAT converter6bsupplies the output current Iob of the magnitude I2.

Thereafter, the first power device5supplies the output current Io1of the optimal current magnitude Im to the DC device102(S6). The first power device5compensates for a shortage until the current having the total magnitude I0is supplied after the PV converter6aand the BAT converter6cvaries their output currents Ioa and Iob. Therefore, power is successfully supplied to the load (the DC device102).

With regard to the step S2, when the magnitude I0of the load current ILis not greater than the optimal current magnitude Im, the control unit73controls each second power device6such that each power device6terminates supplying power. Thus, the first power device5supplies the output current Io1of the magnitude I0to the DC device102(S6).

With regard to the step S4, when the difference current (the load current minus the current having the optimal current magnitude) has the same magnitude as the output current Ioa of the PV converter6a, the control unit73deactivates the second power devices (the BAT converter6band the FC converter6c) other than the PV converter6a. Consequently, the first power device5supplies the output current Io1of the optimal current magnitude Im to the DC device102(S6). In this situation, the control unit73transmits the instruction to the second power device6aand sends the output termination signal to the remaining second power devices6band6c.

The power supply apparatus3of the present embodiment performs the aforementioned operation. Therefore, in response to an increase of the load current IL, the power supply apparatus3increases a total magnitude of the output currents Ioa and Iob of the PV and BAT converters6aand6bby an extent of the increase of the load current IL. Further, in response to a decrease of the load current IL, the power supply apparatus3decreases the total magnitude of the output currents Ioa and Iob of the PV and BAT converters6aand6bby an extent of the decrease of the load current IL. When decreasing the total magnitude of the output currents Ioa and Iob of the PV and BAT converters6aand6b, the power supply apparatus3may decrease the output current Iob of the BAT converter6bprior to the output current Ioa of the PV converter6a.

The power supply apparatus3may be configured to perform the operations respectively defined by the steps S1to S6regularly (at a predetermined time interval). With this arrangement, it is possible to adjust the output current Io2in response to a variation of a supply capacity of the cell (161,162,163) or the magnitude of the load current IL. Besides, the predetermined time interval may be a time interval (e.g., a few milliseconds) enough to enable the load-following operation. In addition, the power supply apparatus3may perform the operations respectively defined by the steps S1to S6irrespective of the predetermined time interval.

The power supply apparatus3of the present embodiment includes the commercial power source dedicated power device (the first power device)5, the one or more other power devices (the second power devices)6, the load current measuring means (the load current meter)70, and the controlling means (the control unit)73. The commercial power source dedicated power device (the first power device)5is configured to receive power from the commercial power source AC and provide the output voltage Vo1of a DC voltage which is a constant voltage irrespective of the magnitude of the output current Io1. The other power device6is configured to receive power and provide the output voltage Vo2of a DC voltage which is monotonically decreased with an increase of the output current Io2. The other power device6is operated simultaneously with the first power device5, thereby supplying DC power to the load device102. The load current meter70is configured to measure the magnitude of the load current ILsupplied to the load device102. The controlling means73is configured to adjust the magnitude of the output current Io2of the second power device6. An optimal current magnitude Im is defined as the magnitude of the output current Io1of the first power device5operating at the maximum conversion efficiency thereof. The second power device6includes the adjusting means64configured to translate the line indicative of the output current-output voltage characteristics defining the relation between the output current Io2and the output voltage Vo2during the power supply to the load device102. Upon acknowledging that the magnitude of the load current ILmeasured by the load current meter70exceeds the optimal current magnitude Im, the control unit73shifts the line indicative of the output current-output voltage characteristics of each second power device6by use of the adjusting means64such that the total magnitude of the output currents Io2of the second devices6is identical to the difference between the magnitude of the load current ILand the optimal current magnitude Im.

In other words, the power supply apparatus of the present embodiment includes the first power device5, the second power unit8, the load current measuring means (load current meter)70, the judging means (judgment unit)72, and the controlling means (control unit)73. Each of the first power device5and the second power unit8is adapted to be connected to the DC supply line Wdc to be connected to the load device102and is configured to supply DC power to the load device102through the DC supply line Wdc. The load current measuring means70is configured to measure the current (load current) ILflowing through the DC supply line Wdc and output the measurement I0indicative of the magnitude (current magnitude)10of the measured current (load current) IL. The judging means72is configured to, upon obtaining the measurement I0from the load current measuring means70, judge whether or not the obtained measurement I0exceeds the optimal current magnitude Im. The first power device5is adapted to be connected to the commercial power source AC. The first power device5is configured to convert power obtained from the commercial power source AC to the DC power so as to perform the constant voltage control of supplying the constant voltage (output voltage) Vo1to the DC supply line Wdc irrespective of the current (output current) Io1supplied therefrom to the DC supply line Wdc. The optimal current magnitude Im is defined as the magnitude of the current (output current) Io1which is outputted to the DC supply line Wdc from the first power device5operating the maximum power-conversion efficiency thereof. The second power unit8includes at least one of the second power devices6. The second power device6is configured to perform the inclination control of monotonically decreasing the output voltage Vo1supplied therefrom to the DC supply line Wdc with an increase of the output current Io2supplied therefrom to the DC supply line Wdc, and of monotonically increasing the output voltage Vo2with a decrease of the output current Io2. The controlling means is configured to, upon acknowledging that the judging means72determines that the measurement I0exceeds the optimal current magnitude Im, send the instruction to the second power device6such that the magnitude of the current supplied from the second power unit8to the DC power line Wdc is identical to the difference (the magnitude of the difference current) between the measurement I0and the optimal current magnitude Im. The second power device6includes the adjusting means64configured to adjust the output current Io2on the basis of the instruction received from the controlling means73. The adjusting means64is configured to, upon receiving the instruction from the controlling means73, modify the condition of the inclination control so as to adjust the output current Io2to the current corresponding to the instruction without varying the output voltage Vo2.

As described in the above, upon acknowledging that the load current ILhas its magnitude greater than the magnitude (optimal current magnitude Im) of the output current Io1supplied from the first power device5connected to the commercial power source AC and operating at the maximum conversion efficiency, the apparatus of the present embodiment adjusts the magnitude of the output current Io2of each power device6such that the output current Io1of the first power device5has the same magnitude as the optimal current magnitude Im. Consequently, it is possible to operate the first power device at the maximum conversion efficiency.

Further, upon acknowledging that the magnitude of the load current ILmeasured by the load current meter70is not greater than the optimal current magnitude Im, the control unit73controls the second power unit6so as to terminate supplying the current. In other words, the control unit73is configured to, upon acknowledging that the judgment unit72determines that the measurement I0does not exceed the optimal current magnitude Im, control the second power unit8so as to terminate supplying the current to the DC supply line Wdc.

Accordingly, the present embodiment can operate the first power device5at a condition as close to the maximum conversion efficiency as possible, even when the magnitude of the load current ILis less than the optimal current magnitude Im.

Besides, in the present embodiment, when the magnitude of the load current ILis greater than the optimal current magnitude Im, the PV converter6aand the BAT converter6bcooperate to compensate for the difference current (the load current minus the current having the optimal current magnitude). Alternatively, instead of a combination of the PV converter6aand the BAT converter6b, the PV converter6aand the FC converter6cmay cooperate to compensate for the difference current, or the BAT converter6band the FC converter6cmay cooperate to compensate for the difference current.